View source for Biology

{{short description|Science that studies life}}
{{other uses}}
{{redirect|Biological|other uses}}

{{multiple image|perrow=2|total_width = 270|image1=EscherichiaColi NIAID.jpg|image2=Thompson's Gazelle.jpeg|image3=Goliath beetle.jpg|image4=Tree Fern.jpg|footer=Biology is the study of life.
* top: [[Escherichia coli|''E. coli'' bacteria]] and [[gazelle]]
* bottom: [[Goliathus|Goliath beetle]] and [[tree fern]]}}
{{TopicTOC-Biology}}
'''Biology''' is the [[science|scientific]] study of [[life]].<ref name = "urry2017a">{{cite book | last1 = Urry | first1 = Lisa | last2 = Cain | first2 = Michael | last3 = Wasserman | first3 =  Steven | last4 = Minorsky | first4 = Peter |  last5 = Reece | first5 = Jane | chapter = Evolution, the themes of biology, and scientific inquiry | title = Campbell Biology | publisher = Pearson | edition = 11th | date = 2017 | location = New York | pages = 2–26 | isbn = 978-0134093413}}</ref><ref name = "hillis2020">{{cite book | last1 = Hillis | first1 = David M. | last2 = Heller | first2 =  H. Craig | last3 = Hacker  | first3 = Sally D. | last4 = Laskowski | first4 = Marta J. | last5 = Sadava | first5 = David E. | chapter = Studying life | title = Life: The Science of Biology | publisher = W. H. Freeman | edition = 12th | date = 2020 | isbn = 978-1319017644}}</ref><ref name = "freeman2017a">{{cite book | last1 = Freeman| first1 = Scott | last2 = Quillin | first2 = Kim | last3 = Allison | first3 = Lizabeth | last4 = Black | first4 = Michael |  last5 = Podgorski | first5 = Greg | last6 = Taylor | first6 = Emily | last7 = Carmichael | first7 = Jeff | chapter = Biology and the three of life | title = Biological Science | publisher = Pearson | edition = 6th | date = 2017 | location = Hoboken, N.J. | pages = 1–18 | isbn = 978-0321976499}}</ref> It is a [[natural science]] with a broad scope but has several unifying themes that tie it together as a single, coherent field.<ref name = "urry2017a"/><ref name = "hillis2020"/><ref name = "freeman2017a"/> For instance, all [[organism]]s are made up of [[Cell (biology)|cell]]s that process hereditary information encoded in [[gene]]s, which can be transmitted to future generations. Another major theme is [[evolution]], which explains the unity and diversity of life.<ref name = "urry2017a"/><ref name = "hillis2020"/><ref name = "freeman2017a"/> [[Metabolism|Energy processing]] is also important to life as it allows organisms to [[Animal locomotion|move]], grow, and [[reproduce]].<ref name = "urry2017a"/><ref name = "hillis2020"/><ref name = "freeman2017a"/> Finally, all organisms are able to regulate their own [[internal environment]]s.<ref name = "urry2017a"/><ref name = "hillis2020"/><ref name = "freeman2017a"/><ref name = "modell2015">{{Cite journal | last1 = Modell |first1 = Harold | last2 = Cliff | first2 = William | last3 = Michael | first3 = Joel | last4 = McFarland | first4 = Jenny | last5 = Wenderoth | first5 = Mary Pat | last6 = Wright | first6 = Ann | date = December 2015 | title = A physiologist's view of homeostasis | journal = Advances in Physiology Education | volume = 39 | issue = 4 | pages = 259–266 | doi=10.1152/advan.00107.2015 | issn = 1043-4046 | pmc = 4669363 | pmid = 26628646}}</ref><ref name = "davies2013">{{cite journal |author1=Davies, PC |author2=Rieper, E |author3=Tuszynski, JA | title=Self-organization and entropy reduction in a living cell | journal=Bio Systems | volume=111 | issue=1 | pages=1–10 | date=January 2013 | pmid=23159919 | pmc=3712629 | doi=10.1016/j.biosystems.2012.10.005 }}</ref>

[[Biologist]]s are able to study life at multiple [[Biological organisation|levels of organization]],<ref name="urry2017a" /> from the [[molecular biology]] of a cell to the [[anatomy]] and [[physiology]] of [[plants]] and [[animals]], and evolution of [[population]]s.<ref name = "urry2017a"/><ref name=aquarenagloss>Based on definition from: {{cite web |url=http://www.bio.txstate.edu/~wetlands/Glossary/glossary.html |archive-url=https://web.archive.org/web/20040608113114/http://www.bio.txstate.edu/~wetlands/Glossary/glossary.html |archive-date=2004-06-08 |title=Aquarena Wetlands Project glossary of terms |author=<!--Staff writer(s); no by-line.--> |publisher=Texas State University at San Marcos}}</ref> Hence, there are multiple [[Outline of biology#Branches|subdisciplines within biology]], each defined by the nature of their [[research question]]s and the [[List of research methods in biology|tool]]s that they use.<ref>{{Cite book|title=Molecular Biology, Principles of Genome Function|last=Craig|first=Nancy|year=2014|isbn=978-0-19-965857-2}}</ref><ref>{{Cite journal|last1=Mosconi|first1=Francesco|last2=Julou|first2=Thomas|last3=Desprat|first3=Nicolas|last4=Sinha|first4=Deepak Kumar|last5=Allemand|first5=Jean-François|last6=Vincent Croquette|last7=Bensimon|first7=David|date=2008|title=Some nonlinear challenges in biology|journal=Nonlinearity|language=en|volume=21|issue=8|page=T131|doi=10.1088/0951-7715/21/8/T03|issn=0951-7715|bibcode=2008Nonli..21..131M|s2cid=119808230 }}</ref><ref name="AB-20141208">{{cite web|url=https://www.astrobio.net/origin-and-evolution-of-life/life-become-complex-happen-beyond-earth/|title=How Did Life Become Complex, And Could It Happen Beyond Earth?|last=Howell|first=Elizabeth|date=8 December 2014|work=[[Astrobiology Magazine]]|access-date=14 February 2018|archive-date=17 August 2018|archive-url=https://web.archive.org/web/20180817193332/https://www.astrobio.net/origin-and-evolution-of-life/life-become-complex-happen-beyond-earth/|url-status=usurped}}</ref> Like other [[scientist]]s, biologists use the [[scientific method]] to make [[observation]]s, pose questions, generate [[hypothesis|hypotheses]], perform [[experiment]]s, and form conclusions about the world around them.<ref name = "urry2017a"/>

Life on [[Earth]], which emerged more than 3.7 billion years ago,<ref name="Pearce 343–364">{{cite journal |last1=Pearce |first1=Ben K.D. |last2=Tupper |first2=Andrew S. |last3=Pudritz |first3=Ralph E. |author3-link=Ralph Pudritz |last4=Higgs |first4=Paul G. |display-authors=3 |date=March 1, 2018 |title=Constraining the Time Interval for the Origin of Life on Earth |journal=[[Astrobiology (journal)|Astrobiology]] |volume=18 |issue=3 |pages=343–364 |arxiv=1808.09460 |s2cid=4419671 |bibcode=2018AsBio..18..343P |doi=10.1089/ast.2017.1674 |issn=1531-1074 |pmid=29570409}}</ref> is immensely diverse. Biologists have sought to study and classify the various forms of life, from [[prokaryote|prokaryotic]] organisms such as [[archaea]] and [[bacteria]] to [[Eukaryote|eukaryotic]] organisms such as [[protist]]s, [[fungi]], plants, and animals. These various organisms contribute to the [[biodiversity]] of an [[ecosystem]], where they play specialized roles in the [[Biogeochemical cycle|cycling]] of [[nutrient]]s and [[energy]] through their [[biophysical environment]].

==History==
{{Further | History of biology}}
{{For timeline|Timeline of biology and organic chemistry}}
[[File:Hooke-bluefly.jpg|thumb|right|alt=A drawing of a fly from facing up, with wing detail|Diagram of a fly from [[Robert Hooke]]'s innovative ''[[Micrographia]]'', 1665.]]

The earliest of roots of [[science]], which included [[medicine]], can be traced to [[ancient Egypt]] and [[Mesopotamia]] in around 3000 to 1200 [[Common Era|BCE]].<ref name= "Lindberg1">{{cite book | last= Lindberg | first= David C. | year = 2007 | chapter = Science before the Greeks | title= The beginnings of Western science: the European Scientific tradition in philosophical, religious, and institutional context | pages = 1–20 | edition = Second | location = Chicago, Illinois | publisher = University of Chicago Press | isbn= 978-0-226-48205-7}}</ref><ref name= "Grant2007a">{{cite book | last= Grant| first= Edward | year = 2007 | chapter = Ancient Egypt to Plato | title= A History of Natural Philosophy: From the Ancient World to the Nineteenth Century | url= https://archive.org/details/historynaturalph00gran| url-access= limited| pages = [https://archive.org/details/historynaturalph00gran/page/n16 1]–26 | edition = First | location = New York, New York | publisher = Cambridge University Press | isbn= 978-052-1-68957-1}}</ref> Their contributions later entered and shaped Greek [[natural philosophy]] of [[classical antiquity]].<ref name= "Lindberg1"/><ref name= "Grant2007a"/><ref>{{cite book|last=Magner|first=Lois N.|title=A History of the Life Sciences, Revised and Expanded|url=https://books.google.com/books?id=YKJ6gVYbrGwC|year=2002|publisher=CRC Press|isbn=978-0-203-91100-6|url-status=live|archive-url=https://web.archive.org/web/20150324125937/http://books.google.com/books?id=YKJ6gVYbrGwC|archive-date=2015-03-24}}</ref><ref>{{cite book | first=Anthony | last=Serafini | title=The Epic History of Biology | date=2013 | url=https://books.google.com/books?id=Z3oECAAAQBAJ&q=biology%20egypt&pg=PA2 | access-date=14 July 2015 | isbn=978-1-4899-6327-7 | archive-date=15 April 2021 | archive-url=https://web.archive.org/web/20210415122005/https://books.google.com/books?id=Z3oECAAAQBAJ&q=biology%20egypt&pg=PA2 | url-status=live }}</ref> [[ancient Greece|Ancient Greek]] philosophers such as [[Aristotle]] (384–322 BCE) contributed extensively to the development of biological knowledge. His works such as ''[[History of Animals]]'' were especially important because they revealed his naturalist leanings, and later more empirical works that focused on biological causation and the diversity of life. Aristotle's successor at the [[Lyceum]], [[Theophrastus]], wrote a series of books on [[botany]] that survived as the most important contribution of antiquity to the plant sciences, even into the [[Middle Ages]].<ref name="eb1911">{{EB1911|wstitle=Theophrastus|inline=1}}</ref>

Scholars of the [[Science in the medieval Islamic world|medieval Islamic world]] who wrote on biology included [[al-Jahiz]] (781–869), [[Al-Dīnawarī]] (828–896), who wrote on botany,<ref name="Fahd-815">{{cite book | last=Fahd | first=Toufic |contribution=Botany and agriculture|page=815|editor-last1=Morelon |editor-first1=Régis |editor-last2=Rashed |editor-first2=Roshdi |year=1996 |title=Encyclopedia of the History of Arabic Science |volume=3 |publisher=[[Routledge]] | isbn=978-0-415-12410-2| title-link=Encyclopedia of the History of Arabic Science }}</ref> and [[Muhammad ibn Zakarīya Rāzi|Rhazes]] (865–925) who wrote on [[anatomy]] and [[physiology]]. [[Medicine]] was especially well studied by Islamic scholars working in Greek philosopher traditions, while natural history drew heavily on Aristotelian thought, especially in upholding a fixed hierarchy of life.

Biology began to quickly develop and grow with [[Anton van Leeuwenhoek]]'s dramatic improvement of the [[microscope]]. It was then that scholars discovered [[spermatozoa]], [[bacteria]], [[infusoria]] and the diversity of microscopic life. Investigations by [[Jan Swammerdam]] led to new interest in [[entomology]] and helped to develop the basic techniques of microscopic [[dissection]] and [[staining]].<ref>{{cite book |last=Magner |first=Lois N. |title=A History of the Life Sciences, Revised and Expanded |url=https://books.google.com/books?id=YKJ6gVYbrGwC |year=2002 |publisher=CRC Press |isbn=978-0-203-91100-6 |pages=133–44 |url-status=live |archive-url=https://web.archive.org/web/20150324125937/http://books.google.com/books?id=YKJ6gVYbrGwC |archive-date=2015-03-24 }}</ref>

Advances in [[microscopy]] also had a profound impact on biological thinking. In the early 19th century, a number of biologists pointed to the central importance of the [[cell (biology)|cell]]. Then, in 1838, [[Matthias Jakob Schleiden|Schleiden]] and [[Theodor Schwann|Schwann]] began promoting the now universal ideas that (1) the basic unit of organisms is the cell and (2) that individual cells have all the characteristics of [[life]], although they opposed the idea that (3) all cells come from the division of other cells. However, [[Robert Remak]] and [[Rudolf Virchow]] were able to reify the third tenet, and by the 1860s most biologists accepted all three tenets which consolidated into [[cell theory]].<ref>{{cite book | first=Jan | last=Sapp | author-link=Jan Sapp | year=2003 | title=Genesis: The Evolution of Biology | chapter=7 | publisher=Oxford University Press | location=New York | isbn=978-0-19-515618-8 }}</ref><ref>{{cite book | last=Coleman | first=William | year=1977 | title=Biology in the Nineteenth Century: Problems of Form, Function, and Transformation | publisher=Cambridge University Press | location=New York | isbn=978-0-521-29293-1 }}</ref>

Meanwhile, taxonomy and classification became the focus of natural historians. [[Carl Linnaeus]] published a basic [[Taxonomy (biology)|taxonomy]] for the natural world in 1735 (variations of which have been in use ever since), and in the 1750s introduced [[Binomial nomenclature|scientific names]] for all his species.<ref>Mayr, Ernst. ''The Growth of Biological Thought'', chapter 4</ref> [[Georges-Louis Leclerc, Comte de Buffon]], treated species as artificial categories and living forms as malleable—even suggesting the possibility of [[common descent]]. Although he was opposed to evolution, Buffon is a key figure in the [[history of evolutionary thought]]; his work influenced the evolutionary theories of both [[Lamarck]] and [[Charles Darwin|Darwin]].<ref>Mayr, Ernst. ''The Growth of Biological Thought'', chapter 7</ref>

[[File:Charles Darwin aged 51.jpg|thumb|In 1842, [[Charles Darwin]] penned his first sketch of ''[[On the Origin of Species]]''.<ref>{{harvnb|Darwin|1909|p=53}}</ref>]]
Serious evolutionary thinking originated with the works of [[Jean-Baptiste Lamarck]], who was the first to present a coherent theory of evolution.<ref name="Gould 2002 187">[[Stephen Jay Gould|Gould, Stephen Jay]]. ''The Structure of Evolutionary Theory''. The Belknap Press of Harvard University Press: Cambridge, 2002. {{ISBN|0-674-00613-5}}. p. 187.</ref> He posited that evolution was the result of environmental stress on properties of animals, meaning that the more frequently and rigorously an organ was used, the more complex and efficient it would become, thus adapting the animal to its environment. Lamarck believed that these acquired traits could then be passed on to the animal's offspring, who would further develop and perfect them.<ref name=Lam1914>[[#Lamarck1914|Lamarck (1914)]]</ref> However, it was the British naturalist [[Charles Darwin]], combining the biogeographical approach of [[Alexander von Humboldt|Humboldt]], the uniformitarian geology of [[Charles Lyell|Lyell]], [[Thomas Malthus|Malthus's]] writings on population growth, and his own morphological expertise and extensive natural observations, who forged a more successful evolutionary theory based on [[natural selection]]; similar reasoning and evidence led [[Alfred Russel Wallace]] to independently reach the same conclusions.<ref>Mayr, Ernst. ''The Growth of Biological Thought'', chapter 10: "Darwin's evidence for evolution and common descent"; and chapter 11: "The causation of evolution: natural selection"</ref><ref>{{cite book |last=Larson |first=Edward J. |title=Evolution: The Remarkable History of a Scientific Theory |chapter-url=https://books.google.com/books?id=xzLRvxlJhzkC |year=2006 |publisher=Random House Publishing Group |isbn=978-1-58836-538-5 |chapter=Ch. 3 |url-status=live |archive-url=https://web.archive.org/web/20150324124009/http://books.google.com/books?id=xzLRvxlJhzkC |archive-date=2015-03-24 }}</ref> Darwin's theory of evolution by natural selection quickly spread through the scientific community and soon became a central axiom of the rapidly developing science of biology.

The basis for modern genetics began with the work of [[Gregor Mendel]], who presented his paper, "''Versuche über Pflanzenhybriden''" ("[[Experiments on Plant Hybridization]]"), in 1865,<ref>{{Cite book|last=Henig|url=http://archive.org/details/monkingardenlost00heni|title=Op. cit|date=2000|pages=134–138}}</ref> which outlined the principles of biological inheritance, serving as the basis for modern genetics.<ref name = "miko2008a">{{cite journal | last = Miko | first = Ilona | title = Gregor Mendel's principles of inheritance form the cornerstone of modern genetics. So just what are they? | journal = Nature Education | volume = 1 | issue = 1 | pages = 134 | date = 2008 | url = https://www.nature.com/scitable/topicpage/gregor-mendel-and-the-principles-of-inheritance-593/ | access-date = 2021-05-13 | archive-date = 2019-07-19 | archive-url = https://web.archive.org/web/20190719224056/http://www.nature.com/scitable/topicpage/gregor-mendel-and-the-principles-of-inheritance-593 | url-status = live }}</ref> However, the significance of his work was not realized until the early 20th century when evolution became a unified theory as the [[Modern synthesis (20th century)|modern synthesis]] reconciled Darwinian evolution with [[classical genetics]].<ref name="Futuyma2017a">{{cite book |last1 = Futuyma | first1 = Douglas J. | last2 = Kirkpatrick | first2 = Mark | date = 2017| pages=3–26 | chapter = Evolutionary Biology | title = Evolution | edition = 4th | publisher = Sinauer Associates | location = Sunderland, Mass.}}</ref> In the 1940s and early 1950s, a [[Hershey–Chase experiment|series of experiments]] by [[Alfred Hershey]] and [[Martha Chase]] pointed to [[DNA]] as the component of [[chromosomes]] that held the trait-carrying units that had become known as [[genes]]. A focus on new kinds of model organisms such as [[viruses]] and [[bacteria]], along with the discovery of the double-helical structure of DNA by [[James Watson]] and [[Francis Crick]] in 1953, marked the transition to the era of [[molecular genetics]]. From the 1950s onwards, biology has been vastly extended in the [[Molecular Biology|molecular]] domain. The [[genetic code]] was cracked by [[Har Gobind Khorana]], [[Robert W. Holley]] and [[Marshall Warren Nirenberg]] after DNA was understood to contain [[codons]]. Finally, the [[Human Genome Project]] was launched in 1990 with the goal of mapping the general human [[genome]]. This project was essentially completed in 2003,<ref>{{cite news | url=http://news.bbc.co.uk/2/hi/science/nature/2940601.stm | title=Human genome finally complete | access-date=2006-07-22 | date=2003-04-14 | work=BBC News | first=Ivan | last=Noble | url-status=live | archive-url=https://web.archive.org/web/20060614141605/http://news.bbc.co.uk/2/hi/science/nature/2940601.stm | archive-date=2006-06-14 }}</ref> with further analysis still being published. The Human Genome Project was the first step in a globalized effort to incorporate accumulated knowledge of biology into a functional, molecular definition of the human body and the bodies of other organisms.

==Chemical basis==
===Atoms and molecules===
{{Further | Chemistry}}

All organisms are made up of [[chemical element]]s;<ref name = "urry2017b">{{cite book | last1 = Urry | first1 = Lisa | last2 = Cain | first2 = Michael | last3 = Wasserman | first3 =  Steven | last4 = Minorsky | first4 = Peter |  last5 = Reece | first5 = Jane | chapter = The chemical context of life | title = Campbell Biology | publisher = Pearson | edition = 11th | date = 2017 | location = New York | pages = 28–43 | isbn = 978-0134093413}}</ref> [[oxygen]], [[carbon]], [[hydrogen]], and [[nitrogen]] account for 96%{{explain|reason=by mass?|date=October 2022}} of all organisms, with [[calcium]], [[phosphorus]], [[sulfur]], [[sodium]], [[chlorine]], and [[magnesium]] constituting essentially all the remainder. Different elements can combine to form [[chemical compound|compound]]s such as water, which is fundamental to life.<ref name = "urry2017b"/> [[Biochemistry]] is the study of [[Chemical process|chemical processes]] within and relating to living [[Organism|organisms]]. [[Molecular biology]] is the branch of biology that seeks to understand the molecular basis of biological activity in and between cells, including [[Biomolecule|molecular]] synthesis, modification, mechanisms, and interactions.

===Water===
[[Image:3D model hydrogen bonds in water.svg|right|thumb|Model of hydrogen bonds (1) between molecules of [[Properties of water|water]]]]{{See also|Planetary habitability|Circumstellar habitable zone|Water distribution on Earth}}
Life arose from the Earth's first [[ocean]], which was formed approximately 3.8 billion years ago.<ref name="freeman2017b">{{cite book |last1=Freeman |first1=Scott |title=Biological Science |last2=Quillin |first2=Kim |last3=Allison |first3=Lizabeth |last4=Black |first4=Michael |last5=Podgorski |first5=Greg |last6=Taylor |first6=Emily |last7=Carmichael |first7=Jeff |date=2017 |publisher=Pearson |isbn=978-0321976499 |edition=6th |location=Hoboken, N.J. |pages=55–77 |chapter=Water and carbon: The chemical basis of life}}</ref> Since then, [[water]] continues to be the most abundant molecule in every organism. Water is important to life because it is an effective [[solvent]], capable of dissolving solutes such as [[sodium]] and [[chloride]] ions or other small molecules to form an [[aqueous solution|aqueous]] [[Solution (chemistry)|solution]]. Once dissolved in water, these solutes are more likely to come in contact with one another and therefore take part in [[chemical reaction]]s that sustain life.<ref name = "freeman2017b"/>

In terms of its [[Properties of water#Molecular structure|molecular structure]], water is a small [[Chemical polarity#Polar molecules|polar molecule]] with a bent shape formed by the polar covalent bonds of two hydrogen (H) atoms to one oxygen (O) atom (H<sub>2</sub>O).<ref name = "freeman2017b"/> Because the O–H bonds are polar, the oxygen atom has a slight negative charge and the two hydrogen atoms have a slight positive charge.<ref name = "freeman2017b"/> This polar [[Properties of water|property]] of water allows it to attract other water molecules via hydrogen bonds, which makes water [[Cohesion (chemistry)|cohesive]].<ref name = "freeman2017b"/> [[Surface tension]] results from the cohesive force due to the attraction between molecules at the surface of the liquid.<ref name = "freeman2017b"/> Water is also [[Adhesion|adhesive]] as it is able to adhere to the surface of any polar or charged non-water molecules.<ref name = "freeman2017b"/>

Water is [[Density|denser]] as a [[Water#Liquid water|liquid]] than it is as a solid (or [[ice]]).<ref name = "freeman2017b"/> This unique property of water allows ice to float above liquid water such as ponds, lakes, and oceans, thereby [[Thermal insulation|insulating]] the liquid below from the cold air above.<ref name = "freeman2017b"/> The lower density of ice compared to liquid water is due to the lower number of water molecules that form the [[Crystal structure|crystal lattice structure]] of ice, which leaves a large amount of space between water molecules.<ref name = "freeman2017b"/> In contrast, there is no crystal lattice structure in liquid water, which allows more water molecules to occupy the same amount of volume.<ref name = "freeman2017b"/>

Water also has the capacity to absorb energy, giving it a higher [[specific heat capacity]] than other solvents such as [[ethanol]].<ref name = "freeman2017b"/> Thus, a large amount of energy is needed to break the hydrogen bonds between water molecules to convert liquid water into [[water vapor]].<ref name = "freeman2017b"/>

As a molecule, water is not completely stable as each water molecule continuously dissociates into [[hydrogen]] and [[hydroxyl]] ions before reforming into a water molecule again.<ref name = "freeman2017b"/> In [[Purified water|pure water]], the number of hydrogen ions balances (or equals) the number of hydroxyl ions, resulting in a [[pH]] that is neutral.

===Organic compounds===
{{Further | Organic chemistry}}
[[Image:Glucose Haworth.png|thumb|right|Organic compounds such as [[glucose]] are vital to organisms.]]
[[Organic compound]]s are molecules that contain carbon bonded to another element such as hydrogen.<ref name = "freeman2017b"/> With the exception of water, nearly all the molecules that make up each organism contain carbon.<ref name = "freeman2017b"/><ref name = "urry2017d">{{cite book | last1 = Urry | first1 = Lisa | last2 = Cain | first2 = Michael | last3 = Wasserman | first3 =  Steven | last4 = Minorsky | first4 = Peter |  last5 = Reece | first5 = Jane | chapter = Carbon and the molecular diversity of life | title = Campbell Biology | publisher = Pearson | edition = 11th | date = 2017 | location = New York | pages = 56–65 | isbn = 978-0134093413}}</ref> Carbon can form [[covalent bond]]s with up to four other atoms, enabling it to form diverse, large, and complex molecules.<ref name = "freeman2017b"/><ref name = "urry2017d"/> For example, a single carbon atom can form four single covalent bonds such as in [[methane]], two [[Double bond|double covalent bond]]s such as in [[carbon dioxide]] ({{CO2}}), or a [[triple bond|triple covalent bond]] such as in [[carbon monoxide]] (CO). Moreover, carbon can form very long chains of interconnecting [[carbon–carbon bond]]s such as [[octane]] or ring-like structures such as [[glucose]].

The simplest form of an organic molecule is the [[hydrocarbon]], which is a large family of organic compounds that are composed of [[hydrogen]] atoms bonded to a chain of carbon atoms. A hydrocarbon backbone can be substituted by other elements such as [[oxygen]] (O), [[hydrogen]] (H), [[phosphorus]] (P), and [[sulfur]] (S), which can change the chemical behavior of that compound.<ref name = "freeman2017b"/> Groups of atoms that contain these elements (O-, H-, P-, and S-) and are bonded to a central carbon atom or skeleton are called [[functional group]]s.<ref name = "freeman2017b"/> There are six prominent functional groups that can be found in organisms: [[Amine|amino group]], [[Carboxylic acid|carboxyl group]], [[carbonyl group]], [[Hydroxy group|hydroxyl group]], [[phosphate|phosphate group]], and [[Thiol|sulfhydryl group]].<ref name = "freeman2017b"/>

In 1953, the [[Miller-Urey experiment]] showed that organic compounds could be synthesized abiotically within a closed system mimicking the conditions of [[early Earth]], thus suggesting that complex organic molecules could have arisen spontaneously in early Earth (see [[abiogenesis]]).<ref name="hillisetal2014d">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4 = Price | first4 = Mary V. | chapter = Carbon and molecular diversity of life | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 56–65 | isbn = 978-1464175121}}</ref><ref name = "freeman2017b"/>

===Macromolecules===
{{Further | Biochemistry|Macromolecule|Molecular biology}}
[[File:0302 Phospholipid Bilayer.jpg|thumb|A phospholipid bilayer consists of two adjacent sheets of phospholipids, with the hydrophilic tails facing inwards and the hydrophobic heads facing outwards.]]
[[Macromolecule]]s are large molecules made up of smaller molecular subunits that are joined.<ref name = "freeman2017c">{{cite book | last1 = Freeman| first1 = Scott | last2 = Quillin | first2 = Kim | last3 = Allison | first3 = Lizabeth | last4 = Black | first4 = Michael |  last5 = Podgorski | first5 = Greg | last6 = Taylor | first6 = Emily | last7 = Carmichael | first7 = Jeff | chapter = Protein structure and function | title = Biological Science | publisher = Pearson | edition = 6th | date = 2017 | location = Hoboken, N.J. | pages = 78–92 | isbn = 978-0321976499}}</ref> Small molecules such as sugars, amino acids, and nucleotides can act as single repeating units called [[monomer]]s to form chain-like molecules called [[polymer]]s via a chemical process called [[condensation reaction|condensation]].<ref name = "urry2017e">{{cite book | last1 = Urry | first1 = Lisa | last2 = Cain | first2 = Michael | last3 = Wasserman | first3 =  Steven | last4 = Minorsky | first4 = Peter |  last5 = Reece | first5 = Jane | chapter = The structure and function of large biological molecules | title = Campbell Biology | publisher = Pearson | edition = 11th | date = 2017 | location = New York | pages = 66–92 | isbn = 978-0134093413}}</ref> For example, amino acids can form [[peptide|polypeptide]]s whereas nucleotides can form strands of nucleic acid. Polymers make up three of the four macromolecules ([[polysaccharide]]s, [[lipid]]s, [[protein]]s, and [[nucleic acid]]s) that are found in all organisms. Each of these macromolecules plays a specialized role within any given cell.

[[Carbohydrate]]s (or [[sugar]]) are molecules with the molecular formula {{nowrap|(CH<sub>2</sub>O)<sub>''n''</sub>}}, with ''n'' being the number of carbon-hydrate groups.<ref name = "freeman2017e">{{cite book | last1 = Freeman| first1 = Scott | last2 = Quillin | first2 = Kim | last3 = Allison | first3 = Lizabeth | last4 = Black | first4 = Michael |  last5 = Podgorski | first5 = Greg | last6 = Taylor | first6 = Emily | last7 = Carmichael | first7 = Jeff | chapter = An introduction to carbohydrate | title = Biological Science | publisher = Pearson | edition = 6th | date = 2017 | location = Hoboken, N.J. | pages = 107–118 | isbn = 978-0321976499}}</ref> They include monosaccharides (monomer), oligosaccharides (small polymers), and polysaccharides (large polymers). Monosaccharides can be linked together by [[Glycosidic bond|glycosidic linkage]]s, a type of covalent bond.<ref name = "freeman2017e"/> When two monosaccharides such as [[glucose]] and [[fructose]] are linked together, they can form a [[disaccharide]] such as [[sucrose]].<ref name = "freeman2017e"/> When many monosaccharides are linked together, they can form an oligosaccharide or a polysaccharide, depending on the number of monosaccharides. Polysaccharides can vary in function. Monosaccharides such as glucose can be a source of energy and some polysaccharides can serve as storage material that can be [[hydrolysis|hydrolyzed]] to provide cells with sugar.

Lipids are the only class of macromolecules that are not made up of polymers. The most biologically important lipids are [[steroid]]s, [[phospholipid]]s, and [[fat]]s.<ref name = "urry2017e"/> These lipids are organic compounds that are largely nonpolar and hydrophobic.<ref name = "freeman2017f">{{cite book | last1 = Freeman| first1 = Scott | last2 = Quillin | first2 = Kim | last3 = Allison | first3 = Lizabeth | last4 = Black | first4 = Michael |  last5 = Podgorski | first5 = Greg | last6 = Taylor | first6 = Emily | last7 = Carmichael | first7 = Jeff | chapter = Lipids, membranes, and the first cells | title = Biological Science | publisher = Pearson | edition = 6th | date = 2017 | location = Hoboken, N.J. | pages = 119–141| isbn = 978-0321976499}}</ref> Steroids are organic compounds that consist of four fused rings.<ref name = "freeman2017f"/> Phospholipids consist of glycerol that is linked to a phosphate group and two hydrocarbon chains (or [[fatty acid]]s).<ref name = "freeman2017f"/> The glycerol and phosphate group together constitute the polar and [[hydrophilic]] (or head) region of the molecule whereas the fatty acids make up the nonpolar and hydrophobic (or tail) region.<ref name = "freeman2017f"/> Thus, when in water, phospholipids tend to form a [[Lipid bilayer|phospholipid bilayer]] whereby the hydrophobic heads face outwards to interact with water molecules. Conversely, the hydrophobic tails face inwards towards other hydrophobic tails to avoid contact with water.<ref name = "freeman2017f"/>

[[File:225 Peptide Bond-01.jpg|thumb | right |The (a) primary, (b) secondary, (c) tertiary, and (d) quaternary structures of a [[hemoglobin]] protein]]
Proteins are the most diverse of the macromolecules, which include [[enzyme]]s, [[transport protein]]s, large [[cell signal|signaling]] molecules, [[antibodies]], and [[structural proteins]]. The basic unit (or monomer) of a protein is an amino acid, which has a central carbon atom that is covalently bonded to a hydrogen atom, an [[Amine|amino group]], a [[Carboxylic acid|carboxyl group]], and a [[side chain]] (or R-group, "R" for residue).<ref name = "freeman2017c"/> There are twenty amino acids that make up the building blocks of proteins, with each amino acid having its own unique side chain.<ref name = "freeman2017c"/> The polarity and charge of the side chains affect the solubility of amino acids. An amino acid with a side chain that is polar and electrically charged is soluble as it is hydrophilic whereas an amino acid with a side chain that lacks a charged or an electronegative atom is hydrophobic and therefore tends to coalesce rather than dissolve in water.<ref name = "freeman2017c"/> Proteins have four distinct levels of organization ([[Protein primary structure|primary]], [[Protein secondary structure|secondary]], [[Protein tertiary structure|tertiary]], and [[Protein quaternary structure|quartenary]]). The primary structure consists of a unique sequence of amino acids that are covalently linked together by [[peptide bond]]s.<ref name = "freeman2017c"/> The side chains of the individual amino acids can then interact with each other, giving rise to the secondary structure of a protein.<ref name = "freeman2017c"/> The two common types of secondary structures are [[alpha helix|alpha helices]] and [[beta sheet]]s.<ref name = "freeman2017c"/> The folding of alpha helices and beta sheets gives a protein its three-dimensional or tertiary structure. Finally, multiple tertiary structures can combine to form the quaternary structure of a protein.

Nucleic acids are polymers made up of monomers called nucleotides.<ref name = "freeman2017d">{{cite book | last1 = Freeman| first1 = Scott | last2 = Quillin | first2 = Kim | last3 = Allison | first3 = Lizabeth | last4 = Black | first4 = Michael |  last5 = Podgorski | first5 = Greg | last6 = Taylor | first6 = Emily | last7 = Carmichael | first7 = Jeff | chapter = Nucleic acids and the RNA world | title = Biological Science | publisher = Pearson | edition = 6th | date = 2017 | location = Hoboken, N.J. | pages = 93–106 | isbn = 978-0321976499}}</ref> Their function is to store, transmit, and express hereditary information.<ref name = "urry2017e"/> Nucleotides consist of a phosphate group, a five-carbon sugar, and a nitrogenous base. Ribonucleotides, which contain ribose as the sugar, are the monomers of [[RNA|ribonucleic acid (RNA)]]. In contrast, deoxyribonucleotides contain deoxyribose as the sugar and are constitute the monomers of [[DNA|deoxyribonucleic acid (DNA)]]. RNA and DNA also differ with respect to one of their bases.<ref name = "freeman2017d"/> There are two types of bases: [[purine]]s and [[pyrimidine]]s.<ref name = "freeman2017d"/> The purines include [[guanine]] (G) and [[adenine]] (A) whereas the pyrimidines consist of [[cytosine]] (C), [[uracil]] (U), and [[thymine]] (T). Uracil is used in RNA whereas thymine is used in DNA. Taken together, when the different sugar and bases are take into consideration, there are eight distinct nucleotides that can form two types of nucleic acids: DNA (A, G, C, and T) and RNA (A, G, C, and U).<ref name = "freeman2017d"/>

==Cells==
{{Further|Cell biology}}
[[Cell theory]] states that [[Cell (biology)|cell]]s are the fundamental units of life, that all living things are composed of one or more cells, and that all cells arise from preexisting cells through [[cell division]].<ref name = "mazzarello1999">{{cite journal | author=Mazzarello, P | title=A unifying concept: the history of cell theory | journal=Nature Cell Biology | volume=1 | issue=1 | pages=E13–15 | date=May 1999 | pmid=10559875 | doi=10.1038/8964| s2cid=7338204 }}</ref> Most cells are very small, with diameters ranging from 1 to 100&nbsp;[[micrometre|micrometer]]s and are therefore only visible under a [[light microscope|light]] or [[electron microscope]].<ref>{{cite book|url=http://www.phschool.com/el_marketing.html|title=Biology: Exploring Life|last1=Campbell|first1=Neil A.|first2=Brad|last2=Williamson|first3=Robin J.|last3=Heyden|name-list-style=vanc|publisher=Pearson Prentice Hall|year=2006|isbn=9780132508827|location=Boston|access-date=2021-05-13|archive-date=2014-11-02|archive-url=https://web.archive.org/web/20141102041816/http://www.phschool.com/el_marketing.html|url-status=live}}</ref> There are generally two types of cells: [[eukaryotic]] cells, which contain a [[Cell nucleus|nucleus]], and [[prokaryotic]] cells, which do not. Prokaryotes are [[Unicellular organism|single-celled organisms]] such as [[bacterium|bacteria]], whereas eukaryotes can be single-celled or [[multicellular organism|multicellular]]. In [[multicellular organisms]], every cell in the organism's body is derived ultimately from a [[zygote|single cell]] in a fertilized [[egg (biology)|egg]].

===Cell structure===
[[File:biological cell.svg|thumb|right|Structure of an animal [[cell (biology)|cell]] depicting various [[organelle]]s]]
Every cell is enclosed within a [[cell membrane]] that separates its [[cytoplasm]] from the [[extracellular space]].<ref name = "urry2017g">{{cite book | last1 = Urry | first1 = Lisa | last2 = Cain | first2 = Michael | last3 = Wasserman | first3 =  Steven | last4 = Minorsky | first4 = Peter |  last5 = Reece | first5 = Jane | chapter = Membrane structure and function | title = Campbell Biology | publisher = Pearson | edition = 11th | date = 2017 | location = New York | pages = 126–142 | isbn = 978-0134093413}}</ref> A cell membrane consists of a [[lipid bilayer]], including [[cholesterol]]s that sit between [[phospholipid]]s to maintain their [[Membrane fluidity|fluidity]] at various temperatures. Cell membranes are [[semipermeable membrane|semipermeable]], allowing small molecules such as oxygen, carbon dioxide, and water to pass through while restricting the movement of larger molecules and charged particles such as [[ion]]s.<ref name="MBOC">{{cite book|url=https://www.ncbi.nlm.nih.gov/books/NBK21054/?term=Molecular%20Biology%20of%20the%20Cell|title=Molecular Biology of the Cell|vauthors=Alberts B, Johnson A, Lewis J|publisher=Garland Science|year=2002|isbn=978-0-8153-3218-3|edition=4th|location=New York|display-authors=etal|url-status=live|archive-url=https://web.archive.org/web/20171220092628/https://www.ncbi.nlm.nih.gov/books/NBK21054/?term=Molecular%20Biology%20of%20the%20Cell|archive-date=2017-12-20}}</ref> Cell membranes also contains [[membrane protein]]s, including [[integral membrane protein]]s that go across the membrane serving as [[membrane transporter]]s, and [[peripheral protein]]s that loosely attach to the outer side of the cell membrane, acting as [[enzyme]]s shaping the cell.<ref name="Tom Herrmann 2019">{{cite journal | author1=Tom Herrmann | author2=Sandeep Sharma | journal=StatPearls | date=March 2, 2019 <!-- | location=1 SIU School of Medicine 2 Baptist Regional Medical Center--> | url=https://www.ncbi.nlm.nih.gov/books/NBK538211/ | pmid=30855799 | title=Physiology, Membrane | access-date=May 14, 2021 | archive-date=February 17, 2022 | archive-url=https://web.archive.org/web/20220217034021/https://www.ncbi.nlm.nih.gov/books/NBK538211/ | url-status=live }}</ref> Cell membranes are involved in various cellular processes such as [[cell adhesion]], [[membrane potential|storing electrical energy]], and [[cell signalling]] and serve as the attachment surface for several extracellular structures such as a [[cell wall]], [[glycocalyx]], and [[cytoskeleton]].

[[File:Plant cell structure-en.svg|thumb|left|Structure of a plant cell]]
Within the [[cytoplasm]] of a cell, there are many [[biomolecule]]s such as [[protein]]s and [[nucleic acid]]s.<ref name="Alberts2002">[https://www.ncbi.nlm.nih.gov/books/NBK26863/ Cell Movements and the Shaping of the Vertebrate Body] {{Webarchive|url=https://web.archive.org/web/20200122055346/https://www.ncbi.nlm.nih.gov/books/NBK26863/ |date=2020-01-22 }} in Chapter 21 of ''[https://www.ncbi.nlm.nih.gov/books/NBK21054/ Molecular Biology of the Cell] {{Webarchive|url=https://web.archive.org/web/20170927035510/https://www.ncbi.nlm.nih.gov/books/NBK21054/ |date=2017-09-27 }}'' fourth edition, edited by Bruce Alberts (2002) published by Garland Science.<br /> The Alberts text discusses how the "cellular building blocks" move to shape developing [[embryo]]s. It is also common to describe small molecules such as [[amino acid]]s as "[https://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Search&db=books&doptcmdl=GenBookHL&term=%22all+cells%22+AND+mboc4%5Bbook%5D+AND+372023%5Buid%5D&rid=mboc4.section.4#23 molecular building blocks] {{Webarchive|url=https://web.archive.org/web/20200122055404/https://www.ncbi.nlm.nih.gov/books?cmd=Search&doptcmdl=GenBookHL&term=%22all%2Bcells%22%2BAND%2Bmboc4%5Bbook%5D%2BAND%2B372023%5Buid%5D&rid=mboc4.section.4#23 |date=2020-01-22 }}".</ref> In addition to biomolecules, eukaryotic cells have specialized structures called [[organelle]]s that have their own lipid bilayers or are spatially units.<ref name="hillisetal2014f">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4 = Price | first4 = Mary V. | chapter = Cells: The working units of life | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 60–81| isbn = 978-1464175121}}</ref> These organelles include the [[cell nucleus]], which contains most of the cell's DNA, or [[mitochondrion|mitochondria]], which generates [[adenosine triphosphate]] (ATP) to power cellular processes. Other organelles such as [[endoplasmic reticulum]] and [[Golgi apparatus]] play a role in the synthesis and packaging of proteins, respectively. Biomolecules such as proteins can be engulfed by [[lysosome]]s, another specialized organelle. [[Plant cell]]s have additional organelles that distinguish them from [[animal cell]]s such as a [[cell wall]] that provides support for the plant cell, [[chloroplast]]s that harvest sunlight energy to produce sugar, and [[vacuole]]s that provide storage and structural support as well as being involved in reproduction and breakdown of plant seeds.<ref name="hillisetal2014f"/> Eukaryotic cells also have cytoskeleton that is made up of [[microtubule]]s, [[intermediate filament]]s, and [[microfilament]]s, all of which provide support for the cell and are involved in the movement of the cell and its organelles.<ref name="hillisetal2014f"/> In terms of their structural composition, the microtubules are made up of [[tubulin]] (e.g., [[Tubulin#α-Tubulin|α-tubulin]] and [[Tubulin#β-Tubulin|β-tubulin]] whereas intermediate filaments are made up of fibrous proteins.<ref name="hillisetal2014f"/> Microfilaments are made up of [[actin]] molecules that interact with other strands of proteins.<ref name="hillisetal2014f"/>

===Metabolism===
{{Further | Bioenergetics}}
[[File:Activation2 updated.svg|right|thumb|Example of an enzyme-catalysed [[Exothermic reaction|exothermic]] reaction]]
All cells require [[energy]] to sustain cellular processes. Energy is the capacity to do [[Work (physics)|work]], which, in [[thermodynamics]], can be calculated using [[Gibbs free energy]]. According to the [[first law of thermodynamics]], energy is [[Conservation law|conserved]], i.e., cannot be created or destroyed. Hence, [[chemical reaction]]s in a cell do not create new energy but are involved instead in the transformation and transfer of energy.<ref name = "freeman2017h">{{cite book | last1 = Freeman| first1 = Scott | last2 = Quillin | first2 = Kim | last3 = Allison | first3 = Lizabeth | last4 = Black | first4 = Michael |  last5 = Podgorski | first5 = Greg | last6 = Taylor | first6 = Emily | last7 = Carmichael | first7 = Jeff | chapter = Energy and enzymes: An introduction to metabolism | title = Biological Science | publisher = Pearson | edition = 6th | date = 2017 | location = Hoboken, N.J. | pages = 171–188 | isbn = 978-0321976499}}</ref> Nevertheless, all energy transfers lead to some loss of usable energy, which increases [[entropy]] (or state of disorder) as stated by the [[second law of thermodynamics]]. As a result, an organism requires continuous input of energy to maintain a low state of entropy. In cells, energy can be transferred as electrons during [[Redox|redox (reduction–oxidation)]] reactions, stored in covalent bonds, and generated by the movement of ions (e.g., hydrogen, sodium, potassium) across a membrane.

[[Metabolism]] is the set of [[life]]-sustaining [[chemical reactions]] in [[organisms]]. The three main purposes of metabolism are: the conversion of food to [[energy]] to run cellular processes; the conversion of food/fuel to building blocks for [[protein]]s, [[lipid]]s, [[nucleic acid]]s, and some [[carbohydrate]]s; and the elimination of [[metabolic waste]]s. These [[enzyme]]-catalyzed reactions allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolic reactions may be categorized as [[catabolic]]—the breaking down of compounds (for example, the breaking down of glucose to pyruvate by [[cellular respiration]]); or [[anabolic]]—the building up ([[biosynthesis|synthesis]]) of compounds (such as proteins, carbohydrates, lipids, and nucleic acids). Usually, catabolism releases energy, and anabolism consumes energy.

The chemical reactions of metabolism are organized into [[metabolic pathway]]s, in which one chemical is transformed through a series of steps into another chemical, each step being facilitated by a specific [[enzyme]]. Enzymes are crucial to metabolism because they allow organisms to drive desirable reactions that require [[energy]] that will not occur by themselves, by [[Coupling (physics)|coupling]] them to [[spontaneous process|spontaneous reactions]] that release energy. Enzymes act as [[Catalysis|catalysts]]—they allow a reaction to proceed more rapidly without being consumed by it—by reducing the amount of [[activation energy]] needed to convert [[reactant]]s into [[Product (chemistry)|product]]s. Enzymes also allow the [[Metabolic pathway#Regulation|regulation]] of the rate of a metabolic reaction, for example in response to changes in the [[cell (biology)|cell's]] environment or to [[cell signaling|signals]] from other cells.

===Cellular respiration===
{{Further information|Cellular respiration}}[[Image:CellRespiration.svg|thumb|right|300px|Respiration in a [[eukaryote|eukaryotic cell]]]]
Cellular respiration is a set of [[metabolism|metabolic]] reactions and processes that take place in the [[cell (biology)|cell]]s of [[organism]]s to convert [[Energy#Energy and life|chemical energy]] from [[nutrients]] into [[adenosine triphosphate]] (ATP), and then release waste products.<ref>{{cite web|last=Bailey|first=Regina|title=Cellular Respiration|url=http://biology.about.com/od/cellularprocesses/a/cellrespiration.htm|url-status=live|archive-url=https://web.archive.org/web/20120505043947/http://biology.about.com/od/cellularprocesses/a/cellrespiration.htm|archive-date=2012-05-05}}</ref> The reactions involved in respiration are [[catabolism|catabolic reactions]], which break large molecules into smaller ones, releasing energy. Respiration is one of the key ways a cell releases chemical energy to fuel cellular activity. The overall reaction occurs in a series of biochemical steps, some of which are [[redox]] reactions. Although cellular respiration is technically a [[combustion reaction]], it clearly does not resemble one when it occurs in a cell because of the slow, controlled release of energy from the series of reactions.

Sugar in the form of [[glucose]] is the main nutrient used by animal and plant cells in respiration. Cellular respiration involving oxygen is called aerobic respiration, which has four stages: [[glycolysis]], [[citric acid cycle]] (or Krebs cycle), [[electron transport chain]], and [[oxidative phosphorylation]].<ref name = "lodish2008l">{{cite book | last1 = Lodish | first1 = Harvey | last2 = Berk | first2 = Arnold. | last3 = Kaiser | first3 =  Chris A.  | last4 = Krieger | first4 = Monty | last5 = Scott | first5 = Matthew P. | last6 = Bretscher | first6 = Anthony | last7 = Ploegh | first7 = Hidde | last8 = Matsudaira | first8 = Paul | chapter = Cellular energetics | title = Molecular Cell Biology | publisher = W.H. Freeman and Company | edition = 6th | date = 2008 | location = New York | pages = 479–532 | isbn = 978-0716776017}}</ref> Glycolysis is a metabolic process that occurs in the cytoplasm whereby glucose is converted into two [[pyruvic acid|pyruvate]]s, with two net molecules of ATP being produced at the same time.<ref name = "lodish2008l"/> Each pyruvate is then oxidized into [[acetyl-CoA]] by the [[pyruvate dehydrogenase complex]], which also generates [[Nicotinamide adenine dinucleotide|NADH]] and carbon dioxide. Acetyl-Coa enters the citric acid cycle, which takes places inside the mitochondrial matrix. At the end of the cycle, the total yield from 1 glucose (or 2 pyruvates) is 6 NADH, 2 FADH<sub>2</sub>, and 2 ATP molecules. Finally, the next stage is oxidative phosphorylation, which in eukaryotes, occurs in the [[Crista|mitochondrial cristae]]. Oxidative phosphorylation comprises the [[electron transport chain]], which is a series of four [[protein complex]]es that transfer electrons from one complex to another, thereby releasing energy from NADH and FADH<sub>2</sub> that is coupled to the pumping of protons (hydrogen ions) across the inner mitochondrial membrane ([[chemiosmosis]]), which generates a [[Chemiosmosis#Proton-motive force|proton motive force]].<ref name = "lodish2008l"/> Energy from the proton motive force drives the enzyme [[ATP synthase]] to synthesize more ATPs by [[Phosphorylation|phosphorylating]] [[Adenosine diphosphate|ADP]]s. The transfer of electrons terminates with molecular oxygen being the final [[electron acceptor]].

If oxygen were not present, pyruvate would not be metabolized by cellular respiration but undergoes a process of [[fermentation]]. The pyruvate is not transported into the mitochondrion but remains in the cytoplasm, where it is converted to [[cellular waste product|waste products]] that may be removed from the cell. This serves the purpose of oxidizing the electron carriers so that they can perform glycolysis again and removing the excess pyruvate. Fermentation oxidizes NADH to NAD<sup>+</sup> so it can be re-used in glycolysis.  In the absence of oxygen, fermentation prevents the buildup of NADH in the cytoplasm and provides NAD<sup>+</sup> for glycolysis.  This waste product varies depending on the organism. In skeletal muscles, the waste product is [[lactic acid]]. This type of fermentation is called [[lactic acid fermentation]]. In strenuous exercise, when energy demands exceed energy supply, the respiratory chain cannot process all of the hydrogen atoms joined by NADH. During anaerobic glycolysis, NAD<sup>+</sup> regenerates when pairs of hydrogen combine with pyruvate to form lactate. Lactate formation is catalyzed by lactate dehydrogenase in a reversible reaction. Lactate can also be used as an indirect precursor for liver glycogen. During recovery, when oxygen becomes available, NAD<sup>+</sup> attaches to hydrogen from lactate to form ATP. In yeast, the waste products are [[ethanol]] and [[carbon dioxide]]. This type of fermentation is known as alcoholic or [[ethanol fermentation]]. The ATP generated in this process is made by [[substrate-level phosphorylation]], which does not require oxygen.

===Photosynthesis===
[[File:Simple photosynthesis overview.svg|thumb|Photosynthesis changes sunlight into chemical energy, splits water to liberate O<sub>2</sub>, and fixes CO<sub>2</sub> into sugar.]]{{Main|Photosynthesis}}
Photosynthesis is a process used by plants and other organisms to [[Energy transformation|convert]] [[light energy]] into [[chemical energy]] that can later be released to fuel the organism's metabolic activities via [[cellular respiration]]. This chemical energy is stored in [[carbohydrate]] molecules, such as [[sugar]]s, which are synthesized from [[carbon dioxide]] and [[water]].<ref name="OnlineEtDict_photosynthesis">{{cite web |title=photosynthesis |url=http://www.etymonline.com/index.php?term=photosynthesis&allowed_in_frame=0 |work=[[Online Etymology Dictionary]] |access-date=2013-05-23 |archive-url=https://web.archive.org/web/20130307020959/http://www.etymonline.com/index.php?term=photosynthesis&allowed_in_frame=0 |archive-date=2013-03-07 |url-status=live }}</ref><ref name="LSJ1">{{LSJ|fw{{=}}s2|φῶς|ref}}</ref><ref name="LSJ2">{{LSJ|su/nqesis|σύνθεσις|ref}}</ref> In most cases, [[oxygen]] is also released as a waste product. Most [[plant]]s, [[algae]], and [[cyanobacteria]] perform photosynthesis, which is largely responsible for producing and maintaining the [[atmospheric oxygen|oxygen content]] of the Earth's atmosphere, and supplies most of the energy necessary for [[life]] on Earth.<ref name="bryantfrigaard">{{cite journal |vauthors=Bryant DA, Frigaard NU |title=Prokaryotic photosynthesis and phototrophy illuminated |journal=[[Trends in Microbiology]] |volume=14 |issue=11 |pages=488–496 |date=Nov 2006 |pmid=16997562 |doi=10.1016/j.tim.2006.09.001}}</ref>

Photosynthesis has four stages: [[Light absorption]], electron transport, ATP synthesis, and [[carbon fixation]].<ref name = "lodish2008l"/> Light absorption is the initial step of photosynthesis whereby light energy is absorbed by [[chlorophyll]] pigments attached to proteins in the [[Thylakoid#Membrane|thylakoid membrane]]s. The absorbed light energy is used to remove electrons from a donor (water) to a primary electron acceptor, a [[quinone]] designated as Q. In the second stage, electrons move from the quinone primary electron acceptor through a series of electron carriers until they reach a final electron acceptor, which is usually the oxidized form of NADP<sup>+</sup>, which is [[Redox|reduced]] to NADPH, a process that takes place in a protein complex called [[photosystem I]] (PSI). The transport of electrons is coupled to the movement of protons (or hydrogen) from the stroma to the thylakoid membrane, which forms a pH gradient across the membrane as hydrogen becomes more concentrated in the lumen than in the stroma. This is analogous to the proton-motive force generated across the inner mitochondrial membrane in aerobic respiration.<ref name = "lodish2008l"/>

During the third stage of photosynthesis, the movement of protons down their [[concentration gradient]]s from the thylakoid lumen to the stroma through the ATP synthase is coupled to the synthesis of ATP by that same ATP synthase.<ref name = "lodish2008l"/> The NADPH and ATPs generated by the [[light-dependent reaction]]s in the second and third stages, respectively, provide the energy and electrons to drive the synthesis of glucose by fixing atmospheric carbon dioxide into existing organic carbon compounds, such as [[ribulose bisphosphate]] (RuBP) in a sequence of light-independent (or dark) reactions called the [[Calvin cycle]].<ref name="isbn0-321-73975-2">{{cite book |vauthors=Reece J, Urry L, Cain M, Wasserman S, Minorsky P, Jackson R |title=Biology |edition=International |publisher=[[Pearson Education]] |location=Upper Saddle River, N.J. |isbn=978-0-321-73975-9 |pages=[https://archive.org/details/isbn_9781256158769/page/235 235, 244] |quote=This initial incorporation of carbon into organic compounds is known as carbon fixation. |year=2011 |url=https://archive.org/details/isbn_9781256158769/page/235 }}</ref>

===Cell signaling===
{{Further|Cell signaling}}
Cell signaling (or communication) is the ability of [[Cell (biology)|cell]]s to receive, process, and transmit signals with its environment and with itself.<ref name="neitzelrasband2021">{{cite journal | last1 = Neitzel | first1 = James | last2 = Rasband | first2 = Matthew | title = Cell communication | journal = Nature Education | access-date = 29 May 2021 | url = https://www.nature.com/scitable/topic/cell-communication-14122659/ | archive-date = 29 September 2010 | archive-url = https://web.archive.org/web/20100929110101/https://www.nature.com/scitable/topic/cell-communication-14122659/ | url-status = live }}</ref><ref name="cellsignalling">{{cite journal | title = Cell signaling | journal = Nature Education | access-date = 29 May 2021 | url = https://www.nature.com/scitable/topicpage/cell-signaling-14047077/ | archive-date = 31 October 2010 | archive-url = https://web.archive.org/web/20101031053612/https://www.nature.com/scitable/topicpage/cell-signaling-14047077/ | url-status = live }}</ref> Signals can be non-chemical such as light, [[Action potential|electrical impulses]], and heat, or chemical signals (or [[ligand]]s) that interact with [[Receptor (biochemistry)|receptors]], which can be found [[Cell surface receptor|embedded]] in the [[cell membrane]] of another cell or [[Intracellular receptor|located deep inside]] a cell.<ref name="hillisetal2014e">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4 = Price | first4 = Mary V. | chapter = Cell membranes and signaling | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 82–104 | isbn = 978-1464175121}}</ref><ref name="cellsignalling" /> There are generally four types of chemical signals: [[Autocrine signaling|autocrine]], [[Paracrine signaling|paracrine]], [[Juxtacrine signalling|juxtacrine]], and [[hormone]]s.<ref name="hillisetal2014e" /> In autocrine signaling, the ligand affects the same cell that releases it. [[Neoplasm|Tumor]] cells, for example, can reproduce uncontrollably because they release signals that initiate their own self-division. In paracrine signaling, the ligand diffuses to nearby cells and affects them. For example, brain cells called [[neuron]]s release ligands called [[neurotransmitter]]s that diffuse across a [[chemical synapse#Structure|synaptic cleft]] to bind with a receptor on an adjacent cell such as another neuron or [[muscle cell]]. In juxtacrine signaling, there is direct contact between the signaling and responding cells. Finally, hormones are ligands that travel through the [[circulatory system]]s of animals or [[vascular system]]s of plants to reach their target cells. Once a ligand binds with a receptor, it can influence the behavior of another cell, depending on the type of receptor. For instance, neurotransmitters that bind with an [[Ligand-gated ion channel|inotropic receptor]] can alter the [[Cell excitability|excitability]] of a target cell. Other types of receptors include [[protein kinase]] receptors (e.g., [[Insulin receptor|receptor]] for the hormone [[insulin]]) and [[G protein-coupled receptor]]s. Activation of G protein-coupled receptors can initiate [[Second messenger system|second messenger]] cascades. The process by which a chemical or physical signal is transmitted through a cell as a [[biochemical cascade|series of molecular events]] is called [[signal transduction]]

===Cell cycle===
[[File:Meiosis Overview new.svg|thumb|300px|In meiosis, the chromosomes duplicate and the [[homologous chromosome]]s exchange genetic information during meiosis I. The daughter cells divide again in meiosis II to form haploid [[gamete]]s.]]{{Main|Cell cycle}}
The cell cycle is a series of events that take place in a [[cell (biology)|cell]] that cause it to divide into two daughter cells. These events include the [[DNA replication|duplication of its DNA]] and some of its [[organelle]]s, and the subsequent partitioning of its cytoplasm into two daughter cells in a process called [[cell division]].<ref>{{Cite book|title=A dictionary of biology|date=2020 | vauthors = Martin EA, Hine R | publisher = Oxford University Press |isbn=9780199204625|edition=6th|location=Oxford|oclc=176818780}}</ref> In [[eukaryote]]s (i.e., [[animal]], [[plant]], [[fungal]], and [[protist]] cells), there are two distinct types of cell division: [[mitosis]] and [[meiosis]].<ref name=":0">{{Cite book|title=Introduction to genetic analysis|date=2012| publisher=W.H. Freeman and Co.| vauthors = Griffiths AJ | isbn=9781429229432 |edition=10th |location=New York |oclc=698085201 }}</ref> Mitosis is part of the cell cycle, in which replicated [[chromosomes]] are separated into two new nuclei. Cell division gives rise to genetically identical cells in which the total number of chromosomes is maintained. In general, mitosis (division of the nucleus) is preceded by the S stage of [[interphase]] (during which the DNA is replicated) and is often followed by [[telophase]] and [[cytokinesis]]; which divides the [[cytoplasm]], [[organelle]]s and [[cell membrane]] of one cell into two new [[Cell (biology)|cells]] containing roughly equal shares of these cellular components. The different stages of mitosis all together define the mitotic phase of an animal cell cycle—the division of the mother cell into two genetically identical daughter cells.<ref>{{Cite web|title=10.2 The Cell Cycle - Biology 2e {{!}} OpenStax|url=https://openstax.org/books/biology-2e/pages/10-2-the-cell-cycle|access-date=2020-11-24|website=openstax.org|language=en|archive-date=2020-11-29|archive-url=https://web.archive.org/web/20201129223722/https://openstax.org/books/biology-2e/pages/10-2-the-cell-cycle|url-status=live}}</ref> The cell cycle is a vital process by which a single-celled [[fertilized egg]] develops into a mature organism, as well as the process by which [[hair]], [[skin]], [[blood cell]]s, and some [[viscus|internal organs]] are renewed. After cell division, each of the daughter cells begin the [[interphase]] of a new cycle. In contrast to mitosis, meiosis results in four haploid daughter cells by undergoing one round of DNA replication followed by two divisions.<ref name="freeman2017m">{{cite book | last1 = Freeman| first1 = Scott | last2 = Quillin | first2 = Kim | last3 = Allison | first3 = Lizabeth | last4 = Black | first4 = Michael |  last5 = Podgorski | first5 = Greg | last6 = Taylor | first6 = Emily | last7 = Carmichael | first7 = Jeff | chapter = Meiosis | title = Biological Science | publisher = Pearson | edition = 6th | date = 2017 | location = Hoboken, N.J. | pages = 271–289 | isbn = 978-0321976499}}</ref> [[Homologous chromosome]]s are separated in the first division ([[Meiosis#Meiosis I|meiosis I]]), and sister chromatids are separated in the second division ([[Meiosis#Meiosis II|meiosis II]]). Both of these cell division cycles are used in the process of sexual reproduction at some point in their life cycle. Both are believed to be present in the last eukaryotic common ancestor.

[[Prokaryote]]s (i.e., [[archaea]] and [[bacteria]]) can also undergo cell division (or [[binary fission]]). Unlike the processes of [[mitosis]] and [[meiosis]] in eukaryotes, binary fission takes in prokaryotes takes place without the formation of a [[spindle apparatus]] on the cell. Before binary fission, DNA in the bacterium is tightly coiled. After it has uncoiled and duplicated, it is pulled to the separate poles of the bacterium as it increases the size to prepare for splitting. Growth of a new cell wall begins to separate the bacterium (triggered by [[FtsZ]] polymerization and "Z-ring" formation)<ref name=":32046082">{{cite journal | vauthors = Casiraghi A, Suigo L, Valoti E, Straniero V | title = Targeting Bacterial Cell Division: A Binding Site-Centered Approach to the Most Promising Inhibitors of the Essential Protein FtsZ | journal = Antibiotics | volume = 9 | issue = 2 | pages = 69 | date = February 2020 | pmid = 32046082 | doi = 10.3390/antibiotics9020069 | pmc = 7167804 | doi-access = free }}</ref> The new cell wall ([[Septum (cell biology)|septum]]) fully develops, resulting in the complete split of the bacterium. The new daughter cells have tightly coiled DNA rods, [[ribosome]]s, and [[plasmid]]s.

==Genetics==
===Inheritance===
{{Further | Classical genetics}}
[[File:Punnett square mendel flowers.svg|thumb|right| [[Punnett square]] depicting a cross between two pea plants heterozygous for purple (B) and white (b) blossoms]]
[[Genetics]] is the scientific study of inheritance.<ref name="griffithsetal2014l">{{cite book | last1 = Griffiths | first1 = Anthony J. | last2 = Wessler | first2 = Susan R. | last3 = Carroll | first3 = Sean B. | last4 = Doebley | first4 = John | chapter = The genetics revolution | title = An Introduction to Genetic Analysis | publisher = W.H. Freeman & Company | edition = 11th | date = 2015 | location = Sunderland, Mass. | pages = 1–30 | isbn = 978-1464109485}}</ref><ref name=griffiths2000sect60>{{cite book |editor1-first=Anthony J.F. |editor1-last=Griffiths |editor2-first=Jeffrey H. |editor2-last=Miller |editor3-first=David T. |editor3-last=Suzuki |editor4-first=Richard C. |editor4-last=Lewontin |editor5-first=William M.|editor5-last=Gelbart | title=An Introduction to Genetic Analysis |year=2000 |isbn=978-0-7167-3520-5 |edition=7th |publisher=W. H. Freeman |location=New York |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=iga.section.60 |chapter=Genetics and the Organism: Introduction }}</ref><ref name=Hartl_and_Jones>{{Cite book |author1=Hartl, D |author2=Jones, E |title=Genetics: Analysis of Genes and Genomes |edition=6th |publisher=Jones & Bartlett |year=2005 |isbn=978-0-7637-1511-3 |url=https://archive.org/details/genetics00dani }}</ref> [[Mendelian inheritance]], specifically, is the process by which genes and traits are passed on from parents to offspring.<ref name = "miko2008a"/> It was formulated by [[Gregor Mendel]], based on his work with pea plants in the mid-nineteenth century. Mendel established several principles of inheritance. The first is that genetic characteristics, which are now called [[allele]]s, are discrete and have alternate forms (e.g., purple vs. white or tall vs. dwarf), each inherited from one of two parents. Based on his [[Mendelian inheritance#Law of dominance and uniformity|law of dominance and uniformity]], which states that some alleles are [[Dominance (genetics)|dominant]] while others are [[Dominance (genetics)|recessive]]; an organism with at least one dominant allele will display the [[phenotype]] of that dominant allele.<ref name="Mendelian Principles">Rutgers: [http://lifesci.dls.rutgers.edu/~mcguire/Toolbox-Demo/Basic%20Genetics/Mendelian%20Principles.htm Mendelian Principles] {{Webarchive|url=https://web.archive.org/web/20210514212942/http://lifesci.dls.rutgers.edu/~mcguire/Toolbox-Demo/Basic%20Genetics/Mendelian%20Principles.htm |date=2021-05-14 }}</ref> Exceptions to this rule include [[penetrance]] and [[Expressivity (genetics)|expressivity]].<ref name = "miko2008a"/> Mendel noted that during gamete formation, the alleles for each gene segregate from each other so that each gamete carries only one allele for each gene, which is stated by his [[Mendelian inheritance#Law of segregation|law of segregation]]. [[Heterozygous|Heterozygotic]] individuals produce gametes with an equal frequency of two alleles. Finally, Mendel formulated the [[Mendelian inheritance#Law of Independent Assortment|law of independent assortment]], which states that genes of different traits can segregate independently during the formation of gametes, i.e., genes are unlinked. An exception to this rule would include traits that are [[sex linkage|sex-linked]]. [[Test cross]]es can be performed to experimentally determine the underlying [[genotype]] of an organism with a dominant phenotype.<ref name = "miko2008b">{{Citation | last = Miko | first = Ilona | title = Test crosses | journal = Nature Education | volume = 1 | issue = 1 | pages = 136 | date = 2008 | url = https://www.nature.com/scitable/topicpage/test-crosses-585/ | access-date = 2021-05-28 | archive-date = 2021-05-21 | archive-url = https://web.archive.org/web/20210521003428/https://www.nature.com/scitable/topicpage/test-crosses-585/ | url-status = live }}</ref> A [[Punnett square]] can be used to predict the results of a test cross. The [[Boveri–Sutton chromosome theory|chromosome theory of inheritance]], which states that genes are found on chromosomes, was supported by [[Thomas Hunt Morgan|Thomas Morgans]]'s experiments with [[Drosophila melanogaster|fruit flies]], which established the [[sex linkage]] between eye color and [[sex]] in these insects.<ref name = "miko2008c">{{Citation | last = Miko | first = Ilona | title = Thomas Hunt Morgan and sex linkage | journal = Nature Education | volume = 1 | issue = 1 | pages = 143 | date = 2008 | url = https://www.nature.com/scitable/topicpage/thomas-hunt-morgan-and-sex-linkage-452/ | access-date = 2021-05-28 | archive-date = 2021-05-20 | archive-url = https://web.archive.org/web/20210520234008/https://www.nature.com/scitable/topicpage/thomas-hunt-morgan-and-sex-linkage-452/ | url-status = live }}</ref> In humans and other mammals (e.g., dogs), it is not feasible or practical to conduct test cross experiments. Instead, [[Pedigree chart|pedigree]]s, which are genetic representations of family trees,<ref name = "genome2021">{{cite web | url = https://www.genome.gov/genetics-glossary/Pedigree | title = Pedigree | publisher = National Human Genome Research Institute | access-date = 28 May 2021 | quote = A pedigree is a genetic representation of a family tree that diagrams the inheritance of a trait or disease though several generations. The pedigree shows the relationships between family members and indicates which individuals express or silently carry the trait in question. | archive-date = 16 June 2021 | archive-url = https://web.archive.org/web/20210616193945/https://www.genome.gov/genetics-glossary/Pedigree | url-status = live }}</ref> are used instead to trace the inheritance of a specific trait or disease through multiple generations.<ref name = "urry2017n">{{cite book | last1 = Urry | first1 = Lisa | last2 = Cain | first2 = Michael | last3 = Wasserman | first3 =  Steven | last4 = Minorsky | first4 = Peter |  last5 = Reece | first5 = Jane | chapter = Mendel and the gene idea | title = Campbell Biology | publisher = Pearson | edition = 11th | date = 2017 | location = New York | pages = 269–293 | isbn = 978-0134093413}}</ref>

===DNA===
[[File:DNA animation.gif|thumb|Bases lie between two spiraling DNA strands.]]{{See also|Gene|DNA|Genetics}}
A gene is a unit of [[heredity]] that corresponds to a region of deoxyribonucleic acid (DNA) that carries genetic information that influences the form or function of an organism in specific ways. DNA is a [[molecule]] composed of two [[polynucleotide]] chains that coil around each other to form a [[Nucleic acid double helix|double helix]], which was first described by [[James Watson]] and [[Francis Crick]] in 1953.<ref name="hillisetal2014i">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4 = Price | first4 = Mary V. | chapter = DNA and its role in heredity | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 172–193 | isbn = 978-1464175121}}</ref> It is found as linear [[chromosome]]s in [[eukaryote]]s, and circular chromosomes in [[prokaryote]]s. A chromosome is an organized structure consisting of DNA and [[histone]]s. The set of chromosomes in a cell and any other hereditary information found in the [[Mitochondrion|mitochondria]], [[chloroplast]]s, or other locations is collectively known as a cell's [[genome]]. In eukaryotes, genomic DNA is localized in the [[cell nucleus]], or with small amounts in mitochondria and chloroplasts.<ref name="russell2001">{{cite book |last= Russell |first= Peter | name-list-style = vanc | title= iGenetics |url= https://archive.org/details/igenetics0000russ_v6o1 |url-access= registration |publisher= Benjamin Cummings |location= New York |year= 2001 |isbn= 0-8053-4553-1}}</ref> In prokaryotes, the DNA is held within an irregularly shaped body in the cytoplasm called the [[nucleoid]].<ref>{{cite journal |author1=Thanbichler, M |author2=Wang, SC |author3=Shapiro, L | title=The bacterial nucleoid: a highly organized and dynamic structure | journal=Journal of Cellular Biochemistry | volume=96 | issue=3 | pages=506–21 | date=October 2005 | pmid=15988757 | doi=10.1002/jcb.20519 |s2cid=25355087 | doi-access=free }}</ref> The genetic information in a genome is held within genes, and the complete assemblage of this information in an organism is called its [[genotype]].<ref>{{cite web |url=http://www.medterms.com/script/main/art.asp?articlekey=8472 |title=Genotype definition – Medical Dictionary definitions |publisher=Medterms.com |date=2012-03-19 |access-date=2013-10-02 |url-status=live |archive-url=https://web.archive.org/web/20130921054803/http://www.medterms.com/script/main/art.asp?articlekey=8472 |archive-date=2013-09-21 }}</ref> Genes encode the information needed by cells for the synthesis of proteins, which in turn play a central role in influencing the final [[phenotype]] of the organism.

The two polynucleotide strands that make up DNA run in opposite directions to each other and are thus [[antiparallel (biochemistry)|antiparallel]]. Each strand is composed of [[nucleotide]]s,<ref>{{Cite book |vauthors= Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P | title= Molecular Biology of the Cell |edition= 6th |publisher= Garland |year= 2014 |url= http://www.garlandscience.com/product/isbn/9780815344322 |page= Chapter 4: DNA, Chromosomes and Genomes |isbn= 978-0-8153-4432-2 |url-status=live |archive-url= https://web.archive.org/web/20140714210549/http://www.garlandscience.com/product/isbn/9780815344322 |archive-date= 14 July 2014 |df= dmy-all }}</ref><ref>{{cite web |last1=Purcell |first1=Adam |name-list-style=vanc |title=DNA |url=http://basicbiology.net/micro/genetics/dna|website=Basic Biology |url-status=live |archive-url=https://web.archive.org/web/20170105045651/http://basicbiology.net/micro/genetics/dna/ |archive-date=5 January 2017}}</ref> with each nucleotide containing one of four nitrogenous [[nucleobase|base]]s ([[cytosine]] [C], [[guanine]] [G], [[adenine]] [A] or [[thymine]] [T]), a [[monosaccharide|sugar]] called [[deoxyribose]], and a [[Organophosphate|phosphate group]]. The nucleotides are joined to one another in a chain by [[covalent bond]]s between the sugar of one nucleotide and the phosphate of the next, resulting in an alternating [[backbone chain|sugar-phosphate backbone]]. It is the [[Nucleic acid sequence|sequence]] of these four bases along the backbone that encodes genetic information. Bases of the two polynucleotide strands are bound together by [[hydrogen bond]]s, according to [[base pair]]ing rules (A with T and C with G), to make double-stranded DNA. The bases are divided into two groups: [[pyrimidine]]s and [[purine]]s. In DNA, the pyrimidines are thymine and cytosine whereas the purines are adenine and guanine.

There are [[Nucleic acid double helix#Grooves|grooves]] that run along the entire length of the double helix due to the uneven spacing of the DNA strands relative to each other.<ref name="hillisetal2014i"/> Both grooves differ in size, with the major groove being larger and therefore more accessible to the binding of proteins than the minor groove.<ref name="hillisetal2014i"/> The outer edges of the bases are exposed to these grooves and are therefore accessible for additional hydrogen bonding.<ref name="hillisetal2014i"/> Because each groove can have two possible base-pair configurations (G-C and A-T), there are four possible base-pair configurations within the entire double helix, each of which is chemically distinct from another.<ref name="hillisetal2014i"/> As a result, protein molecules are able to recognize and bind to specific base-pair sequences, which is the basis of specific DNA-protein interactions.

[[DNA replication]] is a [[Semiconservative replication|semiconservative]] process whereby each strand serves as a template for a new strand of DNA.<ref name="hillisetal2014i"/> The process begins with the unwounding of the double helix at an [[origin of replication]], which separates the two strands, thereby making them available as two templates. This is then followed by the binding of the enzyme [[primase]] to the template to synthesize a starter RNA (or DNA in some viruses) strand called a [[Primer (molecular biology)|primer]] from the 5' to 3' location.<ref name="hillisetal2014i"/> Once the primer is completed, the primase is released from the template, followed by the binding of the enzyme [[DNA polymerase]] to the same template to synthesize new DNA.  The rate of DNA replication in a living cell was measured as 749 nucleotides added per second under ideal conditions.<ref>McCarthy D, Minner C, Bernstein H, Bernstein C (October 1976). "DNA elongation rates and growing point distributions of wild-type phage T4 and a DNA-delay amber mutant". Journal of Molecular Biology. 106 (4): 963–81. doi:10.1016/0022-2836(76)90346-6. PMID 789903</ref>

DNA replication is not perfect as the DNA polymerase sometimes insert bases that are not complementary to the template (e.g., putting in A in the strand opposite to G in the template strand).<ref name="hillisetal2014i"/> In eukaryotes, the initial error or [[mutation rate]] is about 1 in 100,000.<ref name="hillisetal2014i"/> [[Proofreading (biology)|Proofreading]] and [[DNA mismatch repair|mismatch repair]] are the two mechanisms that repair these errors, which reduces the mutation rate to 10<sup>−10</sup>, particularly before and after a cell cycle.<ref name="hillisetal2014i"/>

Mutations are heritable changes in DNA.<ref name="hillisetal2014i"/> They can arise [[Mutation#Spontaneous mutation|spontaneously]] as a result of replication errors that were not corrected by proofreading or can be [[Mutation#Induced mutation|induced]] by an environmental [[mutagen]] such as a chemical (e.g., [[nitrous acid]], [[benzopyrene]]) or radiation (e.g., [[x-ray]], [[gamma ray]], [[Ultraviolet|ultraviolet radiation]], particles emitted by unstable isotopes).<ref name="hillisetal2014i"/> Mutations can appear as a change in single base or at a larger scale involving chromosomal mutations such as [[Deletion (genetics)|deletion]]s, [[Chromosomal inversion|inversion]]s, or [[Chromosomal translocation|translocation]]s.<ref name="hillisetal2014i"/>

In multicellular organisms, mutations can occur in [[Somatic cell|somatic]] or [[germline]] cells.<ref name="hillisetal2014i"/> In somatic cells, the mutations are passed on to daughter cells during mitosis.<ref name="hillisetal2014i"/> In a germline cell such as a sperm or an egg, the mutation will appear in an organism at fertilization.<ref name="hillisetal2014i"/> Mutations can lead to several types of phenotypic effects such as silent, loss-of-function, [[Gain-of-function research|gain-of-function]], and conditional mutations.<ref name="hillisetal2014i"/>

Some mutations can be beneficial, as they are a source of [[genetic variation]] for evolution.<ref name="hillisetal2014i"/> Others can be harmful if they were to result in a loss of function of genes needed for survival.<ref name="hillisetal2014i"/> Mutagens such as [[carcinogen]]s are typically avoided as a matter of [[health policy|public health policy]] goals.<ref name="hillisetal2014i"/> One example is the banning of [[chlorofluorocarbon]]s (CFC) by the [[Montreal Protocol]], as CFCs tend to deplete the [[ozone layer]], resulting in more ultraviolet radiation from the sun passing through the Earth's upper atmosphere, thereby causing somatic mutations that can lead to [[skin cancer]].<ref name="hillisetal2014i"/> Similarly, [[smoking ban]]s have been enforced throughout the world in an effort to reduce the incidence of [[lung cancer]].<ref name="hillisetal2014i"/>

===Gene expression===
[[File:Extended Central Dogma with Enzymes.jpg|thumb| 300px | right | The extended [[central dogma of molecular biology]] includes all the processes involved in the flow of genetic information.]]{{Main|Gene expression}}
Gene expression is the molecular process by which a [[genotype]] gives rise to a [[phenotype]], i.e., observable trait. The genetic information stored in [[DNA]] represents the genotype, whereas the phenotype results from the synthesis of proteins that control an organism's structure and development, or that act as [[enzyme]]s catalyzing specific metabolic pathways. This process is summarized by the [[central dogma of molecular biology]], which was formulated by [[Francis Crick]] in 1958.<ref>{{cite journal | vauthors = Crick FH | title = On protein synthesis | journal = Symposia of the Society for Experimental Biology | volume = 12 | pages = 138–63 | date = 1958 | pmid = 13580867 }}</ref><ref>{{cite journal | vauthors = Crick F | title = Central dogma of molecular biology | journal = Nature | volume = 227 | issue = 5258 | pages = 561–3 | date = August 1970 | pmid = 4913914 | doi = 10.1038/227561a0 | bibcode = 1970Natur.227..561C | s2cid = 4164029 }}</ref><ref>{{cite journal | title = Central dogma reversed | journal = Nature | volume = 226 | issue = 5252 | pages = 1198–9 | date = June 1970 | pmid = 5422595 | doi = 10.1038/2261198a0 | bibcode = 1970Natur.226.1198. | s2cid = 4184060 }}</ref> According to the Central Dogma, genetic information flows from DNA to RNA to protein. Hence, there are two gene expression processes: [[transcription (genetics)|transcription]] (DNA to RNA) and [[translation (genetics)|translation]] (RNA to protein).<ref name="linelowitz2015">{{cite journal | last1 = Lin | first1 = Yihan | last2 = Elowitz | first2 = Michael B. | title = Central Dogma Goes Digital | journal = Molecular Cell | volume = 61 | issue = 6 | pages = 791–792 | date = 2016 | url = https://www.sciencedirect.com/science/article/pii/S1097276516001830 | doi = 10.1016/j.molcel.2016.03.005 | pmid = 26990983 | access-date = 2021-10-03 | archive-date = 2021-10-03 | archive-url = https://web.archive.org/web/20211003191145/https://www.sciencedirect.com/science/article/pii/S1097276516001830 | url-status = live }}</ref> These processes are used by all life—[[eukaryotes]] (including [[multicellular organisms]]), [[prokaryotes]] ([[bacteria]] and [[archaea]]), and are exploited by [[virus]]es—to generate the [[macromolecule|macromolecular]] machinery for life.

During transcription, [[messenger RNA]] (mRNA) strands are created using DNA strands as a template, which is initiated when [[RNA polymerase]] binds to a DNA sequence called a [[Promoter (genetics)|promoter]], which instructs the RNA to begin transcription of one of the two DNA strands.<ref name="hillisetal2014j">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4 = Price | first4 = Mary V. | chapter = From DNA to protein: Gene expression | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 194–214 | isbn = 978-1464175121}}</ref> The DNA bases are exchanged for their corresponding bases except in the case of thymine (T), for which RNA substitutes [[uracil]] (U).<ref>{{Cite web|url=https://www.genome.gov/genetics-glossary/Uracil|title=Uracil|website=Genome.gov|language=en|access-date=2019-11-21|archive-date=2019-10-19|archive-url=https://web.archive.org/web/20191019050405/https://www.genome.gov/genetics-glossary/Uracil|url-status=live}}</ref> In eukaryotes, a large part of DNA (e.g., >98% in humans) contain [[non-coding DNA|non-coding]] called [[intron]]s, which do not serve as patterns for [[Primary protein structure|protein sequences]]. The coding regions or [[exon]]s are interspersed along with the introns in the [[primary transcript]] (or pre-mRNA).<ref name="hillisetal2014j"/> Before translation, the pre-mRNA undergoes further processing whereby the introns are removed (or spliced out), leaving only the spliced exons in the mature mRNA strand.<ref name="hillisetal2014j"/>

The translation of mRNA to protein occurs in [[ribosome]]s, whereby the transcribed mRNA strand specifies the sequence of [[amino acid]]s within proteins using the [[genetic code]]. [[Gene product]]s are often [[protein]]s, but in non-protein-coding genes such as [[Transfer RNA|transfer RNA (tRNA)]] and [[Small nuclear RNA|small nuclear RNA (snRNA)]], the product is a functional [[List of RNAs|non-coding RNA]].<ref>{{cite journal | vauthors = Temin HM, Mizutani S | title = RNA-dependent DNA polymerase in virions of Rous sarcoma virus | journal = Nature | volume = 226 | issue = 5252 | pages = 1211–3 | date = June 1970 | pmid = 4316301 | doi = 10.1038/2261211a0 | s2cid = 4187764 }}</ref><ref>{{cite journal | vauthors = Baltimore D | title = RNA-dependent DNA polymerase in virions of RNA tumour viruses | journal = Nature | volume = 226 | issue = 5252 | pages = 1209–11 | date = June 1970 | pmid = 4316300 | doi = 10.1038/2261209a0 | s2cid = 4222378 }}</ref>

===Gene regulation===
[[File:Gene expression control.png|thumb|Regulation of various stages of gene expression]]{{Main|Regulation of gene expression}}
The regulation of gene expression (or gene regulation) by environmental factors and during different stages of [[Developmental biology|development]] can occur at each step of the process such as [[Transcription (biology)|transcription]], [[RNA splicing]], [[Translation (biology)|translation]], and [[post-translational modification]] of a protein.<ref name="hillisetal2014k">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4 = Price | first4 = Mary V. | chapter = Regulation of gene expression | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 215–233 | isbn = 978-1464175121}}</ref>

The ability of gene transcription to be regulated allows for the conservation of energy as cells will only make proteins when needed.<ref name="hillisetal2014k"/> Gene expression can be influenced by positive or negative regulation, depending on which of the two types of regulatory proteins called [[transcription factor]]s bind to the DNA sequence close to or at a promoter.<ref name="hillisetal2014k"/> A cluster of genes that share the same promoter is called an [[operon]], found mainly in prokaryotes and some lower eukaryotes (e.g., ''[[Caenorhabditis elegans]]'').<ref name="hillisetal2014k"/><ref name="keeneandtenenbaum2002">{{cite journal | last1 = Keene | first1 = Jack D. | last2 = Tenenbaum | first2 = Scott A. | title = Eukaryotic mRNPs may represent posttranscriptional operons | journal = Molecular Cell | year = 2002 | volume = 9 | issue = 6 | pages = 1161–1167 | doi = 10.1016/s1097-2765(02)00559-2| pmid = 12086614 }}</ref> It was first identified in ''[[Escherichia coli]]''—a prokaryotic cell that can be found in the [[intestine]]s of humans and other animals—in the 1960s by [[François Jacob]] and [[Jacques Monod]].<ref name="hillisetal2014k"/> They studied the prokaryotic cell's [[lac operon|''lac'' operon]], which is part of three genes (''lacZ'', ''lacY'', and ''lacA'') that encode three lactose-metabolizing enzymes ([[Beta-galactosidase|β-galactosidase]], [[Beta-galactoside permease|β-galactoside permease]], and [[Galactoside acetyltransferase|β-galactoside transacetylase]]).<ref name="hillisetal2014k"/> In positive regulation of gene expression, the [[Activator (genetics)|activator]] is the transcription factor that stimulates transcription when it binds to the sequence near or at the promoter. In contrast, negative regulation occurs when another transcription factor called a [[repressor]] binds to a DNA sequence called an [[Operon#Operator|operator]], which is part of an operon, to prevent transcription. When a repressor binds to a repressible operon (e.g., [[trp operon|''trp'' operon]]), it does so only in the presence of a [[corepressor]]. Repressors can be inhibited by compounds called [[inducer]]s (e.g., [[allolactose]]), which exert their effects by binding to a repressor to prevent it from binding to an operator, thereby allowing transcription to occur.<ref name="hillisetal2014k"/> Specific genes that can be activated by inducers are called [[Gene expression#Regulation of gene expression|inducible gene]]s (e.g., ''lacZ'' or ''lacA'' in ''E. coli''), which are in contrast to [[Gene expression#Regulation of gene expression|constitutive gene]]s that are almost always active.<ref name="hillisetal2014k"/> In contrast to both, [[structural gene]]s encode proteins that are not involved in gene regulation.<ref name="hillisetal2014k"/>

In prokaryotic cells, transcription is regulated by proteins called [[sigma factor]]s, which bind to RNA polymerase and direct it to specific promoters.<ref name="hillisetal2014k"/> Similarly, transcription factors in eukaryotic cells can also coordinate the expression of a group of genes, even if the genes themselves are located on different chromosomes.<ref name="hillisetal2014k"/> Coordination of these genes can occur as long as they share the same regulatory DNA sequence that bind to the same transcription factors.<ref name="hillisetal2014k"/> Promoters in eukaryotic cells are more diverse but tend to contain a core sequence that RNA polymerase can bind to, with the most common sequence being the [[TATA box]], which contains multiple repeating A and T bases.<ref name="hillisetal2014k"/> Specifically, [[RNA polymerase II]] is the RNA polymerase that binds to a promoter to initiate transcription of protein-coding genes in eukaryotes, but only in the presence of multiple [[general transcription factors]], which are distinct from the transcription factors that have regulatory effects, i.e., activators and repressors.<ref name="hillisetal2014k"/> In eukaryotic cells, DNA sequences that bind with activators are called enhances whereas those sequences that bind with repressors are called silencers.<ref name="hillisetal2014k"/> Transcription factors such as [[NFAT|nuclear factor of activated T-cells]] (NFAT) are able to identify specific nucleotide sequence based on the base sequence (e.g., CGAGGAAAATTG for NFAT) of the binding site, which determines the arrangement of the chemical groups within that sequence that allows for specific DNA-protein interactions.<ref name="hillisetal2014k"/> The expression of transcription factors is what underlies [[cellular differentiation]] in a developing [[embryo]].<ref name="hillisetal2014k"/>

In addition to regulatory events involving the promoter, gene expression can also be regulated by [[Epigenetics|epigenetic]] changes to [[chromatin]], which is a complex of DNA and protein found in eukaryotic cells.<ref name="hillisetal2014k"/>

Post-transcriptional control of mRNA can involve the [[alternative splicing]] of [[primary transcript|primary mRNA transcript]]s, resulting in a single gene giving rise to different mature mRNAs that encode a family of different proteins.<ref name="hillisetal2014k"/><ref name = "freeman2017s">{{cite book | last1 = Freeman| first1 = Scott | last2 = Quillin | first2 = Kim | last3 = Allison | first3 = Lizabeth | last4 = Black | first4 = Michael |  last5 = Podgorski | first5 = Greg | last6 = Taylor | first6 = Emily | last7 = Carmichael | first7 = Jeff | chapter = Control of gene expression in eukaryotes | title = Biological Science | publisher = Pearson | edition = 6th | date = 2017 | location = Hoboken, N.J. | pages = 379–397 | isbn = 978-0321976499}}</ref> A well-studied example is the ''Sxl'' gene in ''[[Drosophila]]'', which determines the [[sex]] in these animals. The gene itself contains four exons and alternative splicing of its pre-mRNA transcript can generate two active forms of the Sxl protein in female flies and one in inactive form of the protein in males.<ref name="hillisetal2014k"/> Another example is the [[human immunodeficiency virus]] (HIV), which has a single pre-mRNA transcript that can generate up to nine proteins as a result of alternative splicing.<ref name="hillisetal2014k"/> In humans, eighty percent of all 21,000 genes are alternatively spliced.<ref name="hillisetal2014k"/> Given that both chimpanzees and humans have a similar number of genes, it is thought that alternative splicing might have contributed to the latter's complexity due to the greater number of alternative splicing in the human brain than in the brain of chimpanzees.<ref name="hillisetal2014k"/>

Translation can be regulated in three known ways, one of which involves the binding of tiny RNA molecules called [[microRNA]] (miRNA) to a target mRNA transcript, which inhibits its translation and causes it to degrade.<ref name="hillisetal2014k"/> Translation can also be inhibited by the modification of the 5' cap by substituting the modified guanosine triphosphate (GTP) at the 5' end of an mRNA for an unmodified GTP molecule.<ref name="hillisetal2014k"/> Finally, translational repressor proteins can bind to mRNAs and prevent them from attaching to a ribosome, thereby blocking translation.<ref name="hillisetal2014k"/>

Once translated, the stability of proteins can be regulated by being targeted for degradation.<ref name="hillisetal2014k"/> A common example is when an enzyme attaches a regulatory protein called [[ubiquitin]] to the [[lysine]] [[Amino acid|residue]] of a targeted protein.<ref name="hillisetal2014k"/> Other ubiquitins then attached to the primary ubiquitin to form a polyubiquitinated protein, which then enters a much larger protein complex called [[proteasome]].<ref name="hillisetal2014k"/> Once the polyubiquitinated protein enters the proteasome, the polyubiquitin detaches from the target protein, which is unfolded by the proteasome in an ATP-dependent manner, allowing it to be hydrolyzed by three [[protease]]s.<ref name="hillisetal2014k"/>

===Genomes===
{{Further | Genomics }}
[[File:Components of the human genome.png|right|thumb|Composition of the human genome]]
A [[genome]] is an organism's complete set of [[DNA]], including all of its genes.<ref>{{cite web|url=https://www.who.int/genomics/geneticsVSgenomics/en/ |archive-url=https://web.archive.org/web/20040630170807/http://www.who.int/genomics/geneticsVSgenomics/en/ |url-status=dead |archive-date=June 30, 2004 |publisher=World Health Organization |title=WHO definitions of genetics and genomics}}</ref> Sequencing and analysis of genomes can be done using high throughput [[DNA sequencing]] and [[bioinformatics]] to assemble and analyze the function and structure of entire genomes.<ref name="klug2012">{{cite book | edition = 10th | publisher = Pearson Education | isbn = 978-0-321-72412-0 | title = Concepts of genetics | location = San Francisco | year = 2012}}</ref><ref name="culver2002">{{cite encyclopedia | publisher = Macmillan Reference USA | isbn = 978-0-02-865606-9 | editor-first = Richard | editor-last = Robinson | last1 = Culver | first1 = Kenneth W | first2 = Mark A | last2 = Labow | title = Genomics | encyclopedia = Genetics | series = Macmillan Science Library | date = 8 November 2002 | name-list-style = vanc | url-access = registration | url = https://archive.org/details/genetics0000unse }}</ref><ref name = "kadakkuzha2013">{{cite journal | vauthors = Kadakkuzha BM, Puthanveettil SV | title = Genomics and proteomics in solving brain complexity | journal = Molecular BioSystems | volume = 9 | issue = 7 | pages = 1807–21 | date = July 2013 | pmid = 23615871 | doi = 10.1039/C3MB25391K | pmc = 6425491 }}</ref> The genomes of prokaryotes are small, compact, and diverse. In contrast, the genomes of eukaryotes are larger and more complex such as having more [[regulatory sequence]]s and much of its genome are made up of non-coding DNA sequences for functional RNA ([[Ribosomal RNA|rRNA]], [[Transfer RNA|tRNA]], and [[Messenger RNA|mRNA]]) or regulatory sequences. The genomes of various [[model organism]]s such as [[arabidopsis]], [[Drosophila melanogaster|fruit fly]], mice, [[Caenorhabditis elegans|nematodes]], and [[yeast]] have been sequenced. The [[Human Genome Project]] was a major undertaking by the international scientific community to sequence the entire [[human genome]], which was completed in 2003.<ref name="hillisetal2014l" /> The sequencing of the human genome has yielded practical applications such as [[DNA fingerprinting]], which can be used for [[DNA paternity testing|paternity testing]] and [[forensics]]. In [[medicine]], sequencing of the entire human genome has allowed for the identification of [[mutation]]s that cause [[neoplasm|tumor]]s as well as genes that cause a specific [[genetic disorder]].<ref name="hillisetal2014l" /> The sequencing of genomes from various organisms has led to the emergence of [[comparative genomics]], which aims to draw comparisons of genes from the genomes of those different organisms.<ref name="hillisetal2014l" />

Many genes encode more than one protein, with [[posttranslational modification]]s increasing the diversity of proteins within a cell. An organism's [[proteome]] is its entire set of proteins expressed by its genome and [[proteomics]] seeks to study the complete set of proteins produced by an organism.<ref name="hillisetal2014l">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = Genomes | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 234–252 | isbn = 978-1464175121}}</ref> Because many proteins are enzymes, their activities tend to affects the concentrations of substrates and products. Thus, as the proteome changes, so do the amount of small molecules or [[metabolite]]s.<ref name="hillisetal2014l"/> The complete set of small molecules in a cell or organism is called a [[metabolome]] and [[metabolomics]] is the study of the metabolome in relation to the physiological activity of a cell or organism.<ref name="hillisetal2014l"/>

===Biotechnology===
{{Main|Biotechnology}}
[[Image:recombinant formation of plasmids.svg|thumb | right | Construction of recombinant DNA, in which a foreign DNA fragment is inserted into a [[plasmid vector]]]]
Biotechnology is the use of cells or organisms to develop products for humans.<ref name="hillisetal2014m">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = Biotechnology | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 253–272 | isbn = 978-1464175121}}</ref> One commonly used technology with wide applications is the creation of [[recombinant DNA]], which is a DNA molecule assembled from two or more sources in a laboratory. Before the advent of [[polymerase chain reaction]], biologists would manipulate DNA by cutting it into smaller fragments using [[restriction enzyme]]s. They would then purify and analyze the fragments using [[gel electrophoresis]] and then later recombine the fragments into a novel DNA sequence using [[DNA ligase]].<ref name="hillisetal2014m"/> The recombinant DNA is then [[molecular cloning|cloned]] by inserting it into a host cell, a process known as [[Transformation (genetics)|transformation]] if the host cells were bacteria such as ''[[Escherichia coli|E. coli]]'', or [[transfection]] if the host cells were eukaryotic cells like [[yeast]], plant, or animal cells. Once the host cell or organism has received and integrated the recombinant DNA, it is described as [[transgene|transgenic]].<ref name="hillisetal2014m"/>

A recombinant DNA can be inserted in one of two ways. A common method is to insert the DNA into a host chromosome, with the site of insertion being random.<ref name="hillisetal2014m"/> Another approach would be to insert the recombinant DNA as part of another DNA sequence called a [[Vector (molecular biology)|vector]], which then integrates into the host chromosome or has its own origin of DNA replication, thereby allowing to replicate independently of the host chromosome.<ref name="hillisetal2014m"/> [[Plasmid]]s from bacterial cells such as ''E. coli'' are typically used as vectors due to their relatively small size (e.g. 2000–6000 base pairs in ''E. coli''), presence of restriction enzymes, genes that are resistant to [[antibiotic]]s, and the presence of an origin of replication.<ref name="hillisetal2014m"/> A gene coding for a [[selectable marker]] such as antibiotic resistance is also incorporated into the vector.<ref name="hillisetal2014m"/> Inclusion of this market allows for the selection of only those host cells that contained the recombinant DNA while discarding those that do not.<ref name="hillisetal2014m"/> Moreover, the marker also serves as a [[reporter gene]] that once expressed, can be easily detected and measured.<ref name="hillisetal2014m"/>

Once the recombinant DNA is inside individual bacterial cells, those cells are then [[Culture plate|plated]] and allowed to grow into a [[Colony (biology)|colony]] that contains millions of transgenic cells that carry the same recombinant DNA.<ref name="griffithsetal2014j">{{cite book | last1 = Griffiths | first1 = Anthony J. | last2 = Wessler | first2 = Susan R. | last3 = Carroll | first3 = Sean B. | last4 = Doebley | first4 = John | chapter = Gene isolation and manipulation | title = An Introduction to Genetic Analysis | publisher = W.H. Freeman & Company | edition = 11th | date = 2015 | location = Sunderland, Mass. | pages = 351–395 | isbn = 978-1464109485}}</ref> These transgenic cells then produce large quantities of the transgene product such as human [[insulin]], which was the first medicine to be made using recombinant DNA technology.<ref name="hillisetal2014m" />

One of the goals of [[molecular cloning]] is to identify the function of specific DNA sequences and the proteins they encode.<ref name="hillisetal2014m" /> For a specific DNA sequence to be studied and manipulated, millions of copies of DNA fragments containing that DNA sequence need to be made.<ref name="hillisetal2014m" /> This involves breaking down an intact genome, which is much too large to be introduced into a host cell, into smaller DNA fragments. Although no longer intact, the collection of these DNA fragments still make up an organism's genome, with the collection itself being referred to as a [[genomic library]], due to the ability to search and retrieve specific DNA fragments for further study, analogous to the process of retrieving a book from a regular [[library]].<ref name="hillisetal2014m" /> DNA fragments can be obtained using [[restriction enzyme]]s and other processes such as [[DNA fragmentation|mechanical shearing]]. Each obtained fragment is then inserted into a vector that is taken up by a bacterial host cell. The host cell is then allowed to proliferate on a selective [[Growth medium|medium]] (e.g., antibiotic resistance), which produces a colony of these recombinant cells, each of which contains many copies of the same DNA fragment.<ref name="hillisetal2014m" /> These colonies can be grown by spreading them over a solid medium in [[Petri dish]]es, which are [[Incubator (culture)|incubated]] at a suitable temperature. One dish alone can hold thousands of bacterial colonies, which can be easily screened for a specific DNA sequence.<ref name="hillisetal2014m" /> The sequence can be identified by first duplicating a Petri dish with bacterial colonies and then exposing the DNA of the duplicated colonies for [[Nucleic acid hybridization|hybridization]], which involves labeling them with complementary [[Isotopic labeling|radioactive]] or [[Fluorophore|fluorescent]] nucleotides.<ref name="hillisetal2014m" />

Smaller DNA libraries that contain genes from a specific tissue can be created using [[complementary DNA]] (cDNA).<ref name="hillisetal2014m" /> The collection of these cDNAs from a specific tissue at a particular time is called a [[cDNA library]], which provides a "snapshot" of transcription patterns of cells at a specific location and time.<ref name="hillisetal2014m" />

Other biotechnology tools include [[DNA microarray]]s, [[expression vector]]s, [[synthetic genomics]], and [[CRISPR gene editing]].<ref name="hillisetal2014m" /><ref name = "freeman2017t">{{cite book | last1 = Freeman| first1 = Scott | last2 = Quillin | first2 = Kim | last3 = Allison | first3 = Lizabeth | last4 = Black | first4 = Michael |  last5 = Podgorski | first5 = Greg | last6 = Taylor | first6 = Emily | last7 = Carmichael | first7 = Jeff | chapter = Biology and the three of life | title = Biological Science | publisher = Pearson | edition = 6th | date = 2017 | location = Hoboken, N.J. | pages = 398–417 | isbn = 978-0321976499}}</ref> Other approaches such as [[Pharming (genetics)|pharming]] can produce large quantities of medically useful products through the use of [[genetically modified organism]]s.<ref name="hillisetal2014m" /> Many of these other tools also have wide applications such as creating medically useful proteins, or improving [[Plant#Cultivation|plant cultivation]] and [[animal husbandry]].<ref name="hillisetal2014m" />

===Genes, development, and evolution===
{{Further | Evolutionary developmental biology}}
[[File:Concs.gif|thumb|Model of concentration gradient building up; fine yellow-orange outlines are cell boundaries.<ref>{{cite conference |author=Knabe J.F. |title=Evolution and Morphogenesis of Differentiated Multicellular Organisms: Autonomously Generated Diffusion Gradients for Positional Information |conference=Artificial Life XI: Proceedings of the Eleventh International Conference on the Simulation and Synthesis of Living Systems |date=2008 |url=http://panmental.de/ALifeXIflag |display-authors=etal |access-date=2021-10-04 |archive-date=2018-09-13 |archive-url=https://web.archive.org/web/20180913090900/http://panmental.de/ALifeXIflag |url-status=live }}</ref>]]
[[Developmental biology|Development]] is the process by which a [[multicellular organism]] ([[plant]] or [[animal]]) goes through a series of a changes, starting from a single cell, and taking on various forms that are characteristic of its life cycle.<ref name="hillisetal2014n">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = Genes, development, and evolution | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 273–298 | isbn = 978-1464175121}}</ref> There are four key processes that underlie development: [[Cell fate determination|Determination]], [[Cellular differentiation|differentiation]], [[morphogenesis]], and growth. Determination sets the developmental fate of a cell, which becomes more restrictive during development. Differentiation is the process by which specialized cells from less specialized cells such as [[stem cell]]s.<ref>Slack, J.M.W. (2013) Essential Developmental Biology. Wiley-Blackwell, Oxford.</ref><ref>{{cite journal | last1 = Slack | first1 = J.M.W. | year = 2007 | title = Metaplasia and transdifferentiation: from pure biology to the clinic | journal = Nature Reviews Molecular Cell Biology | volume = 8 | issue = 5| pages = 369–378 | doi=10.1038/nrm2146 | pmid=17377526| s2cid = 3353748 }}</ref> Stem cells are [[Cellular differentiation|undifferentiated]] or partially differentiated [[Cell (biology)|cells]] that can differentiate into various [[types of cells]] and [[Cell proliferation|proliferate]] indefinitely to produce more of the same stem cell.<ref name=":7">{{cite book|last1=Atala|first1=Anthony|last2=Lanza|first2=Robert|name-list-style=vanc|url=https://books.google.com/books?id=wm-K_dKpjBAC&pg=RA1-PA451|title=Handbook of Stem Cells|date=2012-12-31|publisher=Academic Press|isbn=978-0-12-385943-3|pages=452|language=en|access-date=2021-05-28|archive-date=2021-04-12|archive-url=https://web.archive.org/web/20210412065854/https://books.google.com/books?id=wm-K_dKpjBAC&pg=RA1-PA451|url-status=live}}</ref> Cellular differentiation dramatically changes a cell's size, shape, [[membrane potential]], [[metabolism|metabolic activity]], and responsiveness to signals, which are largely due to highly controlled modifications in [[gene expression]] and [[epigenetics]].  With a few exceptions, cellular differentiation almost never involves a change in the [[DNA]] sequence itself.<ref>{{Cite journal|last1=Yanes|first1=Oscar|last2=Clark|first2=Julie|last3=Wong|first3=Diana M.|last4=Patti|first4=Gary J.|last5=Sánchez-Ruiz|first5=Antonio|last6=Benton|first6=H. Paul|last7=Trauger|first7=Sunia A.|last8=Desponts|first8=Caroline|last9=Ding|first9=Sheng|last10=Siuzdak|first10=Gary|date=June 2010|title=Metabolic oxidation regulates embryonic stem cell differentiation|url= |journal=Nature Chemical Biology|language=en|volume=6|issue=6|pages=411–417|doi=10.1038/nchembio.364|pmid=20436487|issn=1552-4469|pmc=2873061}}</ref> Thus, different cells can have very different physical characteristics despite having the same [[genome]]. Morphogenesis, or the development of body form, is the result of spatial differences in gene expression.<ref name="hillisetal2014n" /> Specially, the organization of differentiated tissues into specific structures such as arms or wings, which is known as [[Pattern formation#Biology|pattern formation]], is governed by [[morphogen]]s, signaling molecules that move from one group of cells to surrounding cells, creating a morphogen gradient as described by the [[French flag model]]. [[Apoptosis]], or programmed cell death, also occurs during morphogenesis, such as the death of cells between digits in human embryonic development, which frees up individual fingers and toes. Expression of [[transcription factor]] genes can determine organ placement in a plant and a cascade of transcription factors themselves can establish body segmentation in a fruit fly.<ref name="hillisetal2014n" />

A small fraction of the genes in an organism's genome called the [[Evo-devo gene toolkit|developmental-genetic toolkit]] control the development of that organism. These toolkit genes are highly conserved among [[Phylum|phyla]], meaning that they are ancient and very similar in widely separated groups of animals. Differences in deployment of toolkit genes affect the body plan and the number, identity, and pattern of body parts. Among the most important toolkit genes are the [[Hox gene|''Hox'' genes]]. Hox genes determine where repeating parts, such as the many [[vertebra]]e of [[snake]]s, will grow in a developing embryo or larva.<ref name=CarrollNatHist>{{cite web |last1=Carroll |first1=Sean B. |author-link=Sean B. Carroll |title=The Origins of Form |url=http://www.naturalhistorymag.com/features/061488/the-origins-of-form |website=Natural History |access-date=9 October 2016 |quote=Biologists could say, with confidence, that forms change, and that natural selection is an important force for change. Yet they could say nothing about how that change is accomplished. How bodies or body parts change, or how new structures arise, remained complete mysteries. |archive-date=9 October 2018 |archive-url=https://web.archive.org/web/20181009154501/http://www.naturalhistorymag.com/features/061488/the-origins-of-form |url-status=live }}</ref> Variations in the toolkit may have produced a large part of the morphological evolution of animals. The toolkit can drive evolution in two ways. A toolkit gene can be expressed in a different pattern, as when the beak of Darwin's [[large ground-finch]] was enlarged by the ''[[Bone morphogenetic protein|BMP]]'' gene,<ref>{{cite journal |author1=Abzhanov, A. |author2=Protas, M. |author3=Grant, B.R. |author4=Grant, P.R. |author5=Tabin, C.J. |year=2004 |title=Bmp4 and Morphological Variation of Beaks in Darwin's Finches |journal=Science |volume=305 |issue=5689| doi=10.1126/science.1098095 |pages=1462–1465 |pmid=15353802 |bibcode=2004Sci...305.1462A|s2cid=17226774 }}</ref> or when snakes lost their legs as ''[[DLX gene family|Distal-less (Dlx)]]'' genes became under-expressed or not expressed at all in the places where other reptiles continued to form their limbs.<ref>{{cite journal |author1=Cohn, M.J. |author2=Tickle, C. |title=Developmental basis of limblessness and axial patterning in snakes |year=1999 |journal=Nature |volume=399 |pages=474–479 |pmid=10365960 |issue=6735 |doi=10.1038/20944 |bibcode=1999Natur.399..474C|s2cid=4309833 }}</ref> Or, a toolkit gene can acquire a new function, as seen in the many functions of that same gene, ''distal-less'', which controls such diverse structures as the mandible in vertebrates,<ref name="Beverdam2002">{{cite journal |title=Jaw Transformation With Gain of Symmetry After DLX5/DLX6 Inactivation: Mirror of the Past? |journal=Genesis |date=August 2002 |volume=34 |pages=221–227 |author=Beverdam, A. |author2=Merlo, G.R. |author3=Paleari, L. |author4=Mantero, S. |author5=Genova, F. |author6=Barbieri, O. |author7=Janvier, P. |author8=Levi, G. |doi=10.1002/gene.10156 |issue=4 |pmid=12434331 |hdl=2318/87307 |s2cid=19592597 |url=https://iris.unito.it/bitstream/2318/87307/1/Beverdam_2002.pdf |hdl-access=free |access-date=2021-05-28 |archive-date=2022-07-30 |archive-url=https://web.archive.org/web/20220730085745/https://iris.unito.it/bitstream/2318/87307/1/Beverdam_2002.pdf |url-status=live }}</ref><ref name="Depew2002">{{cite journal |title=Specification of jaw subdivisions by DLX genes |journal=Science |date=October 2002 |volume=298 |issue=5592 |pages=381–385 |author=Depew, M.J. |author2=Lufkin, T. |author3=Rubenstein, J.L. |pmid=12193642 |doi=10.1126/science.1075703 |s2cid=10274300 }}</ref> legs and antennae in the fruit fly,<ref>{{cite journal |last1=Panganiban |first1=Grace |last2=Rubenstein |first2=John L. R. |title=Developmental functions of the Distal-less/Dlx homeobox genes |journal=Development |date=2002 |volume=129 |issue=19 |pages=4371–4386 |doi=10.1242/dev.129.19.4371 |pmid=12223397 |url=http://dev.biologists.org/content/129/19/4371 |access-date=2021-05-28 |archive-date=2020-08-10 |archive-url=https://web.archive.org/web/20200810233645/https://dev.biologists.org/content/129/19/4371 |url-status=live }}</ref> and [[eyespot (mimicry)|eyespot pattern]] in [[butterfly]] [[wing]]s.<ref>{{cite journal |author1=Beldade, P. |author2=Brakefield, P.M. |author3=Long, A.D. |year=2002 |title=Contribution of Distal-less to quantitative variation in butterfly eyespots |journal=Nature |volume=415 |issue=6869 |pages=315–318 |pmid=11797007 |doi=10.1038/415315a|s2cid=4430563 }}</ref> Given that small changes in toolbox genes can cause significant changes in body structures, they have often enabled [[Convergent evolution|convergent]] or [[parallel evolution]].

==Evolution==
===Evolutionary processes===
{{Further | Evolutionary biology}}
[[File:Mutation and selection diagram.svg|thumb|right|[[Natural selection]] for darker traits]]
A central organizing concept in biology is that life changes and develops through [[evolution]], which is the change in [[Heredity|heritable]] [[Phenotypic trait|characteristics]] of [[population]]s over successive [[generation]]s.<ref>{{harvnb|Hall|Hallgrímsson|2007|pp=[https://books.google.com/books?id=jrDD3cyA09kC&pg=PA4 4–6]}}</ref><ref>{{cite web |title=Evolution Resources |location=Washington, D.C. |publisher=[[National Academies of Sciences, Engineering, and Medicine]] |year=2016 |url=http://www.nas.edu/evolution/index.html |url-status=live |archive-url=https://web.archive.org/web/20160603230514/http://www.nas.edu/evolution/index.html |archive-date=2016-06-03}}</ref> Evolution is now used to explain the great variations of life on Earth. The term ''evolution'' was introduced into the scientific lexicon by [[Jean-Baptiste de Lamarck]] in 1809.<ref name="hillisetal2014o">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = Processes of evolution | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 299–324 | isbn = 978-1464175121}}</ref><ref name="p15">{{cite book |last=Packard |first=Alpheus Spring |title= Lamarck, the founder of Evolution: his life and work with translations of his writings on organic evolution|url=https://archive.org/details/lamarckfoundere05packgoog |year= 1901 |publisher= Longmans, Green |location=New York | isbn=978-0-405-12562-1 }}</ref> He proposed that evolution occurred as a result of [[Lamarckism|inheritance of acquired characteristics]], which was unconvincing but there were no alternative explanations at the time.<ref name="hillisetal2014o"/> [[Charles Darwin]], an English [[naturalist]], had returned to England in 1836 from his [[second voyage of HMS Beagle|five-year travels on the HMS Beagle]] where he studied rocks and collected plants and animals from various parts of the world such as the [[Galápagos Islands]].<ref name="hillisetal2014o"/> He had also read ''[[Principles of Geology]]'' by [[Charles Lyell]] and ''[[An Essay on the Principle of Population]]'' by [[Thomas Robert Malthus|Thomas Malthus]] and was influenced by them.<ref name = "urry2017v">{{cite book | last1 = Urry | first1 = Lisa | last2 = Cain | first2 = Michael | last3 = Wasserman | first3 =  Steven | last4 = Minorsky | first4 = Peter |  last5 = Reece | first5 = Jane | chapter = Descent with modifications: A Darwinian view of life | title = Campbell Biology | publisher = Pearson | edition = 11th | date = 2017 | location = New York | pages = 466–483 | isbn = 978-0134093413}}</ref> Based on his observations and readings, Darwin began to formulate his [[Natural selection|theory of evolution by natural selection]] to explain the diversity of plants and animals in different parts of the world.<ref name="hillisetal2014o"/><ref name = "urry2017v"/> [[Alfred Russel Wallace]], another English naturalist who had studied plants and animals in the [[Malay Archipelago]], also came to the same idea, but later and independently of Darwin.<ref name="hillisetal2014o"/> Both Darwin and Wallace jointly presented their essay and manuscript, respectively, at the [[Linnaean Society of London]] in 1858, giving them both credit for their discovery of evolution by natural selection.<ref name="hillisetal2014o"/><ref>{{cite web | url=http://darwin-online.org.uk/biography.html | title=The Complete Works of Darwin Online – Biography | publisher=darwin-online.org.uk | access-date=2006-12-15 | url-status=live | archive-url=https://web.archive.org/web/20070107165048/http://darwin-online.org.uk/biography.html | archive-date=2007-01-07 }}</ref><ref>{{cite journal |last1=Dobzhansky |first1=T. | year=1973 |title=Nothing in biology makes sense except in the light of evolution |journal=The American Biology Teacher |volume=35 |issue=3 |pages=125–29 |doi= 10.2307/4444260|jstor=4444260 |citeseerx=10.1.1.525.3586|s2cid=207358177 }} - see also [[Nothing in Biology Makes Sense Except in the Light of Evolution]]</ref><ref>{{cite book |title=On the origin of species by means of natural selection | editor-last=Carroll | editor-first=Joseph |year=2003 |publisher=Broadview |location= Peterborough, Ontario | isbn=978-1-55111-337-1 | page=15 | quote=As Darwinian scholar Joseph Carroll of the University of Missouri–St. Louis puts it in his introduction to a modern reprint of Darwin's work: "''The Origin of Species'' has special claims on our attention. It is one of the two or three most significant works of all time—one of those works that fundamentally and permanently alter our vision of the world ... It is argued with a singularly rigorous consistency but it is also eloquent, imaginatively evocative, and rhetorically compelling." }}</ref><ref>Shermer p. 149.</ref> Darwin would later publish his book ''[[On the Origin of Species]]'' in 1859, which explained in detail how the process of evolution by natural selection works.<ref name="hillisetal2014o"/>

To explain natural selection, Darwin drew an analogy with humans modifying animals through [[artificial selection]], whereby animals were selectively bred for specific [[Phenotypic trait|trait]]s, which has given rise to individuals that no longer resemble their wild ancestors.<ref name = "urry2017v"/> Darwin argued that in the natural world, it was nature that played the role of humans in selecting for specific traits. He came to this conclusion based on two observations and two inferences.<ref name = "urry2017v"/> First, members of any population tend to vary with respect to their [[heredity|heritable]] traits. Second, all species tend to produce more offspring than can be supported by their respective environments, resulting in many individuals not surviving and reproducing.<ref name = "urry2017v"/> Based on these observations, Darwin inferred that those individuals who possessed heritable traits that are better adapted to their environments are more likely to survive and produce more offspring than other individuals.<ref name = "urry2017v"/> He further inferred that the unequal or differential survival and reproduction of certain individuals over others will lead to the accumulation of favorable traits over successive generations, thereby increasing the match between the organisms and their environment.<ref name = "urry2017v"/><ref name="lewontin70">{{cite journal |last=Lewontin |first=Richard C. |author-link=Richard Lewontin |date=November 1970 |title=The Units of Selection |url=http://joelvelasco.net/teaching/167/lewontin%2070%20-%20the%20units%20of%20selection.pdf |journal=[[Annual Review of Ecology, Evolution, and Systematics|Annual Review of Ecology and Systematics]] |volume=1 |pages=1–18 |doi=10.1146/annurev.es.01.110170.000245 |jstor=2096764 |issn=1545-2069 |url-status=live |archive-url=https://web.archive.org/web/20150206172942/http://joelvelasco.net/teaching/167/lewontin%2070%20-%20the%20units%20of%20selection.pdf |archive-date=2015-02-06}}</ref><ref>[[Charles Darwin|Darwin, Charles]] (1859). ''[[On the Origin of Species]]'', John Murray.</ref> Thus, taken together, natural selection is the differential survival and reproduction of individuals in subsequent generations due to differences in or more heritable traits.<ref name="futuyma2017a">{{cite book | last1 = Futuyma | first1 = Douglas J. | last2 = Kirkpatrick | first2 = Mark | date = 2017 | pages = 3–26 | chapter = Evolutionary biology | title = Evolution | edition = 4th | location = Sunderland, Mass. | publisher = Sinauer Associates }}</ref><ref name = "urry2017v"/><ref name="hillisetal2014o"/>

Darwin was not aware of Mendel's work of inheritance and so the exact mechanism of inheritance that underlie natural selection was not well-understood<ref name = "Charlesworth2009">{{cite journal | last1 = Charlesworth | first1 = Brian | last2 = Charlesworth | first2 = Deborah | title = Darwin and genetics | journal = Genetics | volume = 183 | issue = 3 | pages = 757–766 | date = 2009| doi = 10.1534/genetics.109.109991 | pmid = 19933231 | pmc = 2778973 }}</ref> until the early 20th century when the [[Modern synthesis (20th century)|modern synthesis]] reconciled [[Darwinism|Darwinian evolution]] with [[classical genetics]], which established a [[Neo-Darwinism|neo-Darwinian]] perspective of evolution by natural selection.<ref name="futuyma2017a"/> This perspective holds that evolution occurs when there are changes in the [[allele frequency|allele frequencies]] within a population of interbreeding organisms. In the absence of any evolutionary process acting on a large random mating population, the allele frequencies will remain constant across generations as described by the [[Hardy–Weinberg principle]].<ref name="futuyma2017d">{{cite book | last1 = Futuyma | first1 = Douglas J. | last2 = Kirkpatrick | first2 = Mark | date = 2017 | pages = 79–101 | chapter = Mutation and variation | title = Evolution | edition = 4th | location = Sunderland, Mass. | publisher = Sinauer Associates }}</ref>

Another process that drives evolution is [[genetic drift]], which is the random fluctuations of allele frequencies within a population from one generation to the next.<ref name=GGS>{{Cite book | last=Simpson | first=George Gaylord | author-link=George Gaylord Simpson | year=1967 | title=The Meaning of Evolution | publisher=Yale University Press | edition=Second | isbn=978-0-300-00952-1 }}</ref> When selective forces are absent or relatively weak, allele frequencies are equally likely to ''drift'' upward or downward at each successive generation because the alleles are subject to [[sampling error]].<ref name="Masel 2011">{{cite journal |last=Masel |first=Joanna |s2cid=17619958 |date=October 25, 2011 |title=Genetic drift |journal=Current Biology |volume=21 |issue=20 |pages=R837–R838 |doi=10.1016/j.cub.2011.08.007 |issn=0960-9822 |pmid=22032182|doi-access=free }}</ref> This drift halts when an allele eventually becomes fixed, either by disappearing from the population or replacing the other alleles entirely. Genetic drift may therefore eliminate some alleles from a population due to chance alone.

===Speciation===
[[File:Speciation modes.svg|right|thumb|300px|Comparison of [[allopatric speciation|allopatric]], [[peripatric speciation|peripatric]], [[parapatric speciation|parapatric]] and [[sympatric speciation]]]]{{See also|Species|Speciation}}
A species is a group of organisms that mate with one another and speciation is the process by which one lineage splits into two lineages as a result of having evolved independently from each other.<ref name="hillisetal2014q">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = Speciation | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 343–356 | isbn = 978-1464175121}}</ref> For speciation to occur, there has to be [[reproductive isolation]].<ref name="hillisetal2014q" /> Reproductive isolation can result from incompatibilities between genes as described by [[Bateson–Dobzhansky–Muller model]]. Reproductive isolation also tends to increase with [[genetic divergence]]. Speciation can occur when there are physical barriers that divide an ancestral species, a process known as [[allopatric speciation]].<ref name="hillisetal2014q" /> In contrast, [[sympatric speciation]] occurs in the absence of physical barriers.

[[Reproductive isolation#Pre-zygotic isolation|Pre-zygotic isolation]] such as [[Reproductive isolation#Mechanical isolation|mechanical]], [[Reproductive isolation#Mechanical isolation|temporal]], [[Reproductive isolation#Behavioral isolation|behavioral]], habitat, and [[Reproductive isolation#Gametic isolation|gametic isolation]]s can prevent different species from [[Hybrid (biology)|hybridizing]].<ref name="hillisetal2014q" /> Similarly, [[Reproductive isolation#Post-zygotic isolation|post-zygotic isolation]]s can result in hybridization being selected against due to the lower viability of hybrids or hybrid infertility (e.g., [[mule]]). [[Hybrid zone]]s can emerge if there were to be incomplete reproductive isolation between two closely related species.

===Phylogeny===
{{Further|Phylogenetics|Biodiversity}}
{{PhylomapB|caption = Phylogenetic tree showing the domains of [[bacterium|bacteria]], [[archaea]], and [[eukaryote]]s}}
A phylogeny is an evolutionary history of a specific group of organisms or their genes.<ref name="hillisetal2014p">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = Reconstructing and using phylogenies | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 325–342 | isbn = 978-1464175121}}</ref> It can be represented using a [[phylogenetic tree]], which is a diagram showing lines of descent among organisms or their genes. Each line drawn on the time axis of a tree represents a [[Lineage (evolution)|lineage]] of descendants of a particular species or population. When a lineage divides into two, it is represented as a node (or split) on the phylogenetic tree. The more splits there are over time, the more branches there will be on the tree, with the common ancestor of all the organisms in that tree being represented by the root of that tree. Phylogenetic trees may portray the evolutionary history of all life forms, a major evolutionary group (e.g., [[insect]]s), or an even smaller group of closely related [[species]]. Within a tree, any group of species designated by a name is a [[taxon]] (e.g., humans, primates, mammals, or vertebrates) and a taxon that consists of all its evolutionary descendants is a [[clade]], otherwise known as a [[monophyletic]] taxon.<ref name="hillisetal2014p" /> Closely related species are referred to as [[Sister group|sister species]] and closely related clades are sister clades. In contrast to a monophyletic group, a [[polyphyly|polyphyletic]] group does not include its common ancestor whereas a [[paraphyly|paraphyletic]] group does not include all the descendants of a common ancestor.<ref name="hillisetal2014p" />

Phylogenetic trees are the basis for comparing and grouping different species.<ref name="hillisetal2014p" /> Different species that share a feature inherited from a common ancestor are described as having [[Homology (biology)|homologous]] features (or [[Apomorphy and synapomorphy|synapomorphy]]).<ref name = "kitchingetal2001">{{cite encyclopedia | last1 = Kitching | first1 = Ian J. | last2 = Forey | first2 = Peter L. | last3 = Williams | first3 = David M. | editor-last = Levin | editor-first = Simon A. | title = Cladistics | encyclopedia = Encyclopedia of Biodiversity | year = 2001 | edition = 2nd | pages = 33–45 | publisher = Elsevier | doi = 10.1016/B978-0-12-384719-5.00022-8 | isbn = 9780123847201 | access-date = 29 August 2021 | url = https://www.sciencedirect.com/science/article/pii/B9780123847195000228 | archive-date = 29 August 2021 | archive-url = https://web.archive.org/web/20210829234556/https://www.sciencedirect.com/science/article/pii/B9780123847195000228 | url-status = live }})</ref><ref name="Futuyma2017p">{{cite book |last1 = Futuyma | first1 = Douglas J. | last2 = Kirkpatrick | first2 = Mark | date = 2017| pages=401–429 | chapter = Phylogeny: The unity and diversity of life | title = Evolution | edition = 4th | publisher = Sinauer Associates | location = Sunderland, Mass.}}</ref><ref name="hillisetal2014p" /> Homologous features may be any [[Heredity|heritable]] [[Phenotypic trait|traits]] such as [[Nucleic acid sequence|DNA sequence]], protein structures, anatomical features, and behavior patterns. A [[vertebral column]] is an example of a homologous feature shared by all vertebrate animals. Traits that have a similar form or function but were not derived from a common ancestor are described as [[analogous structures|analogous feature]]s. Phylogenies can be reconstructed for a group of organisms of primary interests, which are called the ingroup. A species or group that is closely related to the ingroup but is phylogenetically outside of it is called the [[Outgroup (cladistics)|outgroup]], which serves a reference point in the tree. The root of the tree is located between the ingroup and the outgroup.<ref name="hillisetal2014p" /> When phylogenetic trees are reconstructed, multiple trees with different evolutionary histories can be generated. Based on the principle of [[Maximum parsimony (phylogenetics)|Parsimony (or Occam's razor)]], the tree that is favored is the one with the fewest evolutionary changes needed to be assumed over all traits in all groups. [[Computational phylogenetics|Computational algorithms]] can be used to determine how a tree might have evolved given the evidence.<ref name="hillisetal2014p" />

Phylogeny provides the basis of biological classification, which is based on [[Linnaean taxonomy]] that was developed by [[Carl Linnaeus]] in the 18th century.<ref name="hillisetal2014p" /> This classification system is rank-based, with the highest rank being the [[Domain (biology)|domain]] followed by [[Kingdom (biology)|kingdom]], [[phylum]], [[Class (biology)|class]], [[Order (biology)|order]], [[Family (biology)|family]], [[genus]], and [[species]].<ref name="hillisetal2014p" /> All organisms can be classified as belonging to one of [[three-domain system|three domain]]s: [[Archaea]] (originally Archaebacteria); [[bacteria]] (originally eubacteria), or [[Eukaryote|eukarya]] (includes the [[protist]], [[fungi]], [[plant]], and [[animal]] kingdoms).<ref name="domain">{{cite journal |author1=Woese, CR |author2=Kandler, O |author3=Wheelis, ML | title=Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya | journal=Proceedings of the National Academy of Sciences of the United States of America | volume=87 | issue=12 | pages=4576–79 | date=June 1990 | pmid=2112744 | pmc=54159 | doi=10.1073/pnas.87.12.4576 | bibcode=1990PNAS...87.4576W |doi-access=free }}</ref> A [[binomial nomenclature]] is used to classify different species. Based on this system, each species is given two names, one for its genus and another for its species.<ref name="hillisetal2014p" /> For example, humans are ''[[Homo sapiens]]'', with ''Homo'' being the genus and ''sapiens'' being the species. By convention, the scientific names of organisms are italicized, with only the first letter of the genus capitalized.<ref>{{cite book |author1=McNeill, J|author2=Barrie, FR|author3=Buck, WR|author4=Demoulin, V|author5=Greuter, W|author6=Hawksworth, DL|author7=Herendeen, PS|author8=Knapp, S|author9=Marhold, K|author10=Prado, J|author11=Prud'homme Van Reine, WF|author12=Smith, GF|author13=Wiersema, JH|author14=Turland, NJ |display-authors=6 |year=2012 <!-- |volume=Regnum Vegetabile 154 --> |title=International Code of Nomenclature for algae, fungi and plants (Melbourne Code) adopted by the Eighteenth International Botanical Congress Melbourne, Australia, July 2011 |publisher=A.R.G. Gantner Verlag KG |isbn=978-3-87429-425-6 |url=http://www.iapt-taxon.org/nomen/main.php?page=title |url-status=live |archive-url=https://web.archive.org/web/20131104060236/http://www.iapt-taxon.org/nomen/main.php?page=title |archive-date=2013-11-04 }} Recommendation 60F</ref><ref>{{cite book | url=https://books.google.com/books?id=hVUU7Gq8QskC&pg=PA198 | page=198 | title=Writing for Science and Engineering: Papers, Presentation | last=Silyn-Roberts | first=Heather | year=2000 | isbn=978-0-7506-4636-9 | publisher=Butterworth-Heinemann | location=Oxford | access-date=2020-08-24 | archive-date=2020-10-02 | archive-url=https://web.archive.org/web/20201002180221/https://books.google.com/books?id=hVUU7Gq8QskC&lpg=PA198 | url-status=live }}</ref>

===History of life===
{{Life timeline}}
{{Main|History of life}}

The history of life on [[Earth]] traces the processes by which [[organism]]s have evolved from the earliest emergence of life to present day. Earth formed about 4.5 billion years ago and all life on [[Earth]], both living and extinct, descended from a [[last universal common ancestor]] that lived about [[Timeline of evolution|3.5 billion years ago]].<ref>{{cite journal |author1=Montévil, M |author2=Mossio, M |author3=Pocheville, A |author4=Longo, G | title=Theoretical principles for biology: Variation | journal=Progress in Biophysics and Molecular Biology | volume=122 | issue=1 | pages=36–50 | date=October 2016 | pmid=27530930 | doi=10.1016/j.pbiomolbio.2016.08.005 | url=https://www.academia.edu/27942089 | series=From the Century of the Genome to the Century of the Organism: New Theoretical Approaches |s2cid=3671068 | url-status=live | archive-url=https://web.archive.org/web/20180320150224/http://www.academia.edu/27942089/Theoretical_principles_for_biology_Variation | archive-date=2018-03-20 }}</ref><ref name="deduve2002">{{cite book | title=Life Evolving: Molecules, Mind, and Meaning | url=https://archive.org/details/lifeevolvingmole00duve_331 | url-access=limited | last=De Duve | first=Christian | location=New York | publisher=Oxford University Press | year=2002| page=[https://archive.org/details/lifeevolvingmole00duve_331/page/n60 44] | isbn=978-0-19-515605-8}}</ref> The dating of the Earth's history can be done using several geological methods such as [[stratigraphy]], [[radiometric dating]], and [[Geochronology#Paleomagnetic dating|paleomagnetic dating]].<ref name="hillisetal2014r">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = The history of life on Earth | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 357–376 | isbn = 978-1464175121}}</ref> Based on these methods, [[geologist]]s have developed a [[geologic time scale]] that divides the history of the Earth into major divisions, starting with four eons ([[Hadean]], [[Archean]], [[Proterozoic]], and [[Phanerozoic]]), the first three of which are collectively known as the [[Precambrian]], which lasted approximately 4 billion years.<ref name="hillisetal2014r" /> Each eon can be divided into [[era]]s, with the Phanerozoic eon that began 539 million years ago<ref>{{cite web |title=Stratigraphic Chart 2022 |url=https://stratigraphy.org/ICSchart/ChronostratChart2022-02.pdf |publisher=International Stratigraphic Commission |date=February 2022 |access-date=25 April 2022 |archive-date=2 April 2022 |archive-url=https://web.archive.org/web/20220402100018/https://stratigraphy.org/ICSchart/ChronostratChart2022-02.pdf |url-status=live }}</ref> being subdivided into [[Paleozoic]], [[Mesozoic]], and [[Cenozoic]] eras.<ref name="hillisetal2014r" /> These three eras together comprise eleven [[Geologic time scale#Terminology|period]]s ([[Cambrian]], [[Ordovician]], [[Silurian]], [[Devonian]], [[Carboniferous]], [[Permian]], [[Triassic]], [[Jurassic]], [[Cretaceous]], [[Tertiary]], and [[Quaternary]]) and each period into epochs.<ref name="hillisetal2014r" />

The similarities among all known present-day [[species]] indicate that they have diverged through the process of [[evolution]] from their common ancestor.<ref>{{harvnb|Futuyma|2005}}</ref> Biologists regard the ubiquity of the [[genetic code]] as evidence of universal [[common descent]] for all [[bacterium|bacteria]], [[archaea]], and [[eukaryote]]s.<ref name="Futuyma">{{cite book |last=Futuyma |first=DJ |title=Evolution |year=2005 |publisher=Sinauer Associates |isbn=978-0-87893-187-3 |oclc=57311264 |url=https://archive.org/details/evolution0000futu }}</ref><ref name="Pearce 343–364" /><ref name="Rosing 674–676">{{cite journal |last=Rosing |first=Minik T. |date=January 29, 1999 |title=<sup>13</sup>C-Depleted Carbon Microparticles in &gt;3700-Ma Sea-Floor Sedimentary Rocks from West Greenland |journal=[[Science (journal)|Science]] |volume=283 |issue=5402 |pages=674–676 |bibcode=1999Sci...283..674R |doi=10.1126/science.283.5402.674 |issn=0036-8075 |pmid=9924024}}</ref><ref name="Ohtomo 25–28">{{cite journal |last1=Ohtomo |first1=Yoko |last2=Kakegawa |first2=Takeshi |last3=Ishida |first3=Akizumi |last4=Nagase |first4=Toshiro |last5=Rosing |first5=Minik T. |display-authors=3 |date=January 2014 |title=Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks |journal=[[Nature Geoscience]] |volume=7 |issue=1 |pages=25–28 |bibcode=2014NatGe...7...25O |doi=10.1038/ngeo2025 |issn=1752-0894}}</ref> [[Microbial mat|Microbal mats]] of coexisting bacteria and archaea were the dominant form of life in the early [[Archean]] epoch and many of the major steps in early evolution are thought to have taken place in this environment.<ref name="NisbetFowler1999">{{cite journal |last1=Nisbet |first1=Euan G. |last2=Fowler |first2=C.M.R. |author2-link=Mary Fowler (geologist) |date=December 7, 1999 |title=Archaean metabolic evolution of microbial mats |journal=[[Proceedings of the Royal Society#Proceedings of the Royal Society B|Proceedings of the Royal Society B]] |volume=266 |issue=1436 |pages=2375–2382 |doi=10.1098/rspb.1999.0934 |issn=0962-8452 |pmc=1690475}}</ref> The earliest evidence of [[eukaryote]]s dates from 1.85 billion years ago,<ref>{{cite journal |last1=Knoll |first1=Andrew H. |author-link=Andrew H. Knoll |last2=Javaux |first2=Emmanuelle J. |last3=Hewitt |first3=David |last4=Cohen |first4=Phoebe |display-authors=3 |date=June 29, 2006 |title=Eukaryotic organisms in Proterozoic oceans |journal=[[Philosophical Transactions of the Royal Society B]] |volume=361 |issue=1470 |pages=1023–1038 |doi=10.1098/rstb.2006.1843 |issn=0962-8436 |pmc=1578724 |pmid=16754612}}</ref><ref name="fedonkin2003">{{cite journal |last=Fedonkin |first=Mikhail A. |author-link=Mikhail Fedonkin |date=March 31, 2003 |title=The origin of the Metazoa in the light of the Proterozoic fossil record |url=http://www.vend.paleo.ru/pub/Fedonkin_2003.pdf |url-status=dead |journal=Paleontological Research |volume=7 |issue=1 |pages=9–41 |doi=10.2517/prpsj.7.9 |s2cid=55178329 |issn=1342-8144 |archive-url=https://web.archive.org/web/20090226122725/http://www.vend.paleo.ru/pub/Fedonkin_2003.pdf |archive-date=2009-02-26 |access-date=2008-09-02}}</ref> and while they may have been present earlier, their diversification accelerated when they started using oxygen in their [[metabolism]]. Later, around 1.7 billion years ago, [[multicellular organism]]s began to appear, with [[Cellular differentiation|differentiated cell]]s performing specialised functions.<ref name="bonner1999">{{cite journal |last=Bonner |first=John Tyler |author-link=John Tyler Bonner |date=January 7, 1998 |title=The origins of multicellularity |journal=[[Integrative Biology]] |volume=1 |issue=1 |pages=27–36 |doi=10.1002/(SICI)1520-6602(1998)1:1<27::AID-INBI4>3.0.CO;2-6 |issn=1757-9694}}</ref>

Algae-like multicellular land plants are dated back even to about 1 billion years ago,<ref>{{cite journal |last1=Strother |first1=Paul K. |last2=Battison |first2=Leila |last3=Brasier |first3=Martin D. |author3-link=Martin Brasier |last4=Wellman |first4=Charles H. |display-authors=3 |date=May 26, 2011 |title=Earth's earliest non-marine eukaryotes |journal=[[Nature (journal)|Nature]] |volume=473 |issue=7348 |pages=505–509 |bibcode=2011Natur.473..505S |doi=10.1038/nature09943 |s2cid=4418860 |issn=0028-0836 |pmid=21490597}}</ref> although evidence suggests that [[microorganism]]s formed the earliest [[terrestrial ecosystem]]s, at least 2.7 billion years ago.<ref>{{cite journal |last=Beraldi-Campesi |first=Hugo |date=February 23, 2013 |title=Early life on land and the first terrestrial ecosystems |journal=Ecological Processes |volume=2 |issue=1 |pages=1–17 |doi=10.1186/2192-1709-2-1 |doi-access=free |issn=2192-1709 }}</ref> Microorganisms are thought to have paved the way for the inception of land plants in the [[Ordovician]] period. Land plants were so successful that they are thought to have contributed to the [[Late Devonian extinction|Late Devonian extinction event]].<ref name="AlgeoScheckler1998">{{cite journal |last1=Algeo |first1=Thomas J. |last2=Scheckler |first2=Stephen E. |date=January 29, 1998 |title=Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events |journal=[[Philosophical Transactions of the Royal Society B]] |volume=353 |issue=1365 |pages=113–130 |doi=10.1098/rstb.1998.0195 |issn=0962-8436 |pmc=1692181}}</ref>

[[Ediacara biota]] appear during the [[Ediacaran]] period,<ref>{{cite journal |last1=Jun-Yuan |first1=Chen |last2=Oliveri |first2=Paola |last3=Chia-Wei |first3=Li |author4=Gui-Qing Zhou |author5=Feng Gao |last6=Hagadorn |first6=James W. |last7=Peterson |first7=Kevin J. |last8=Davidson |first8=Eric H. |display-authors=3 |date=April 25, 2000 |title=Precambrian animal diversity: Putative phosphatized embryos from the Doushantuo Formation of China |journal=[[Proceedings of the National Academy of Sciences of the United States of America|Proc. Natl. Acad. Sci. U.S.A.]] |volume=97 |issue=9 |pages=4457–4462 |bibcode=2000PNAS...97.4457C |doi=10.1073/pnas.97.9.4457 |issn=0027-8424 |pmid=10781044 |pmc=18256|doi-access=free }}</ref> while [[vertebrate]]s, along with most other modern [[phylum|phyla]] originated about 525 million years ago during the [[Cambrian explosion]].<ref name="D-G.Shu et al. 1999">{{cite journal |last1=D-G. |first1=Shu |last2=H-L. |first2=Luo |last3=Conway Morris |first3=Simon |author3-link=Simon Conway Morris |author4=X-L. Zhang |author5=S-X. Hu |author6=L. Chen |author7=J. Han |author8=M. Zhu |author9=Y. Li |author10=L-Z. Chen |display-authors=3 |date=November 4, 1999 |title=Lower Cambrian vertebrates from south China |url=http://www.bios.niu.edu/davis/bios458/Shu1.pdf |url-status=dead |journal=[[Nature (journal)|Nature]] |volume=402 |pages=42–46 |doi=10.1038/46965 |issue=6757 |bibcode=1999Natur.402...42S |s2cid=4402854 |issn=0028-0836 |archive-url=https://web.archive.org/web/20090226122732/http://www.bios.niu.edu/davis/bios458/Shu1.pdf |archive-date=2009-02-26 |access-date=2015-01-22}}</ref> During the Permian period, [[synapsid]]s, including the ancestors of [[mammal]]s, dominated the land,<ref>{{cite web |url=http://www.csupomona.edu/~dfhoyt/classes/zoo138/SYNAPSID.HTML |title=Synapsid Reptiles |last=Hoyt |first=Donald F. |date=February 17, 1997 |website=ZOO 138 Vertebrate Zoology |publisher=[[California State Polytechnic University, Pomona]] |location=Pomona, Calif. |type=Lecture |archive-url=https://web.archive.org/web/20090520072737/http://www.csupomona.edu/~dfhoyt/classes/zoo138/SYNAPSID.HTML |archive-date=2009-05-20 |access-date=2015-01-22}}</ref> but most of this group became extinct in the [[Permian–Triassic extinction event]] 252 million years ago.<ref>{{cite web |url=https://science.nasa.gov/science-news/science-at-nasa/2002/28jan_extinction/ |url-status=live |title=The Great Dying |last=Barry |first=Patrick L. |date=January 28, 2002 |editor-last=Phillips |editor-first=Tony |work=Science@NASA |publisher=[[Marshall Space Flight Center]] |archive-url=https://web.archive.org/web/20100410015208/https://science.nasa.gov/science-news/science-at-nasa/2002/28jan_extinction/ |archive-date=2010-04-10 |access-date=2015-01-22}}</ref> During the recovery from this catastrophe, [[archosaur]]s became the most abundant land vertebrates;<ref name="TannerLucas2004">{{cite journal |last1=Tanner |first1=Lawrence H. |last2=Lucas |first2=Spencer G. |author2-link=Spencer G. Lucas |last3=Chapman |first3=Mary G. |date=March 2004 |title=Assessing the record and causes of Late Triassic extinctions |url=http://nmnaturalhistory.org/pdf_files/TJB.pdf |journal=[[Earth-Science Reviews]] |volume=65 |issue=1–2 |pages=103–139 |bibcode=2004ESRv...65..103T |doi=10.1016/S0012-8252(03)00082-5 |archive-url=https://web.archive.org/web/20071025225841/http://nmnaturalhistory.org/pdf_files/TJB.pdf |archive-date=2007-10-25 |access-date=2007-10-22}}</ref> one archosaur group, the [[dinosaur]]s, dominated the Jurassic and Cretaceous periods.<ref>{{harvnb|Benton|1997}}</ref> After the [[Cretaceous–Paleogene extinction event]] 66 million years ago killed off the non-avian dinosaurs,<ref>{{cite journal |last1=Fastovsky |first1=David E. |last2=Sheehan |first2=Peter M. |date=March 2005 |title=The Extinction of the Dinosaurs in North America |url=https://www.geosociety.org/gsatoday/archive/15/3/pdf/i1052-5173-15-3-4.pdf |url-status=live |journal=[[Geological Society of America|GSA Today]] |volume=15 |issue=3 |pages=4–10 |doi=10.1130/1052-5173(2005)015<4:TEOTDI>2.0.CO;2 |issn=1052-5173 |archive-url=https://web.archive.org/web/20190322190338/https://www.geosociety.org/gsatoday/archive/15/3/pdf/i1052-5173-15-3-4.pdf |archive-date=2019-03-22 |access-date=2015-01-23}}</ref> mammals [[Adaptive radiation|increased rapidly in size and diversity]].<ref>{{cite news |last=Roach |first=John |date=June 20, 2007 |title=Dinosaur Extinction Spurred Rise of Modern Mammals |url=https://news.nationalgeographic.com/news/2007/06/070620-mammals-dinos.html |url-status=dead |work=National Geographic News |location=Washington, D.C. |publisher=[[National Geographic Society]] |archive-url=https://web.archive.org/web/20080511161825/https://news.nationalgeographic.com/news/2007/06/070620-mammals-dinos.html |archive-date=2008-05-11 |access-date=2020-02-21}}
*{{cite journal |last1=Wible |first1=John R. |last2=Rougier |first2=Guillermo W. |last3=Novacek |first3=Michael J. |last4=Asher |first4=Robert J. |display-authors=3 |date=June 21, 2007 |title=Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary |journal=[[Nature (journal)|Nature]] |volume=447 |issue=7147 |pages=1003–1006 |bibcode=2007Natur.447.1003W |doi=10.1038/nature05854 |issn=0028-0836 |pmid=17581585|s2cid=4334424 }}</ref> Such [[Extinction event|mass extinction]]s may have accelerated evolution by providing opportunities for new groups of organisms to diversify.<ref name="Van Valkenburgh 1999 463–493">{{cite journal |last=Van Valkenburgh |first=Blaire |author-link=Blaire Van Valkenburgh |date=May 1, 1999 |title=Major Patterns in the History of Carnivorous Mammals |url=https://zenodo.org/record/890156 |journal=[[Annual Review of Earth and Planetary Sciences]] |volume=27 |pages=463–493 |bibcode=1999AREPS..27..463V |doi=10.1146/annurev.earth.27.1.463 |issn=1545-4495 |access-date=May 15, 2021 |archive-date=February 29, 2020 |archive-url=https://web.archive.org/web/20200229201201/https://zenodo.org/record/890156 |url-status=live }}</ref>

==Diversity==
===Bacteria and Archaea===
{{Further | Microbiology}}
[[File:Gemmatimonas aurantiaca.jpg|thumb| left | [[Bacteria]] – ''[[Gemmatimonas aurantiaca]]'' (-=1 Micrometer)]]
[[Bacteria]] are a type of [[Cell (biology)|cell]] that constitute a large [[domain (biology)|domain]] of [[prokaryotic]] [[microorganism]]s. Typically a few [[micrometre|micrometer]]s in length, bacteria have a [[Bacterial cell structure#Cell morphology|number of shapes]], ranging from [[coccus|spheres]] to [[bacillus (shape)|rods]] and [[spiral bacteria|spirals]]. Bacteria were among the first life forms to appear on [[Earth]], and are present in most of its [[habitat]]s. Bacteria inhabit soil, water, [[Hot spring|acidic hot springs]], [[radioactive waste]],<ref>{{cite journal | vauthors = Fredrickson JK, Zachara JM, Balkwill DL, Kennedy D, Li SM, Kostandarithes HM, Daly MJ, Romine MF, Brockman FJ | title = Geomicrobiology of high-level nuclear waste-contaminated vadose sediments at the Hanford site, Washington state | journal = Applied and Environmental Microbiology | volume = 70 | issue = 7 | pages = 4230–41 | date = July 2004 | pmid = 15240306 | pmc = 444790 | doi = 10.1128/AEM.70.7.4230-4241.2004 | bibcode = 2004ApEnM..70.4230F }}</ref> and the [[deep biosphere]] of the [[earth's crust]]. Bacteria also live in [[symbiotic]] and [[parasitic]] relationships with plants and animals. Most bacteria have not been characterised, and only about 27 percent of the [[bacterial phyla]] have species that can be [[microbiological culture|grown]] in the laboratory.<ref name=Dudek>{{cite journal | vauthors = Dudek NK, Sun CL, Burstein D | title = Novel Microbial Diversity and Functional Potential in the Marine Mammal Oral Microbiome | journal = Current Biology | volume = 27 | issue = 24 | pages = 3752–3762 | year = 2017 | doi = 10.1016/j.cub.2017.10.040 | pmid = 29153320 | s2cid = 43864355 | url = https://escholarship.org/content/qt1w91s3vq/qt1w91s3vq.pdf?t=pghuwe | doi-access = free | access-date = 2021-05-14 | archive-date = 2021-03-08 | archive-url = https://web.archive.org/web/20210308145807/https://escholarship.org/content/qt1w91s3vq/qt1w91s3vq.pdf?t=pghuwe | url-status = live }}</ref>

[[File:Halobacteria.jpg|thumb| left | [[Archaea]] – [[Halobacteria]]]]
[[Archaea]] constitute the other domain of prokaryotic cells and were initially [[Taxonomy (biology)|classified]] as [[bacteria]], receiving the name archaebacteria (in the Archaebacteria [[Kingdom (biology)|kingdom]]), a term that has fallen out of use.<ref>{{cite journal | vauthors = Pace NR | title = Time for a change | journal = Nature | volume = 441 | issue = 7091 | pages = 289 | date = May 2006 | pmid = 16710401 | doi = 10.1038/441289a | bibcode = 2006Natur.441..289P | s2cid = 4431143 }}</ref> Archaeal cells have unique properties separating them from the other [[three-domain system|two domains]], [[Bacteria]] and [[Eukaryote|Eukaryota]]. Archaea are further divided into multiple recognized [[phylum|phyla]]. Archaea and bacteria are generally similar in size and shape, although a few archaea have very different shapes, such as the flat and square cells of ''[[Haloquadratum walsbyi]]''.<ref>{{cite journal | vauthors = Stoeckenius W | title = Walsby's square bacterium: fine structure of an orthogonal procaryote | journal = Journal of Bacteriology | volume = 148 | issue = 1 | pages = 352–60 | date = October 1981 | pmid = 7287626 | pmc = 216199 | doi = 10.1128/JB.148.1.352-360.1981 }}</ref> Despite this [[Morphology (biology)|morphological]] similarity to bacteria, archaea possess [[gene]]s and several [[metabolic pathway]]s that are more closely related to those of eukaryotes, notably for the [[enzyme]]s involved in [[transcription (genetics)|transcription]] and [[translation (biology)|translation]]. Other aspects of archaeal biochemistry are unique, such as their reliance on [[ether lipid]]s in their [[cell membrane]]s,<ref>{{cite web | title = Archaea Basic Biology | date = March 2018 | url = https://basicbiology.net/micro/microorganisms/archaea | access-date = 2021-05-14 | archive-date = 2021-04-28 | archive-url = https://web.archive.org/web/20210428221114/https://basicbiology.net/micro/microorganisms/archaea | url-status = live }}</ref> including [[archaeol]]s. Archaea use more energy sources than eukaryotes: these range from [[organic compounds]], such as sugars, to [[ammonia]], [[ion|metal ions]] or even [[hydrogen|hydrogen gas]]. [[Halophile|Salt-tolerant]] archaea (the [[Haloarchaea]]) use sunlight as an energy source, and other species of archaea [[carbon fixation|fix carbon]], but unlike plants and [[cyanobacteria]], no known species of archaea does both. Archaea [[asexual reproduction|reproduce asexually]] by [[binary fission]], [[Fragmentation (reproduction)|fragmentation]], or [[budding]]; unlike bacteria, no known species of Archaea form [[endospore]]s.

The first observed archaea were [[extremophile]]s, living in extreme environments, such as [[hot spring]]s and [[salt lake]]s with no other organisms. Improved molecular detection tools led to the discovery of archaea in almost every [[habitat]], including soil, oceans, and [[marshland]]s. Archaea are particularly numerous in the oceans, and the archaea in [[plankton]] may be one of the most abundant groups of organisms on the planet.

Archaea are a major part of [[Life|Earth's life]]. They are part of the [[microbiota]] of all organisms. In the [[human microbiome]], they are important in the [[Colon (anatomy)|gut]], mouth, and on the skin.<ref name=Bang2015>{{cite journal | vauthors = Bang C, Schmitz RA | title = Archaea associated with human surfaces: not to be underestimated | journal = FEMS Microbiology Reviews | volume = 39 | issue = 5 | pages = 631–48 | date = September 2015 | pmid = 25907112 | doi = 10.1093/femsre/fuv010 | doi-access = free }}</ref> Their morphological, metabolic, and geographical diversity permits them to play multiple ecological roles: carbon fixation; nitrogen cycling; organic compound turnover; and maintaining microbial symbiotic and [[Syntrophy|syntrophic]] communities, for example.<ref>{{cite journal | vauthors = Moissl-Eichinger C, Pausan M, Taffner J, Berg G, Bang C, Schmitz RA | title = Archaea Are Interactive Components of Complex Microbiomes | journal = Trends in Microbiology | volume = 26 | issue = 1 | pages = 70–85 | date = January 2018 | pmid = 28826642 | doi = 10.1016/j.tim.2017.07.004 }}</ref>

===Protists===
{{Further | Protistology}}
[[File:Protist collage 2.jpg|thumb|right|Diversity of protists]]
Eukaryotes are hypothesized to have split from archaea, which was followed by their [[Endosymbiont|endosymbioses]] with bacteria (or [[symbiogenesis]]) that gave rise to mitochondria and chloroplasts, both of which are now part of modern-day eukaryotic cells.<ref name="hillisetal2014t"/> The major lineages of eukaryotes diversified in the [[Precambrian]] about 1.5 billion years ago and can be classified into eight major [[clade]]s: [[alveolate]]s, [[Excavata|excavates]], [[stramenopile]]s, [[plant]]s, [[rhizaria]]ns, [[amoebozoa]]ns, [[fungus|fungi]], and [[animal]]s.<ref name="hillisetal2014t"/> Five of these clades are collectively known as [[protist]]s, which are mostly microscopic [[eukaryotic]] [[organism]]s that are not plants, fungi, or animals.<ref name="hillisetal2014t">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = The origin and diversification of eukaryotes | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 402–419 | isbn = 978-1464175121}}</ref> While it is likely that protists share a [[Common descent|common ancestor]] (the [[last eukaryotic common ancestor]]),<ref name="O'Malley Leger Wideman Ruiz-Trillo pp. 338–344">{{cite journal | last1=O'Malley | first1=Maureen A. | last2=Leger | first2=Michelle M. | last3=Wideman | first3=Jeremy G. | last4=Ruiz-Trillo | first4=Iñaki | s2cid=67790751 | title=Concepts of the last eukaryotic common ancestor | journal=Nature Ecology & Evolution | publisher=Springer Science and Business Media LLC | volume=3 | issue=3 | date=2019-02-18 | issn=2397-334X | doi=10.1038/s41559-019-0796-3 | pages=338–344| pmid=30778187 | hdl=10261/201794 | hdl-access=free }}</ref> protists by themselves do not constitute a separate clade as some protists may be more closely related to plants, fungi, or animals than they are to other protists. Like groupings such as [[algae]], [[invertebrate]]s, or [[protozoa]]ns, the protist grouping is not a formal taxonomic group but is used for convenience.<ref name="hillisetal2014t" /><ref name="Taylor pp. 1707–1714">{{cite journal | last=Taylor | first=F. J. R. 'M. | title=The collapse of the two-kingdom system, the rise of protistology and the founding of the International Society for Evolutionary Protistology (ISEP) | journal=International Journal of Systematic and Evolutionary Microbiology | publisher=Microbiology Society | volume=53 | issue=6 | date=2003-11-01 | issn=1466-5026 | doi=10.1099/ijs.0.02587-0 | pages=1707–1714| pmid=14657097 | doi-access=free }}</ref> Most protists are unicellular, which are also known as microbial eukaryotes.<ref name="hillisetal2014t"/>

The alveolates are mostly photosynthetic unicellular protists that possess sacs called alveoli (hence their name alveolates) that are located beneath their cell membrane, providing support for the cell surface.<ref name="hillisetal2014t"/> Alveolates comprise several groups such as [[dinoflagellate]]s, [[apicomplexa]]ns, and [[ciliate]]s. Dinoflagellates are photosynthetic and can be found in the ocean where they play a role as [[Autotroph|primary producer]]s of organic matter.<ref name="hillisetal2014t"/> Apicomplexans are parasitic alveolates that possess an apical complex, which is a group of organelles located in the apical end of the cell.<ref name="hillisetal2014t"/> This complex allows apicomplexans to invade their hosts' tissues. Ciliates are alveolates that possess numerous hair-like structure called cilia. A defining characteristic of ciliates is the presence of two types of nuclei in each ciliate cell. A commonly studied ciliate is the ''[[paramecium]]''.<ref name="hillisetal2014t"/>

The excavates are groups of protists that began to diversify approximately 1.5 billion years ago shortly after the origin of the eukaryotes.<ref name="hillisetal2014t"/> Some excavates do not possess mitochondria, which are thought to have been lost over the course of evolution as these protists still possess nuclear genes that are associated with mitochondria.<ref name="hillisetal2014t"/> The excavates comprise several groups such as [[diplomonad]]s, [[parabasalid]]s, [[Percolozoa|heterolobosean]]s, [[euglenid]]s, and [[Kinetoplastida|kinetoplastid]]s.<ref name="hillisetal2014t"/>

Stramenopiles, most of which can be characterized by the presence of tubular hairs on the longer of their two [[flagella]], include [[diatom]]s and [[brown algae]].<ref name="hillisetal2014t"/> Diatoms are primary producers and contribute about one-fifth of all photosynthetic [[carbon fixation]], making them a major component of [[phytoplankton]].<ref name="hillisetal2014t"/>

Rhizarians are mostly unicellular and aquatic protists that typically contain long, thin [[Pseudopodia|pseudopod]]s.<ref name="hillisetal2014t"/> The rhizarians comprise three main groups: [[cercozoan]]s, [[foraminifera]]ns, and [[radiolarian]]s.<ref name="hillisetal2014t"/>

Amoebozoans are protists with a body form characterized by the presence lobe-shaped pseudopods, which help them to move.<ref name="hillisetal2014t"/> They include groups such as [[lobosa|lobosean]]s and [[slime mold]]s (e.g., [[Myxogastria|plasmodial slime mold]] and cellular slime molds).<ref name="hillisetal2014t"/>

===Plant diversity===
{{Further|Botany}}
[[File:Diversity of plants (Streptophyta) version 2.png|thumb|right|Diversity of plants]]
[[Plant]]s are mainly multicellular organisms, predominantly [[photosynthetic]] [[eukaryote]]s of the [[Kingdom (biology)|kingdom]] Plantae, which would exclude [[fungi]] and some [[algae]]. A shared derived trait (or [[Apomorphy and synapomorphy|synapomorphy]]) of Plantae is the primary endosymbiosis of a [[cyanobacterium]] into an early eukaryote about one billion years ago, which gave rise to chloroplasts.<ref name="hillisetal2014u">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = The evolution of plants | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 420–449 | isbn = 978-1464175121}}</ref> The first several clades that emerged following primary endosymbiosis were aquatic and most of the aquatic [[photosynthesis|photosynthetic]] [[eukaryotic]] [[organism]]s are collectively described as algae, which is a term of convenience as not all algae are closely related.<ref name="hillisetal2014u"/> Algae comprise several distinct clades such as [[glaucophyte]]s, which are microscopic freshwater algae that may have resembled in form to the early unicellular ancestor of Plantae.<ref name="hillisetal2014u"/> Unlike glaucophytes, the other algal clades such as [[Red algae|red]] and [[green algae]] are multicellular. Green algae comprise three major clades: [[chlorophyte]]s, [[Coleochaetophyceae|coleochaetophyte]]s, and [[stonewort]]s.<ref name="hillisetal2014u"/>

Land plants ([[embryophyte]]s) first appeared in terrestrial environments approximately 450 to 500 million years ago.<ref name="hillisetal2014u"/> A synapomorphy of land plants is an embryo that develops under the protection of tissues of its parent plant.<ref name="hillisetal2014u"/> Land plants comprise ten major clades, seven of which constitute a single clade known as [[vascular plant]]s (or tracheophytes) as they all have [[tracheid]]s, which are fluid-conducting cells, and a well-developed system that transports materials throughout their bodies.<ref name="hillisetal2014u"/> In contrast, the other three clades are [[Non-vascular plant|nonvascular plant]]s as they do not have tracheids.<ref name="hillisetal2014u"/> They also do not constitute a single clade.<ref name="hillisetal2014u"/>

Nonvascular plants include [[liverwort]]s, [[moss]]es, and [[hornwort]]s. They tend to be found in areas where water is readily available.<ref name="hillisetal2014u"/> Most live on soil or even on vascular plants themselves. Some can grow on bare rock, tree trunks that are dead or have fallen, and even buildings.<ref name="hillisetal2014u"/> Most nonvascular plants are terrestrial, with a few living in freshwater environments and none living in the oceans.<ref name="hillisetal2014u"/>

The seven clades (or [[Division (biology)|divisions]]) that make up vascular plants include [[Equisetum|horsetails]] and [[fern]]s, which together can be grouped as a single clade called monilophytes.<ref name="hillisetal2014u"/> Seed plants (or [[spermatophyte]]) comprise the other five divisions, four of which are grouped as [[gymnosperms]] and one is angiosperms. Gymnosperms includes [[Pinophyta|conifers]], [[cycad]]s, ''[[Ginkgo]]'', and [[gnetophyta|gnetophyte]]s. Gymnosperm seeds develop either on the surface of scales or leaves, which are often modified to form [[Conifer cone|cones]], or solitary as in [[Taxus|yew]], ''[[Torreya]]'', ''Ginkgo''.<ref name="TPL">{{cite web|url=http://www.theplantlist.org/browse/G/|title=Gymnosperms on The Plant List|publisher=Theplantlist.org|access-date=2013-07-24|archive-date=2013-08-24|archive-url=https://web.archive.org/web/20130824024109/http://www.theplantlist.org/browse/G/|url-status=live}}</ref> Angiosperms are the most diverse group of [[Embryophyte|land plants]], with 64 [[Order (biology)|orders]], 416 [[Family (biology)|families]], approximately 13,000 known [[Genus|genera]] and 300,000 known [[species]].<ref name="Christenhusz-Byng2016">{{cite journal | author1 = Christenhusz, M. J. M. | author2 = Byng, J. W. | name-list-style = amp | year = 2016 | title = The number of known plants species in the world and its annual increase | journal = Phytotaxa | volume = 261 | pages = 201–217 | url = http://biotaxa.org/Phytotaxa/article/download/phytotaxa.261.3.1/20598 | doi = 10.11646/phytotaxa.261.3.1 | issue = 3 | doi-access = free | access-date = 2021-05-15 | archive-date = 2016-07-29 | archive-url = https://web.archive.org/web/20160729085754/http://biotaxa.org/Phytotaxa/article/download/phytotaxa.261.3.1/20598 | url-status = live }}</ref> Like [[gymnosperm]]s, angiosperms are [[Spermatophyte|seed-producing plants]]. They are distinguished from gymnosperms by having characteristics such as [[flower]]s, [[endosperm]] within their [[seed]]s, and production of [[fruit]]s that contain the seeds.

===Fungi===
{{Further | Mycology}}
[[File:Fungi_collage.jpg|thumb | Diversity of fungi. Clockwise from top left: ''[[Amanita muscaria]]'', a basidiomycete; ''[[Sarcoscypha coccinea]]'', an ascomycete; bread covered in [[Mold (fungus)|mold]]; [[Chytridiomycota|chytrid]]; ''[[Aspergillus]]'' [[conidiophore]].]]
[[Fungus|Fungi]] are [[Eukaryote|eukaryotic]] organisms that digest foods outside of their bodies.<ref name="hillisetal2014v">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = The evolution and diversity of fungi | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 451–468 | isbn = 978-1464175121}}</ref> They do so through a process called absorptive heterotrophy whereby they would first secrete digestive enzymes that break down large food molecules before absorbing them through their cell membranes. Many fungi are also [[saprobe]]s as they are able to take in nutrients from dead organic matter and are hence, the principal [[decomposer]]s in ecological systems.<ref name="hillisetal2014v"/> Some fungi are parasites by absorbing nutrients from living hosts while others are mutualists.<ref name="hillisetal2014v"/> Fungi, along with two other lineages, [[choanoflagellate]]s and animals, can be grouped as [[opisthokont]]s. A synapomorphy that distinguishes fungi from other two opisthokonts is the presence of [[chitin]] in their [[cell wall]]s.<ref name="hillisetal2014v"/>

Most fungi are multicellular but some are unicellular such as [[yeast]]s, which live in liquid or moist environments and are able to absorb nutrients directly into their cell surfaces.<ref name="hillisetal2014v"/> Multicellular fungi, on the other hand, have a body called [[mycelium]], which is composed of a mass of individual tubular filaments called [[hypha]]e that allows for nutrient absorption to occur.<ref name="hillisetal2014v"/>

Fungi can be divided into six major groups based on their life cycles: [[microsporidia]], [[Chytridiomycota|chytrids]], zygospore fungi ([[Zygomycota]]), arbuscular mycorrhizal fungi ([[Glomeromycota]]), sac fungi ([[Ascomycota]]), and [[Clavarioid fungi|club fungi]] ([[Basidiomycota]]).<ref name="hillisetal2014v"/>  Fungi are classified by the particular processes of [[sexual reproduction]] they use.  The usual cellular products of [[meiosis]] during sexual reproduction are spores that are adapted to survive inclement times and to spread.  A principal adaptive benefit of meiosis during sexual reproduction in the [[Ascomycota]] and [[Basidiomycota]] was proposed to be the repair of DNA damage through meiotic recombination.<ref>{{Cite journal |last1=Wallen |first1=R. M. |last2=Perlin |first2=Michael H. |date=2018-03-21 |title=An Overview of the Function and Maintenance of Sexual Reproduction in Dikaryotic Fungi |journal=Frontiers in Microbiology |volume=9 |pages=503 |doi=10.3389/fmicb.2018.00503 |pmc=5871698 |pmid=29619017 |doi-access=free}}</ref>

The fungus kingdom encompasses an enormous diversity of [[taxon|taxa]] with varied ecologies, [[biological life cycle|life cycle]] strategies, and [[morphology (biology)|morphologies]] ranging from unicellular aquatic chytrids to large mushrooms. However, little is known of the true [[biodiversity]] of Kingdom Fungi, which has been estimated at 2.2&nbsp;million to 3.8&nbsp;million species.<ref name=Lucking2017>{{Cite book | vauthors=Hawksworth DL, Lücking R | title=The Fungal Kingdom | chapter=Fungal Diversity Revisited: 2.2 to 3.8 Million Species | journal=Microbiology Spectrum | volume=5 | issue=4 | pages=79–95 | date=July 2017 | pmid=28752818 | doi=10.1128/microbiolspec.FUNK-0052-2016 | isbn=978-1-55581-957-6 }}</ref> Of these, only about 148,000 have been described,<ref name="Cheek et al. 2020">{{cite journal |display-authors=6 |last1=Cheek |first1=Martin |last2=Nic Lughadha |first2=Eimear |last3=Kirk |first3=Paul |last4=Lindon |first4=Heather |last5=Carretero |first5=Julia |last6=Looney |first6=Brian |last7=Douglas |first7=Brian |last8=Haelewaters |first8=Danny |last9=Gaya |first9=Ester |last10=Llewellyn |first10=Theo |last11=Ainsworth |first11=A. Martyn |last12=Gafforov |first12=Yusufjon |last13=Hyde |first13=Kevin |last14=Crous |first14=Pedro |last15=Hughes |first15=Mark |last16=Walker |first16=Barnaby E. |last17=Campostrini Forzza |first17=Rafaela |last18=Wong |first18=Khoon Meng |last19=Niskanen |first19=Tuula |title=New scientific discoveries: Plants and fungi |journal=Plants, People, Planet |volume=2 |issue=5 |year=2020 |pages=371–388 |doi=10.1002/ppp3.10148 |doi-access=free}}</ref> with over 8,000 species known to be detrimental to plants and at least 300 that can be pathogenic to humans.<ref>{{cite journal|title=Stop neglecting fungi|date=25 July 2017|journal=Nature Microbiology|volume=2|issue=8|pages=17120|doi=10.1038/nmicrobiol.2017.120|pmid=28741610|doi-access=free}}</ref>

===Animal diversity===
{{Further | Zoology}}
<imagemap>File:Animal diversity.png | right |thumb| Diversity of animals. From top to bottom, first column: [[Echinoderm]], [[cnidaria]], [[bivalve]], [[tardigrade]], [[crustacean]], and [[arachnid]]. Second column: [[Sponge]], [[insect]], [[mammal]], [[bryozoa]], [[acanthocephala]], and  [[flatworm]]. Third column: [[Cephalopod]], [[annelid]], [[tunicate]], [[fish]], [[bird]], and  [[phoronida]].

rect 0 0 118 86 [[Echinoderm]]
rect 0 86 118 172 [[Cnidaria]]
rect 0 172 118 258 [[Bivalve]]
rect 0 258 118 344 [[Tardigrade]]
rect 0 344 118 430 [[Crustacean]]
rect 0 430 118 516 [[Arachnid]]

rect 118 0 236 86 [[Sponge]]
rect 118 86 236 172 [[Insect]]
rect 118 172 236 258 [[Mammal]]
rect 118 258 236 344 [[Bryozoa]]
rect 118 344 236 430 [[Acanthocephala]]
rect 118 430 236 620 [[Flatworm]]

rect 236 0 354 86 [[Cephalopod]]
rect 236 86 354 172 [[Annelid]]
rect 236 172 354 258 [[Tunicate]]
rect 236 258 354 344 [[Fish]]
rect 236 344 354 430 [[Bird]]
rect 236 430 354 620 [[Phoronida]]
</imagemap>
[[Animals]] are multicellular eukaryotic organisms that form the kingdom Animalia. With few exceptions, animals [[Heterotroph|consume organic material]], [[Cellular respiration#Aerobic respiration|breathe oxygen]], are [[Motility|able to move]], can [[Sexual reproduction|reproduce sexually]], and grow from a hollow sphere of [[Cell (biology)|cells]], the [[blastula]], during [[Embryogenesis|embryonic development]]. Over 1.5 million [[Extant taxon|living]] animal [[species]] have been [[Species description|described]]—of which around 1 million are [[Insecta|insects]]—but it has been estimated there are over 7 million animal species in total. They have [[Ecology|complex interactions]] with each other and their environments, forming intricate [[food web]]s.

Animals can be distinguished into two groups based on their developmental characteristics.<ref name="hillisetal2014w">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = Animal origins and diversity | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 469–519 | isbn = 978-1464175121}}</ref> For instance, embryos of [[Eumetazoa|diploblastic]] animals such as ctenophores, placeozoans, and cnidarians have two cell layers ([[ectoderm]] and [[endoderm]]) whereas the embryos of [[Triploblasty|triploblastic]] animals have three tissue layers (ectoderm, [[mesoderm]], and endoderm), which is a synapomorphy of these animals.<ref name="hillisetal2014w"/> Triploblastic animals can be further divided into two major clades based on based on the pattern of [[gastrulation]], whereby a cavity called a [[blastopore]] is formed from the indentation of a [[blastula]]. In [[protostome]]s, the blastopore gives rise to the mouth, which is then followed by the formation of the anus.<ref name="hillisetal2014w"/> In [[deuterostome]]s, the blastopore gives rise to the anus, followed by the formation of the mouth.<ref name="hillisetal2014w"/>

Animals can also be differentiated based on their [[body plan]], specifically with respect to four key features: [[Symmetry in biology|symmetry]], [[body cavity]], [[Segmentation (biology)|segmentation]], and [[appendage]]s.<ref name="hillisetal2014w"/> The bodies of most animals are symmetrical, with symmetry being either [[Symmetry in biology#Radial symmetry|radial]] or [[Symmetry in biology#Bilateral symmetry|bilateral]].<ref name="hillisetal2014w"/> Triploblastic animals can be divided into three types based on their body cavity: [[acoelomate]], [[pseudocoelomate]], and [[coelomate]].<ref name="hillisetal2014w"/> Segmentation can be observed in the bodies of many animals, which allows for specialization of different parts of the body as well as allowing the animal to change the shape of its body to control its movements.<ref name="hillisetal2014w"/> Finally, animals can be distinguished based on the type and location of their appendages such as [[Antenna (biology)|antennae]] for sensing the environment or claws for capturing prey.<ref name="hillisetal2014w"/>

[[Sponge]]s, the members of the [[phylum]] Porifera, are a basal [[Metazoa]] (animal) [[clade]] as a sister of the diploblasts.<ref name = "Feuda_2017">{{cite journal | vauthors = Feuda R, Dohrmann M, Pett W, Philippe H, Rota-Stabelli O, Lartillot N, Wörheide G, Pisani D | display-authors = 6 | title = Improved Modeling of Compositional Heterogeneity Supports Sponges as Sister to All Other Animals | journal = Current Biology | volume = 27 | issue = 24 | pages = 3864–3870.e4 | date = December 2017 | pmid = 29199080 | doi = 10.1016/j.cub.2017.11.008 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Pisani D, Pett W, Dohrmann M, Feuda R, Rota-Stabelli O, Philippe H, Lartillot N, Wörheide G | display-authors = 6 | title = Genomic data do not support comb jellies as the sister group to all other animals | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 112 | issue = 50 | pages = 15402–7 | date = December 2015 | pmid = 26621703 | pmc = 4687580 | doi = 10.1073/pnas.1518127112 | bibcode = 2015PNAS..11215402P | doi-access = free }}</ref><ref name = "Simion_2017">{{cite journal | vauthors = Simion P, Philippe H, Baurain D, Jager M, Richter DJ, Di Franco A, Roure B, Satoh N, Quéinnec É, Ereskovsky A, Lapébie P, Corre E, Delsuc F, King N, Wörheide G, Manuel M | display-authors = 6 | title = A Large and Consistent Phylogenomic Dataset Supports Sponges as the Sister Group to All Other Animals | language = en | journal = Current Biology | volume = 27 | issue = 7 | pages = 958–967 | date = April 2017 | pmid = 28318975 | doi = 10.1016/j.cub.2017.02.031 | url = https://hal.archives-ouvertes.fr/hal-01681528/file/Simion_etal2017_CurrBiol_proofs.pdf | type = <!-- Submitted manuscript --> | doi-access = free | access-date = 2021-05-14 | archive-date = 2020-04-25 | archive-url = https://web.archive.org/web/20200425191351/https://hal.archives-ouvertes.fr/hal-01681528/file/Simion_etal2017_CurrBiol_proofs.pdf | url-status = live }}</ref><ref>{{Cite journal|last=Giribet|first=Gonzalo | name-list-style = vanc |date=1 October 2016|title=Genomics and the animal tree of life: conflicts and future prospects|journal=Zoologica Scripta |volume=45|pages=14–21|doi=10.1111/zsc.12215|issn=1463-6409|doi-access=free}}</ref><ref>{{cite bioRxiv| vauthors = Laumer CE, Gruber-Vodicka H, Hadfield MG, Pearse VB, Riesgo A, Marioni JC, Giribet G |date=2017-10-11|title=Placozoans are eumetazoans related to Cnidaria|biorxiv=10.1101/200972}}</ref> They are [[Multicellular organism|multicellular]] organisms that have bodies full of pores and channels allowing water to circulate through them, consisting of jelly-like [[mesohyl]] sandwiched between two thin layers of [[Cell (biology)|cell]]s.

The majority (~97%) of animal species are [[invertebrates]],<ref>{{cite journal|first=Robert M.|last=May|date=16 September 1988|title=How Many Species Are There on Earth?|journal=Science|volume=241|issue=4872|pages=1441–1449|url=http://www.ciesin.org/docs/002-253/002-253.html|jstor=1702670|pmid=17790039|doi=10.1126/science.241.4872.1441|bibcode=1988Sci...241.1441M|s2cid=34992724|access-date=17 June 2014|archive-url=https://web.archive.org/web/20161115011411/http://www.ciesin.org/docs/002-253/002-253.html|archive-date=15 November 2016|url-status=dead}}</ref> which are animals that do not have a [[vertebral column]] (or backbone or spine), derived from the [[notochord]]. This includes all animals apart from the [[subphylum]] [[vertebrate|Vertebrata]]. Familiar examples of invertebrates include [[sponge]]s, [[cnidaria]]ns ([[hydras]], [[jellyfish]]es, [[sea anemone]]s, and [[coral]]s), [[mollusks]] ([[chitons]], [[snail]], [[bivalves]], [[squid]]s, and [[octopus]]es), [[annelid]]s ([[earthworm]]s and [[leech]]es), and [[arthropod]]s ([[insect]]s, [[arachnid]]s, [[crustacean]]s, and [[myriapod]]s). Many invertebrate [[taxon|taxa]] have a greater number and variety of species than the entire subphylum of Vertebrata.<ref name="isbn0-412-61390-5">{{cite book | author1=Richards, O. W. |author2=Davies, R.G. |title=Imms' General Textbook of Entomology: Volume 1: Structure, Physiology and Development Volume 2: Classification and Biology |publisher=Springer |location=Berlin | year=1977 |isbn=978-0-412-61390-6 }}</ref>

In contrast, [[vertebrate]]s comprise all species of animals within the [[subphylum]] Vertebrata, which are [[chordate]]s with vertebral columns. These animals have four key features, which are an anterior skull with a brain, a rigid internal skeleton supported by a vertebral column that encloses a spinal cord, internal organs suspended in a coelom, and a well-developed circulatory system driven by a single large heart.<ref name="hillisetal2014w"/> Vertebrates represent the overwhelming majority of the phylum [[Chordata]], with currently about 69,963 [[species]] described.<ref>{{cite web |title=Table 1a: Number of species evaluated in relation to the overall number of described species, and numbers of threatened species by major groups of organisms |url=https://www.iucnredlist.org/resources/summary-statistics |publisher=IUCN Red List |date=18 July 2019 |access-date=14 May 2021 |archive-date=20 December 2020 |archive-url=https://web.archive.org/web/20201220135528/https://www.iucnredlist.org/resources/summary-statistics |url-status=live }}</ref> Vertebrates comprise different major groups that include [[Agnatha|jawless fishes]] (not including [[hagfish]]es), [[Gnathostomata|jawed vertebrates]] such as [[Chondrichthyes|cartilaginous fishes]] ([[shark]]s, [[Batoidea|rays]], and ratfish), [[Osteichthyes|bony fishes]], [[tetrapods]] such as [[amphibian]]s, [[reptile]]s, [[bird]]s, and [[mammal]]s.<ref name="hillisetal2014w"/>

The two remaining groups of jawless fishes that have survived beyond the [[Devonian]] [[Geologic time scale#Terminology|period]] are hagfishes and [[lamprey]], which are collectively known as [[Cyclostomi|cyclostome]]s (for ''circled mouths'').<ref name="hillisetal2014w"/> Both groups of animals have elongated eel-like bodies with no paired fins.<ref name="hillisetal2014w"/> However, because hagfishes have a weak circulatory system with three accessory hearts, a partial skull with no [[cerebellum]], no jaws or stomach, and no jointed vertebrae, some biologists do not classify them as vertebrates but instead as a [[sister group]] of vertebrates.<ref name="hillisetal2014w"/> In contrast, lampreys have a complete skull and a distinct vertebrae that is cartilaginous.<ref name="hillisetal2014w"/>

Mammals have four key features that distinguish them from other animals such as [[sweat gland]]s, [[mammary gland]]s, hair, and a four-chambered heart.<ref name="hillisetal2014w"/> Small and medium-sized mammals used to co-exist with large dinosaurs in much of the [[Mesozoic]] era but soon [[Evolutionary radiation|radiated]] following the [[Cretaceous–Paleogene extinction event|mass extinction of dinosaurs]] at the end of the [[Cretaceous]] period.<ref name="hillisetal2014w"/> There are approximately 57,000 mammal species, which can be divided into two primary groups: [[Yinotheria|prototherian]]s and [[theria]]ns. Prototherians do not possess nipples on their mammary but instead secrete milk onto their skin, allowing their offspring to lap if off their furs.<ref name="hillisetal2014w"/> They also lack a placenta, lays eggs, and have sprawling legs. Currently, there only five known species of prototherians ([[platypus]] and four species of [[echidna]]s).<ref name="hillisetal2014w"/> The therian clade is [[Viviparity|viviparous]] and can be further divided into two groups: [[marsupial]]s and [[eutherian]]s.<ref name="hillisetal2014w"/> Marsupial females have a ventral pouch to carry and feed their offspring. Eutherians form the majority of mammals and include major groups such as [[rodent]]s, [[bat]]s, [[even-toed ungulate]]s and [[cetacea]]ns, [[shrew]]s and [[Mole (animal)|mole]]s, [[primate]]s, [[carnivore]]s, [[rabbit]]s, [[Afrosoricida|African insectivore]]s, [[Erinaceidae|spiny insectivore]]s, [[armadillo]]s, [[treeshrew]]s, [[odd-toed ungulate]]s, [[Elephant shrew|long-nosed insectivore]]s, [[anteater]]s and [[sloth]]s, [[pangolin]]s, [[hyrax]]es, [[sirenian]]s, [[elephant]]s, [[colugo]]s, and [[aardvark]].<ref name="hillisetal2014w"/>

A split in the primate lineage occurred approximately 90 million years ago during the [[Cretaceous]], which brought about two major clades: [[prosimian]]s and [[simian|anthropoid]]s.<ref name="hillisetal2014w"/> The prosimians include [[lemur]]s, [[loris]]es, and [[galago]]s whereas the anthropoids comprise [[tarsier]]s, [[New World monkey]]s, [[Old World monkey]]s, and [[ape]]s.<ref name="hillisetal2014w"/> Apes separated from Old World monkeys about 35 million years ago, with various species living in [[Africa]], [[Europe]], and [[Asia]] between 22 and 5.5 million years ago.<ref name="hillisetal2014w"/> The modern descendants of these animals include [[chimpanzee]]s and [[gorilla]]s in Africa, [[gibbon]]s and [[orangutan]]s in Asia, and humans worldwide. A split in the ape lineage occurred about six million years ago in Africa, which resulted in the emergence of chimpanzees as one group and a hominid clade as another group that includes humans and their extinct relatives.<ref name="hillisetal2014w"/> [[Bipedalism]] emerged in the earliest [[Human evolution|protohominid]]s known as [[Ardipithecus|ardipithecine]]s. As an adaptation, bipedalism conferred three advantages. First, it enabled the ardipithecines to use their forelimbs to manipulate and carry objects while working.<ref name="hillisetal2014w"/> Second, it elevated the animal's eyes to spot preys or predators over tall vegetation.<ref name="hillisetal2014w"/> Finally, bipedalism is more energetically efficient than quadrupedal locomotion.<ref name="hillisetal2014w"/>

===Viruses===
{{Further | Virology}}
[[File:Phage.jpg|thumb|right| [[Bacteriophage]]s attached to a bacterial cell wall]]
[[Virus]]es are [[wikt:submicroscopic|submicroscopic]] [[infectious agent]]s that [[Viral replication|replicate]] inside the [[Cell (biology)|cells]] of [[organism]]s.<ref name="NG-20200415">{{cite news |vauthors=Wu KJ |title=There are more viruses than stars in the universe. Why do only some infect us? – More than a quadrillion quadrillion individual viruses exist on Earth, but most are not poised to hop into humans. Can we find the ones that are? |url=https://www.nationalgeographic.com/science/2020/04/factors-allow-viruses-infect-humans-coronavirus/ |date=15 April 2020 |work=[[National Geographic Society]] |access-date=18 May 2020 |archive-date=28 May 2020 |archive-url=https://web.archive.org/web/20200528154701/https://www.nationalgeographic.com/science/2020/04/factors-allow-viruses-infect-humans-coronavirus/ |url-status=live }}</ref> Viruses infect all types of [[life forms]], from animals and plants to [[microorganism]]s, including [[bacteria]] and [[archaea]].<ref name="pmid16984643">{{cite journal | vauthors = Koonin EV, Senkevich TG, Dolja VV | title = The ancient Virus World and evolution of cells | journal = Biology Direct | volume = 1 | issue = 1 | pages = 29 | date = September 2006 | pmid = 16984643 | pmc = 1594570 | doi = 10.1186/1745-6150-1-29 }}</ref><ref name="NYT-20210226">{{cite news | vauthors = Zimmer C |author-link=Carl Zimmer |title=The Secret Life of a Coronavirus - An oily, 100-nanometer-wide bubble of genes has killed more than two million people and reshaped the world. Scientists don't quite know what to make of it. |newspaper=The New York Times |url=https://www.nytimes.com/2021/02/26/opinion/sunday/coronavirus-alive-dead.html |archive-url=https://ghostarchive.org/archive/20211228/https://www.nytimes.com/2021/02/26/opinion/sunday/coronavirus-alive-dead.html |archive-date=2021-12-28 |url-access=limited |date=26 February 2021 |access-date=28 February 2021 }}{{cbignore}}</ref> More than 6,000 [[virus species]] have been described in detail.<ref name = ictv2019>{{cite web|url=https://talk.ictvonline.org/taxonomy/|title=Virus Taxonomy: 2019 Release|website=talk.ictvonline.org|publisher=International Committee on Taxonomy of Viruses|access-date=25 April 2020|archive-date=20 March 2020|archive-url=https://web.archive.org/web/20200320103754/https://talk.ictvonline.org/taxonomy|url-status=live}}</ref> Viruses are found in almost every [[ecosystem]] on Earth and are the most numerous type of biological entity.<ref name="Lawrence">{{cite journal | vauthors = Lawrence CM, Menon S, Eilers BJ, Bothner B, Khayat R, Douglas T, Young MJ | title = Structural and functional studies of archaeal viruses | journal = The Journal of Biological Chemistry | volume = 284 | issue = 19 | pages = 12599–603 | date = May 2009 | pmid = 19158076 | pmc = 2675988 | doi = 10.1074/jbc.R800078200 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Edwards RA, Rohwer F | s2cid = 8059643 | title = Viral metagenomics | journal = Nature Reviews. Microbiology | volume = 3 | issue = 6 | pages = 504–10 | date = June 2005 | pmid = 15886693 | doi = 10.1038/nrmicro1163 }}</ref>

When infected, a host cell is forced to rapidly produce thousands of identical copies of the original virus. When not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent particles, or ''virions'', consisting of the [[genetic material]] ([[DNA]] or [[RNA]]), a [[protein]] coat called ''[[capsid]]'', and in some cases an outside [[viral envelope|envelope]] of [[lipid]]s. The shapes of these virus particles range from simple [[helix|helical]] and [[icosahedron|icosahedral]] forms to more complex structures. Most virus species have virions too small to be seen with an [[optical microscope]], as they are one-hundredth the size of most bacteria.

The origins of viruses in the [[evolutionary history of life]] are unclear: some may have [[evolution|evolved]] from [[plasmid]]s—pieces of DNA that can move between cells—while others may have evolved from bacteria. In evolution, viruses are an important means of [[horizontal gene transfer]], which increases [[genetic diversity]] in a way analogous to [[sexual reproduction]].<ref name="Canchaya">{{cite journal | vauthors = Canchaya C, Fournous G, Chibani-Chennoufi S, Dillmann ML, Brüssow H | title = Phage as agents of lateral gene transfer | journal = Current Opinion in Microbiology | volume = 6 | issue = 4 | pages = 417–24 | date = August 2003 | pmid = 12941415 | doi = 10.1016/S1369-5274(03)00086-9 }}</ref> Because viruses possess some but not all characteristics of life, they have been described as "organisms at the edge of life",<ref name="ReferenceA">{{cite journal| vauthors = Rybicki EP |year = 1990|title = The classification of organisms at the edge of life, or problems with virus systematics|journal = South African Journal of Science |volume = 86|pages = 182–86}}</ref> and as [[Self-replication|self-replicators]].<ref name="kooninstarokadomskyy2016">{{cite journal | vauthors = Koonin EV, Starokadomskyy P | title = Are viruses alive? The replicator paradigm sheds decisive light on an old but misguided question | journal = Studies in History and Philosophy of Biological and Biomedical Sciences | volume = 59 | pages = 125–34 | date = October 2016 | pmid = 26965225 | pmc = 5406846 | doi = 10.1016/j.shpsc.2016.02.016 }}</ref>

Viruses can spread in many ways. One transmission pathway is through disease-bearing organisms known as [[Vector (epidemiology)|vectors]]: for example, viruses are often transmitted from plant to plant by insects that feed on [[plant sap]], such as [[aphid]]s; and viruses in animals can be carried by [[Hematophagy|blood-sucking]] insects. [[influenza|Influenza viruses]] are spread by coughing and sneezing. [[Norovirus]] and [[rotavirus]], common causes of viral [[gastroenteritis]], are transmitted by the [[fecal–oral route|faecal–oral route]], passed by hand-to-mouth contact or in food or water. Viral infections in animals provoke an [[immune response]] that usually eliminates the infecting virus. Immune responses can also be produced by [[vaccine]]s, which confer an [[immunity (medical)|artificially acquired immunity]] to the specific viral infection.

==Plant form and function==
===Plant body===
{{Further | Plant morphology | Plant anatomy | Plant physiology}}
[[File:Plant.svg|thumb|right| Root and shoot systems in a [[eudicot]]]]
The plant body is made up of [[Organ (biology)|organ]]s that can be organized into two major [[organ system]]s: a [[root |root system]] and a [[shoot |shoot system]].<ref name="hillisetal2014x">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4 = Price | first4 = Mary V. | chapter = The plant body | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 521–536 | isbn = 978-1464175121}}</ref> The root system anchors the plants into place. The roots themselves absorb water and minerals and store photosynthetic products. The shoot system is composed of [[Plant stem|stem]], [[Leaf|leaves]], and [[flower]]s. The stems hold and orient the leaves to the sun, which allow the leaves to conduct photosynthesis. The flowers are [[shoot]]s that have been modified for [[reproduction]]. Shoots are composed of [[phytomer]]s, which are [[functional unit]]s that consist of a node carrying one or more leaves, internode, and one or more [[bud]]s.

A plant body has two basic patterns (apical–basal and radial axes) that been established during [[Plant embryogenesis|embryogenesis]].<ref name="hillisetal2014x" /> Cells and tissues are arranged along the apical-basal axis from root to shoot whereas the three tissue systems ([[Dermal tissue system|dermal]], [[Ground tissue|ground]], and [[Vascular tissue|vascular]]) that make up a plant's body are arranged concentrically around its radial axis.<ref name="hillisetal2014x" /> The dermal tissue system forms the [[epidermis]] (or outer covering) of a plant, which is usually a single cell layer that consists of cells that have differentiated into three specialized structures: [[stomata]] for gas exchange in leaves, [[trichome]]s (or leaf hair) for protection against [[insect]]s and [[Solar irradiance|solar radiation]], and [[root hair]]s for increased surface areas and absorption of water and nutrients. The ground tissue makes up virtually all the tissue that lies between the dermal and vascular tissues in the shoots and roots. It consists of three cell types: [[Parenchyma]], [[collenchyma]], and [[sclerenchyma]] cells. Finally, the vascular tissues are made up of two constituent tissues: [[xylem]] and [[phloem]]. The xylem is made up of two conducting cells called [[tracheid]]s and [[vessel element]]s whereas the phloem is characterized by the presence of [[sieve tube element]]s and [[companion cell]]s.<ref name="hillisetal2014x" />

===Plant nutrition and transport===
{{Further | Vascular plant#Nutrient distribution}}
[[File:Xylem and phloem diagram.svg|thumb|The xylem (blue) transports water and minerals from the roots upwards whereas the phloem (orange) transports carbohydrates between organs.]]
Like all other organisms, plants are primarily made up of water and other molecules containing [[chemical element|element]]s that are essential to life.<ref name="hillisetal2014y">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = Plant nutrition and transport | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 537–554 | isbn = 978-1464175121}}</ref> The absence of specific nutrients (or [[Mineral (nutrient)|essential element]]s), many of which have been identified in [[Hydroponics|hydroponic]] experiments, can disrupt [[plant development|plant growth]] and [[Plant reproduction|reproduction]]. The majority of plants are able to obtain these nutrients from [[Solution (chemistry)|solution]]s that surrounds their [[root]]s in the [[soil]].<ref name="hillisetal2014y" /> Continuous [[Leaching (agriculture)|leaching]] and [[harvest]]ing of [[crop]]s can deplete the soil of its nutrients, which can be restored with the use of [[fertilizer]]s. [[Carnivorous plant]]s such as [[Venus flytrap]]s are able to obtain nutrients by digesting other [[arthropod]]s whereas [[parasitic plant]]s such as [[mistletoe]]s can parasitize other plants for water and nutrients.

Plants need water to conduct [[photosynthesis]], transport [[solute]]s between organs, cool their leaves by [[evaporation]], and maintain internal pressures that support their bodies.<ref name="hillisetal2014y" /> Water is able to [[diffusion|diffuse]] in and out of [[plant cell]]s by [[osmosis]]. The direction of water movement across a [[semipermeable membrane]] is determined by the [[water potential]] across that membrane.<ref name="hillisetal2014y" /> Water is able to diffuse across a root cell's membrane through [[aquaporin]]s whereas solutes are transported across by the membrane by [[ion channel]]s and [[Ion transporter|pump]]s. In [[vascular plant]]s, water and solutes are able to enter the [[xylem]], a [[vascular tissue]], by way of an [[apoplast]] and [[symplast]]. Once in the xylem, the water and minerals are distributed upward by [[transpiration]] from the soil to the aerial parts of the plant.<ref name="hillisetal2014u" /><ref name="hillisetal2014y" /> In contrast, the [[phloem]], another vascular tissue, distributes [[carbohydrate]]s (e.g., [[sucrose]]) and other solutes such as hormones by [[Phloem#Function|translocation]] from a [[Pressure flow hypothesis#Sources and sinks|source]] (e.g., mature [[leaf]] or root) in which they were produced to a [[Pressure flow hypothesis#Sources and sinks|sink]] (e.g., root, [[flower]], or developing [[fruit]]) in which they will be used and stored.<ref name="hillisetal2014y" /> Sources and sinks can switch roles, depending on the amount of carbohydrates accumulated or mobilized for the nourishment of other organs.

===Plant development===
{{Main|Plant development}}
Plant development is regulated by environmental cues and the plant's own [[Receptor (biochemistry)|receptor]]s, [[hormone]]s, and [[genome]].<ref name="hillisetal2014aa">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = Plant growth and development | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 555–572 | isbn = 978-1464175121}}</ref> Morever, they have several characteristics that allow them to obtain resources for growth and reproduction such as [[meristem]]s, post-embryonic organ formation, and differential growth.

Development begins with a [[seed]], which is an [[Plant embryogenesis|embryonic]] [[plant]] enclosed in a [[testa (botany)|protective outer covering]]. Most plant seeds are usually [[Seed dormancy|dormant]], a condition in which the seed's normal activity is suspended.<ref name="hillisetal2014aa" /> Seed dormancy may last may last weeks, months, years, and even centuries. Dormancy is broken once conditions are favorable for growth, and the seed will begin to sprout, a process called [[germination]]. [[Imbibition]] is the first step in germination, whereby water is absorbed by the seed. Once water is absorbed, the seed undergoes metabolic changes whereby [[enzyme]]s are activated and [[RNA]] and [[protein]]s are synthesized. Once the seed germinates, it obtains [[carbohydrate]]s, [[amino acid]]s, and small [[lipid]]s that serve as building blocks for its development. These [[monomer]]s are obtained from the [[hydrolysis]] of [[starch]], [[protein]]s, and lipids that are stored in either the [[cotyledon]]s or [[endosperm]]. Germination is completed once embryonic roots called [[radicle]] have emerged from the [[Seed#Seed coat|seed coat]]. At this point, the developing plant is called a [[seedling]] and its growth is regulated by its own [[photoreceptor protein#Photoreceptors in plants|photoreceptor protein]]s and hormones.<ref name="hillisetal2014aa" />

Unlike [[animal]]s in which growth is determinate, i.e., ceases when the adult state is reached, plant growth is indeterminate as it is an open-ended process that could potentially be lifelong.<ref name="hillisetal2014x" /> Plants grow in two ways: [[secondary growth|primary]] and [[secondary growth|secondary]]. In primary growth, the shoots and roots are formed and lengthened. The [[Meristem#Apical meristems|apical meristem]] produces the primary plant body, which can be found in all [[Spermatophyte|seed plant]]s. During secondary growth, the thickness of the plant increases as the [[Meristem#Secondary meristems|lateral meristem]] produces the secondary plant body, which can be found in woody [[eudicots]] such as trees and shrubs. [[Monocotyledon|Monocots]] do not go through secondary growth.<ref name="hillisetal2014x" /> The plant body is generated by a hierarchy of [[meristem]]s. The apical meristems in the root and shoot systems give rise to primary meristems (protoderm, ground meristem, and [[Meristem#Primary meristems|procambium]]), which in turn, give rise to the three tissue systems ([[Dermal tissue system|dermal]], [[Ground tissue|ground]], and [[Vascular tissue|vascular]]).

===Plant reproduction===
{{Further | Plant reproduction}}
[[File:Figure 11 02 03.jpg|thumb |right |Reproduction and development in [[sporophyte]]s]]
Most [[angiosperms]] (or flowering plants) engage in [[sexual reproduction]].<ref name="hillisetal2014ab">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = Reproduction of flowering plants | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 573–588 | isbn = 978-1464175121}}</ref> Their [[flower]]s are organs that facilitate [[reproduction]], usually by providing a mechanism for the union of [[sperm]] with eggs. Flowers may facilitate two types of [[pollination]]: self-pollination and cross-pollination. [[Self-pollination]] occurs when the pollen from the anther is deposited on the stigma of the same flower, or another flower on the same plant. [[Cross-pollination]] is the transfer of pollen from the anther of one flower to the stigma of another flower on a different individual of the same species. Self-pollination happened in flowers where the stamen and carpel mature at the same time, and are positioned so that the pollen can land on the flower's stigma. This pollination does not require an investment from the plant to provide nectar and pollen as food for pollinators.<ref>{{Cite web|url=https://courses.lumenlearning.com/wm-biology2/chapter/self-pollination-and-cross-pollination/|title=Self-Pollination and Cross-Pollination &#124; Biology for Majors II|website=courses.lumenlearning.com|access-date=2021-05-15|archive-date=2020-07-21|archive-url=https://web.archive.org/web/20200721145554/https://courses.lumenlearning.com/wm-biology2/chapter/self-pollination-and-cross-pollination/|url-status=live}}</ref>

===Plant responses===
{{Further|Plant perception (physiology)|Plant defense against herbivory#Chemical defenses}}
Like animals, plants produce [[plant hormone|hormones]] in one part of its body to signal cells in another part to respond. The [[ripening]] of [[fruit]] and loss of leaves in the winter are controlled in part by the production of the gas [[Ethylene as a plant hormone|ethylene]] by the plant. Stress from water loss, changes in air chemistry, or crowding by other plants can lead to changes in the way a plant functions. These changes may be affected by genetic, chemical, and physical factors.

To function and survive, plants produce a wide array of chemical compounds not found in other organisms. Because they cannot move, plants must also defend themselves chemically from [[herbivore]]s, [[pathogen]]s and competition from other plants. They do this by producing [[toxin]]s and foul-tasting or smelling chemicals. Other compounds defend plants against disease, permit survival during drought, and prepare plants for dormancy, while other compounds are used to attract [[pollinator]]s or herbivores to spread ripe seeds.

Many plant organs contain different types of [[photoreceptor protein#Photoreceptors in plants|photoreceptor protein]]s, each of which reacts very specifically to certain wavelengths of light.<ref name="pmid11687489">{{cite journal | vauthors = Harmer SL, Panda S, Kay SA | title = Molecular bases of circadian rhythms | journal = Annual Review of Cell and Developmental Biology | volume = 17 | pages = 215–53 | year = 2001 | pmid = 11687489 | doi = 10.1146/annurev.cellbio.17.1.215 }}</ref> The photoreceptor proteins relay information such as whether it is day or night, duration of the day, intensity of light available, and the source of light. Shoots generally grow towards light, while roots grow away from it, responses known as [[phototropism]] and skototropism, respectively. They are brought about by light-sensitive pigments like [[phototropin]]s and [[phytochrome]]s and the plant hormone [[auxin]].<ref>{{cite journal|last1=Strong|first1=Donald R.|last2=Ray|first2=Thomas S.|author-link2=Thomas S. Ray|title=Host Tree Location Behavior of a Tropical Vine (''Monstera gigantea'') by Skototropism|journal=[[Science (journal)|Science]]|date=1 January 1975|volume=190|issue=4216|pages=804–806|doi=10.1126/science.190.4216.804 |jstor=1741614|bibcode=1975Sci...190..804S|s2cid=84386403}}</ref> Many [[flowering plant]]s bloom at the appropriate time because of light-sensitive compounds that respond to the length of the night, a phenomenon known as [[photoperiodism]].

In addition to light, plants can respond to other types of stimuli. For instance, plants can sense the direction of [[gravity]] to orient themselves correctly. They can respond to mechanical stimulation.<ref>{{cite journal | vauthors = Jaffe MJ, Forbes S | title = Thigmomorphogenesis: the effect of mechanical perturbation on plants | journal = Plant Growth Regulation | volume = 12 | issue = 3 | pages = 313–24 | date = February 1993 | pmid = 11541741 | doi = 10.1007/BF00027213 | s2cid = 29466083 }}</ref>

==Animal form and function==
===General features===
{{Further|Anatomy|Physiology}}
[[File:105 Negative Feedback Loops.jpg|right|thumb| [[Negative feedback#Biology|Negative feedback]] is necessary for maintaining [[homeostasis]] such as keeping [[Thermoregulation|body temperature]] constant.]]
The cells in each animal body are bathed in [[interstitial fluid]], which make up the cell's environment. This fluid and all its characteristics (e.g., temperature, ionic composition) can be described as the animal's [[internal environment]], which is in contrast to the external environment that encompasses the animal's outside world.<ref name="hillisetal2014ac">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = Fundamentals of animal function | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 605–623 | isbn = 978-1464175121}}</ref> Animals can be classified as either regulators or conformers. Animals such as [[mammal]]s and [[bird]]s are regulators as they are able to maintain a constant internal environment such as body temperature despite their environments changing. These animals are also described as [[homeotherm]]s as they exhibit [[thermoregulation]] by keeping their internal body temperature constant. In contrast, animals such as [[fish]]es and [[frog]]s are conformers as they adapt their internal environment (e.g., body temperature) to match their external environments. These animals are also described as [[poikilotherm]]s or [[ectotherm]]s as they allow their body temperatures to match their external environments. In terms of energy, regulation is more costly than conformity as an animal expands more energy to maintain a constant internal environment such as increasing its [[basal metabolic rate]], which is the rate of energy consumption.<ref name="hillisetal2014ac" /> Similarly, homeothermy is more costly than poikilothermy. [[Homeostasis]] is the stability of an animal's internal environment, which is maintained by [[Negative feedback#Biology|negative feedback]] loops.<ref name="hillisetal2014ac" /><ref>{{cite journal | last=Rodolfo | first=Kelvin | date=January 2000 | url=https://www.scientificamerican.com/article/what-is-homeostasis/ | title=What is homeostasis? | journal=Scientific American | url-status=live | archive-url=https://web.archive.org/web/20131203020456/http://www.scientificamerican.com/article.cfm?id=what-is-homeostasis | archive-date=2013-12-03 }}</ref>

The body size of [[terrestrial animal]]s vary across different species but their use of energy does not [[Allometry#Physiological scaling|scale]] linearly according to their size.<ref name="hillisetal2014ac" /> Mice, for example, are able to consume three times more food than rabbits in proportion to their weights as the basal metabolic rate per unit weight in mice is greater than in rabbits.<ref name="hillisetal2014ac" /> [[Physical activity]] can also increase an animal's metabolic rate. When an animal runs, its metabolic rate increases linearly with speed.<ref name="hillisetal2014ac" /> However, the relationship is non-linear in animals that [[Aquatic locomotion|swim]] or [[Flight|fly]]. When a fish swims faster, it encounters greater water resistance and so its metabolic rates increases exponential.<ref name="hillisetal2014ac" /> Alternatively, the relationship of flight speeds and metabolic rates is U-shaped in birds.<ref name="hillisetal2014ac" /> At low flight speeds, a bird must maintain a high metabolic rates to remain airborne. As it speeds up its flight, its metabolic rate decreases with the aid of air rapidly flows over its wings. However, as it increases in its speed even further, its high metabolic rates rises again due to the increased effort associated with rapid flight speeds. Basal metabolic rates can be measured based on an animal's rate of [[heat]] production.

===Water and salt balance===
{{Further | Osmoregulation | Urinary system}}
[[Image:Bachforelle osmoregulatoin bw en2.png|thumb|Diffusion of water and ions in and out of a freshwater fish]]
An animal's [[body fluid]]s have three properties: [[osmotic pressure]], [[ion]]ic composition, and [[volume]].<ref name="hillisetal2014aj">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = Water and salt balance | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 751–767| isbn = 978-1464175121}}</ref> Osmotic pressures determine the direction of the [[diffusion]] of water (or [[osmosis]]), which moves from a region where osmotic pressure (total solute concentration) is low to a region where osmotic pressure (total solute concentration) is high. Aquatic animals are diverse with respect to their body fluid compositions and their environments. For example, most invertebrate animals in the ocean have body fluids that are [[Tonicity#Isotonicity|isosmotic]] with seawater. In contrast, ocean [[Osteichthyes|bony fish]]es have body fluids that are [[Tonicity#Hypotonic solution|hyposmotic]] to seawater. Finally, freshwater animals have body fluids that are [[Tonicity#Hypertonic solution|hyperosmotic]] to fresh water. Typical ions that can be found in an animal's body fluids are [[sodium]], [[potassium]], [[calcium]], and [[chloride]]. The volume of body fluids can be regulated by [[excretion]]. [[Vertebrate]] animals have [[kidney]]s, which are excretory organs made up of tiny tubular structures called [[nephron]]s, which make [[urine]] from blood plasma. The kidneys' primary function is to regulate the composition and volume of blood plasma by selectively removing material from the blood plasma itself. The ability of [[Deserts and xeric shrublands|xeric]] animals such as [[kangaroo rat]]s to minimize water loss by producing urine that is 10–20 times concentrated than their blood plasma allows them to adapt in [[desert]] environments that receive very little [[precipitation]].<ref name="hillisetal2014aj" />

===Nutrition and digestion===
{{Further | Nutrition}}
[[File:Types of digestive systems in marine fish.jpg|thumb|right|Different digestive systems in marine fishes]]
Animals are [[heterotroph]]s as they feed on other organisms to obtain energy and [[organic compound]]s.<ref name="hillisetal2014ad">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = Nutrition, feeding, and digestion | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 624–642 | isbn = 978-1464175121}}</ref> They are able to obtain food in three major ways such as targeting visible food objects, collecting tiny food particles, or depending on microbes for critical food needs. The amount of [[Food energy|energy stored in food]] can be quantified based on the amount of heat (measured in [[calorie]]s or [[joule|kilojoules]]) emitted when the food is burnt in the presence of oxygen. If an animal were to consume food that contains an excess amount of chemical energy, it will store most of that energy in the form of [[lipid]]s for future use and some of that energy as [[glycogen]] for more immediate use (e.g., meeting the brain's energy needs).<ref name="hillisetal2014ad" /> The molecules in food are chemical building blocks that are needed for growth and development. These molecules include nutrients such as [[carbohydrate]]s, [[fat]]s, and [[protein]]s. [[Vitamin]]s and [[Mineral (nutrient)|mineral]]s (e.g., calcium, magnesium, sodium, and phosphorus) are also essential. The [[digestive system]], which typically consist of a tubular tract that extends from the mouth to the anus, is involved in the breakdown (or [[digestion]]) of food into small molecules as it travels down [[peristalsis|peristaltically]] through the [[Gastrointestinal tract|gut lumen]] shortly after it has been [[ingestion|ingested]]. These small food molecules are then [[Absorption (biology)|absorbed]] into the blood from the lumen, where they are then distributed to the rest of the body as building blocks (e.g., amino acids) or sources of energy (e.g., glucose).<ref name="hillisetal2014ad" />

In addition to their digestive tracts, vertebrate animals have accessory glands such as a liver and pancreas as part of their digestive systems.<ref name="hillisetal2014ad" /> The processing of food in these animals begins in the [[foregut]], which includes the mouth, [[esophagus]], and [[stomach]]. Mechanical digestion of food starts in the mouth with the esophagus serving as a passageway for food to reach the stomach, where it is stored and disintegrated (by the stomach's acid) for further processing. Upon leaving the stomach, food enters into the [[midgut]], which is the first part of the [[intestine]] (or [[small intestine]] in [[mammal]]s) and is the principal site of digestion and absorption. Food that does not get absorbed are stored as indigestible waste (or [[feces]]) in the [[hindgut]], which is the second part of the intestine (or [[large intestine]] in mammals). The hindgut then completes the reabsorption of needed water and salt prior to eliminating the feces from the [[rectum]].<ref name="hillisetal2014ad" />

===Breathing===
[[File:Cranial sinus and postcranial air sac systems in birds.svg|thumb | right | Respiratory system in a [[bird]]]]{{Main|Respiratory system}}
The respiratory system consists of specific [[organs]] and structures used for gas exchange in [[animal]]s. The anatomy and physiology that make this happen varies greatly, depending on the size of the organism, the environment in which it lives and its evolutionary history. In [[terrestrial animal|land animals]] the respiratory surface is internalized as linings of the [[lung]]s.<ref name="Biology">{{cite book |last1=Campbell |first1=Neil A. |title=Biology |date=1990 |publisher=Benjamin/Cummings Pub. Co. |location=Redwood City, Calif. |isbn=0-8053-1800-3 |pages=834–835 |edition=2nd}}</ref> [[Gas exchange]] in the lungs occurs in millions of small air sacs; in mammals and reptiles these are called [[pulmonary alveolus|alveoli]], and in birds they are known as [[Bird anatomy#Respiratory system|atria]]. These microscopic air sacs have a very rich blood supply, thus bringing the air into close contact with the blood.<ref name="Hsia">{{cite journal |last1=Hsia |first1=CC |last2=Hyde |first2=DM |last3=Weibel |first3=ER |author3-link=ER Weibel |title=Lung Structure and the Intrinsic Challenges of Gas Exchange. |journal=Comprehensive Physiology |date=15 March 2016 |volume=6 |issue=2 |pages=827–95 |doi=10.1002/cphy.c150028 |pmid=27065169|pmc=5026132 }}</ref> These air sacs communicate with the external environment via a system of airways, or hollow tubes, of which the largest is the [[trachea]], which branches in the middle of the chest into the two main [[bronchus|bronchi]]. These enter the lungs where they branch into progressively narrower secondary and tertiary bronchi that branch into numerous smaller tubes, the [[bronchiole]]s. In [[bird]]s the bronchioles are termed [[Bird anatomy#Respiratory system|parabronchi]]. It is the bronchioles, or parabronchi that generally open into the microscopic [[pulmonary alveolus|alveoli]] in mammals and [[Bird anatomy#Respiratory system|atria]] in birds. Air has to be pumped from the environment into the alveoli or atria by the process of [[breathing]], which involves the [[muscles of respiration]].

===Circulation===
[[File:Circulatory systems in the animal kingdom.svg|thumb|right | Circulatory systems in arthropods, fish, reptiles, and birds/mammals]]{{Main|Circulatory system}}
A circulatory system usually consists of a muscular pump such as a [[heart]], a fluid ([[blood]]), and system of [[blood vessel]]s that deliver it.<ref name="hillisetal2014af">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = Circulation | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 661–680 | isbn = 978-1464175121}}</ref><ref>{{DorlandsDict|eight/000105264|cardiovascular system}}</ref> Its principal function is to transport [[blood]] and other substances to and from [[cell (biology)|cell]]s and [[Tissue (biology)|tissue]]s. There are two types of circulatory systems: [[Open circulatory system|open]] and [[Closed circulatory system|closed]]. In open circulatory systems, blood exits blood vessels as it circulates throughout the body whereas in closed circulatory system, blood is contained within the blood vessels as it circulates. Open circulatory systems can be observed in [[invertebrate]] animals such as [[arthropod]]s (e.g., [[insect]]s, [[spider]]s, and [[lobster]]s) whereas closed circulatory systems can be found in [[vertebrate]] animals such as [[fish]]es, [[amphibian]]s, and [[mammal]]s. Circulation in animals occur between two types of tissues: [[Circulatory system#Systemic circulation|systemic tissue]]s and [[Respiratory system|breathing (or pulmonary) organ]]s.<ref name="hillisetal2014af" /> Systemic tissues are all the tissues and organs that make up an animal's body other than its breathing organs. Systemic tissues take up oxygen but adds carbon dioxide to the blood whereas a breathing organs takes up carbon dioxide but add oxygen to the blood.<ref name="PubMed">{{cite journal|title=How does the blood circulatory system work?|journal=PubMed Health|date=1 August 2016|url=https://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0072434/|access-date=14 May 2021|archive-date=28 August 2021|archive-url=https://web.archive.org/web/20210828040141/https://www.ncbi.nlm.nih.gov/books/NBK279250/|url-status=live}}</ref> In birds and mammals, the systemic and pulmonary systems are connected in series.

In the circulatory system, blood is important because it is the means by which [[oxygen]], [[carbon dioxide]], [[nutrient]]s, [[hormone]]s, agents of immune system, heat, wastes, and other commodities are transported.<ref name="hillisetal2014af" /> In [[annelid]]s such as [[earthworm]]s and [[leech]]es, blood is propelled by [[Peristalsis|peristaltic wave]]s of [[Muscle contraction|contraction]]s of the heart muscles that make up the blood vessels. Other animals such as crustaceans (e.g., [[crayfish]] and [[lobster]]s), have more than one heart to propel blood throughout their bodies. Vertebrate hearts are [[Heart#Chambers|multichambered]] and are able to pump blood when their [[ventricle (heart)|ventricle]]s contract at each [[cardiac cycle]], which propels blood through the blood vessels.<ref name="hillisetal2014af" /> Although vertebrate hearts are [[Myogenic mechanism|myogenic]], their rate of contraction (or [[heart rate]]) can be modulated by neural input from the body's [[autonomic nervous system]].

===Muscle and movement===
{{Further | Muscle contraction}}
[[File:Motion_of_Insectwing.gif|right|thumb|Asynchronous muscles power flight in most insects. a: Wings b: Wing joint c: Dorsoventral muscles power upstrokes d: Dorsolongitudinal muscles power downstrokes.]]
In [[vertebrate]]s, the [[muscular system]] consists of [[skeletal muscle|skeletal]], [[smooth muscle|smooth]] and [[cardiac muscle|cardiac]] [[muscle]]s. It permits movement of the body, maintains posture and circulates blood throughout the body.<ref name="ross2011">{{Cite book |last1=Pawlina |first1=Wojciech |last2=Ross | first2=Michael H. |title=Histology : a text and atlas : with correlated cell and molecular biology |date=2011 |publisher=Wolters Kluwer/Lippincott Williams & Wilkins Health |isbn=9780781772006 |edition=  6th |location=Philadelphia |oclc=548651322}}</ref> Together with the [[skeletal system]], it forms the [[musculoskeletal system]], which is responsible for the movement of vertebrate animals.<ref>{{Cite book |last=Standring |first=Susan |title=Gray's anatomy : the anatomical basis of clinical practice |isbn=9780702052309 |edition=Forty-first |location=Philadelphia |oclc=920806541 |year = 2016}}</ref> Skeletal muscle contractions are [[neurogenic]] as they require [[Synapse|synaptic input]] from [[motor neurons]]. A single motor neuron is able to innervate multiple muscle fibers, thereby causing the fibers to contract at the same time. Once innervated, the protein filaments within each skeletal muscle fiber slide past each other to produce a contraction, which is explained by the [[sliding filament theory]]. The contraction produced can be described as a twitch, summation, or tetanus, depending on the frequency of [[action potentials]]. Unlike skeletal muscles, contractions of [[smooth muscle|smooth]] and [[cardiac muscle]]s are [[Myogenic contraction|myogenic]] as they are initiated by the smooth or heart muscle cells themselves instead of a motor neuron. Nevertheless, the strength of their contractions can be modulated by input from the [[autonomic nervous system]]. The mechanisms of contraction are similar in all three muscle tissues.

In invertebrates such as [[earthworms]] and [[leeches]], [[Muscle contraction#Circular and longitudinal muscles|circular and longitudinal muscle]]s cells form the body wall of these animals and are responsible for their movement.<ref name="Hillis 2014">{{cite book|title=Principles of Life|last1=Hillis|first1=David M.|last2=Sadava|first2=David E.|last3=Price|first3=Mary V.|date=2014|publisher=Sinauer Associates|isbn=978-1-464-10947-8|edition=2nd|location=Sunderland, Mass. |pages=681–698|chapter=Muscle and movement}}</ref> In an earthworm that is moving through a soil, for example, contractions of circular and longitudinal muscles occur reciprocally while the [[Coelom#Coelomic fluid|coelomic fluid]] serves as a [[Hydrostatic skeleton|hydroskeleton]] by maintaining turgidity of the earthworm.<ref name="Gardner 1976">{{cite journal|last1=Gardner|first1=C.R.|date=1976|title=The neuronal control of locomotion in the earthworm|journal=Biological Reviews of the Cambridge Philosophical Society|volume=51|issue=1|pages=25–52|pmid=766843|doi=10.1111/j.1469-185X.1976.tb01119.x|s2cid=9983649}}</ref> Other animals such as [[Mollusca|mollusks]], and [[nematode]]s, possess obliquely striated muscles, which contain bands of thick and thin filaments that are arranged helically rather than transversely, like in vertebrate skeletal or cardiac muscles.<ref name="Alexander 2003">{{cite book|title=Principles of Animal Locomotion|last1=Alexander|first1=R. McNeill|date=2003|publisher=Princeton University Press|isbn=978-0-691-12634-0|edition=2nd|location=Princeton, N.J. |pages=15–37|chapter=Muscle, the motor}}</ref> Advanced [[insect]]s such as [[wasp]]s, [[Fly|flies]], [[bee]]s, and [[beetle]]s possess [[asynchronous muscles]] that constitute the flight muscles in these animals.<ref name = "Alexander 2003"/> These flight muscles are often called ''fibrillar muscles'' because they contain myofibrils that are thick and conspicuous.<ref>{{Cite journal|last1=Josephson|first1=R. K.|last2=Malamud|first2=J. G.|last3=Stokes|first3=D. R.|date=2000-09-15|title=Asynchronous muscle: a primer|url=http://jeb.biologists.org/content/203/18/2713|journal=Journal of Experimental Biology|language=en|volume=203|issue=18|pages=2713–2722|doi=10.1242/jeb.203.18.2713|issn=0022-0949|pmid=10952872|access-date=2021-05-14|archive-date=2020-10-31|archive-url=https://web.archive.org/web/20201031072610/https://jeb.biologists.org/content/203/18/2713|url-status=live}}</ref>

===Nervous system===
{{Further | Neuroscience | Neuroethology}}
[[File:GFPneuron.png|thumb|right| Mouse [[pyramidal neuron]]s (green) and [[GABA]]ergic neurons (red)<ref>{{cite journal | vauthors = Lee WC, Huang H, Feng G, Sanes JR, Brown EN, So PT, Nedivi E | title = Dynamic remodeling of dendritic arbors in GABAergic interneurons of adult visual cortex | journal = PLOS Biology | volume = 4 | issue = 2 | pages = e29 | date = February 2006 | pmid = 16366735 | pmc = 1318477 | doi = 10.1371/journal.pbio.0040029 |doi-access=free }}</ref>]]
Most multicellular animals have [[nervous system]]s<ref name=Columbia>{{cite encyclopedia | title = Nervous System| encyclopedia = Columbia Encyclopedia| publisher = Columbia University Press}}</ref> that allow them to sense from and respond to their environments. A nervous system is a network of cells that processes [[Sense|sensory]] information and generates [[behavior]]s. At the cellular level, the nervous system is defined by the presence of [[neuron]]s, which are cells specialized to handle information.<ref name = "aidley1998a">{{cite book | last1 = Aidley | first1 = David J. | chapter = Introduction | title = The Physiology of Excitable Cells | publisher = Cambridge University Press | edition = 4th | date = 1998 | location = New York | pages = 1–7 | isbn = 978-0521574211}}</ref> They can transmit or receive information at sites of contacts called [[synapse]]s.<ref name = "aidley1998a"/> More specifically, neurons can conduct nerve impulses (or [[action potential]]s) that travel along their thin fibers called [[axon]]s, which can then be transmitted directly to a neighboring cell through [[electrical synapse]]s or cause chemicals called [[neurotransmitter]]s to be released at [[chemical synapse]]s. According to the sodium theory, these action potentials can be generated by the increased permeability of the neuron's [[cell membrane]] to sodium ions.<ref name = "aidley1998e">{{cite book | last1 = Aidley | first1 = David J. | chapter = The ionic basis of nervous conduction | title = The Physiology of Excitable Cells | publisher = Cambridge University Press | edition = 4th | date = 1998 | location = New York | pages = 54–75 | isbn = 978-0521574211}}</ref> Cells such as neurons or muscle cells may be excited or inhibited upon receiving a signal from another neuron. The connections between neurons can form [[neural pathway]]s, [[neural circuit]]s, and [[large scale brain networks|larger networks]] that generate an organism's perception of the world and determine its behavior. Along with neurons, the nervous system contains other specialized cells called [[glia]] or glial cells, which provide structural and metabolic support.

In vertebrates, the nervous system comprises the [[central nervous system]] (CNS), which includes the [[brain]] and [[spinal cord]], and the [[peripheral nervous system]] (PNS), which consists of [[nerve]]s that connect the CNS to every other part of the body. Nerves that transmit signals from the CNS are called [[motor nerve]]s or [[Efferent nerve fiber|efferent nerves]], while those nerves that transmit information from the body to the CNS are called [[sensory nerve]]s or [[Afferent nerve fiber|afferent nerves]]. [[Spinal nerve]]s are [[mixed nerve]]s that serve both functions. The PNS is divided into three separate subsystems, the [[somatic nervous system|somatic]], [[autonomic nervous system|autonomic]], and [[enteric nervous system|enteric]] nervous systems. Somatic nerves mediate voluntary movement. The autonomic nervous system is further subdivided into the [[sympathetic nervous system|sympathetic]] and the [[parasympathetic nervous system|parasympathetic]] nervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state. The enteric nervous system functions to control the [[gastrointestinal]] system. Both autonomic and enteric nervous systems function involuntarily. Nerves that exit directly from the brain are called [[cranial nerves]] while those exiting from the spinal cord are called spinal nerves.

Many animals have [[Sensory nervous system|sense organ]]s that can detect their environment. These sense organs contain [[sensory receptor]]s, which are sensory neurons that convert stimuli into electrical signals.<ref name="hillisetal2014ah">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = Neurons, sense organs, and nervous systems | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 699–732 | isbn = 978-1464175121}}</ref> [[Mechanoreceptor]]s, for example, which can be found in skin, muscle, and hearing organs, generate action potentials in response to changes in pressures.<ref name="hillisetal2014ah" /><ref name = "freeman2017ar">{{cite book | last1 = Freeman| first1 = Scott | last2 = Quillin | first2 = Kim | last3 = Allison | first3 = Lizabeth | last4 = Black | first4 = Michael |  last5 = Podgorski | first5 = Greg | last6 = Taylor | first6 = Emily | last7 = Carmichael | first7 = Jeff | chapter = Animal sensory systems | title = Biological Science | publisher = Pearson | edition = 6th | date = 2017 | location = Hoboken, N.J. | pages = 922–941 | isbn = 978-0321976499}}</ref> [[Photoreceptor cell]]s such as [[Rod cell|rod]]s and [[Cone cell|cone]]s, which are part of the vertebrate [[retina]], can respond to specific [[Electromagnetic spectrum|wavelengths of light]].<ref name="hillisetal2014ah" /><ref name = "freeman2017ar"/> [[Chemoreceptor]]s detect chemicals in the mouth ([[taste]]) or in the air ([[sense of smell|smell]]).<ref name = "freeman2017ar"/>

===Hormonal control===
{{Further | Endocrinology}}
[[Hormone]]s are signaling molecules transported in the blood to distant organs to regulate their function.<ref name="hillisetal2014ai">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = Control by the endocrine and nervous systems | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 733–750 | isbn = 978-1464175121}}</ref><ref>{{Cite book|title=Biology for a changing world, with physiology| vauthors = Shuster M |isbn= 9781464151132 |edition=Second|location=New York, NY|oclc=884499940|date = 2014-03-14}}</ref> Hormones are secreted by internal [[gland]]s that are part of an [[animal]]'s [[endocrine system]]. In [[vertebrate]]s, the [[hypothalamus]] is the neural control center for all endocrine systems. In [[humans]] specifically, the major [[endocrine gland]]s are the [[thyroid gland]] and the [[adrenal gland]]s. Many other organs that are part of other body systems have secondary endocrine functions, including [[bone]], [[kidney]]s, [[liver]], [[heart]] and [[gonad]]s. For example, kidneys secrete the endocrine hormone [[erythropoietin]]. Hormones can be amino acid complexes, [[steroid]]s, [[eicosanoid]]s, [[leukotriene]]s, or [[prostaglandin]]s.<ref name="Marieb">{{cite book |last=Marieb |first=Elaine |name-list-style=vanc |title=Anatomy & physiology |publisher=Pearson Education, Inc |location=Glenview, IL |year=2014 |isbn=978-0-321-86158-0}}</ref> The endocrine system can be contrasted to both [[exocrine glands]], which secrete hormones to the outside of the body, and [[paracrine signaling]] between cells over a relatively short distance. Endocrine glands have no [[Duct (anatomy)|ducts]], are vascular, and commonly have intracellular vacuoles or granules that store their hormones. In contrast, exocrine glands, such as [[salivary gland]]s, [[sweat gland]]s, and glands within the [[Human gastrointestinal tract|gastrointestinal tract]], tend to be much less vascular and have ducts or a hollow [[Lumen (anatomy)|lumen]].

===Animal reproduction===
[[File:Odonata copulation.jpg|thumb | right | [[Sexual reproduction]] in [[Dragonfly|dragonflies]]]]
Animals can [[Reproduction|reproduce]] in one of two ways: [[Asexual reproduction#Examples in animals|asexual]] and [[Sexual reproduction#Animals|sexual]]. Nearly all animals engage in some form of sexual reproduction.<ref>{{cite book |last=Knobil |first=Ernst |title=Encyclopedia of reproduction, Volume 1 |year=1998 |publisher=Academic Press |isbn=978-0-12-227020-8 |page=[https://archive.org/details/encyclopediaofre0000unse_f1r2/page/315 315] |url=https://archive.org/details/encyclopediaofre0000unse_f1r2/page/315 }}</ref> They produce [[haploid]] [[gamete]]s by [[meiosis]]. The smaller, motile gametes are [[Spermatozoon|spermatozoa]] and the larger, non-motile gametes are [[Egg cell|ova]].<ref>{{cite book |last=Schwartz |first=Jill |title=Master the GED 2011 |year=2010 |publisher=Peterson's |isbn=978-0-7689-2885-3 |page=[https://archive.org/details/petersonsmasterg0000stew_x3f1/page/371 371] |url=https://archive.org/details/petersonsmasterg0000stew_x3f1/page/371 }}</ref> These fuse to form [[zygote]]s,<ref>{{cite book |last=Hamilton |first=Matthew B. |title=Population genetics |url=https://archive.org/details/populationgeneti00hami |url-access=limited |year=2009 |publisher=Wiley-Blackwell |isbn=978-1-4051-3277-0 |page=[https://archive.org/details/populationgeneti00hami/page/n69 55]}}</ref> which develop via [[mitosis]] into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, and develop into a new sponge.<ref>{{cite book |last1=Ville |first1=Claude Alvin |last2=Walker |first2=Warren Franklin |last3=Barnes |first3=Robert D. |title=General zoology |year=1984 |publisher=Saunders College Pub |isbn=978-0-03-062451-3 |page=467}}</ref> In most other groups, the blastula undergoes more complicated rearrangement.<ref>{{cite book |last1=Hamilton |first1=William James |last2=Boyd |first2=James Dixon |last3=Mossman |first3=Harland Winfield |title=Human embryology: (prenatal development of form and function) |year=1945 |publisher=Williams & Wilkins |page=330}}</ref> It first [[Invagination|invaginates]] to form a [[gastrula]] with a digestive chamber and two separate [[germ layer]]s, an external [[ectoderm]] and an internal [[endoderm]].<ref>{{cite book |last=Philips |first=Joy B. |title=Development of vertebrate anatomy |year=1975 |publisher=Mosby |isbn=978-0-8016-3927-2 |page=[https://archive.org/details/developmentofver0000phil/page/176 176] |url=https://archive.org/details/developmentofver0000phil/page/176 }}</ref> In most cases, a third germ layer, the [[mesoderm]], also develops between them.<ref>{{cite book |title=The Encyclopedia Americana: a library of universal knowledge, Volume 10 |year=1918 |publisher=Encyclopedia Americana Corp. |page=281}}</ref> These germ layers then differentiate to form tissues and organs.<ref>{{cite book |last1=Romoser |first1=William S. |author-link1=William S. Romoser |last2=Stoffolano |first2=J. G. |title=The science of entomology |year=1998 |publisher=WCB McGraw-Hill |isbn=978-0-697-22848-2 |page=156}}</ref> Some animals are capable of [[asexual reproduction]], which often results in a genetic clone of the parent. This may take place through [[Fragmentation (reproduction)|fragmentation]]; [[budding]], such as in [[Hydra (genus)|''Hydra'']] and other [[cnidaria]]ns; or [[parthenogenesis]], where fertile eggs are produced without [[mating]], such as in [[aphid]]s.<ref>{{cite book |last1=Adiyodi |first1=K.G. |last2=Hughes |first2=Roger N. |last3=Adiyodi |first3=Rita G. |title=Reproductive Biology of Invertebrates, Volume 11, Progress in Asexual Reproduction |date=July 2002 |publisher=Wiley |page=116 |isbn=978-0-471-48968-9}}</ref><ref>{{cite web |last1=Schatz |first1=Phil |title=Concepts of Biology {{!}} How Animals Reproduce |url=http://philschatz.com/biology-concepts-book/contents/m45547.html |publisher=OpenStax College |access-date=5 March 2018 |archive-url=https://web.archive.org/web/20180306022745/http://philschatz.com/biology-concepts-book/contents/m45547.html |archive-date=6 March 2018 |url-status=live }}</ref>{{-}}

===Animal development===
{{Further | Developmental biology | Embryology}}
[[File:Cleavage.png|right|thumb|[[Cleavage (embryo)|Cleavage]] in zebrafish embryo]]
Animal development begins with the formation of a [[zygote]] that results from the fusion of a [[sperm]] and [[egg cell|egg]] during [[fertilization]].<ref>{{cite journal | vauthors = Jungnickel MK, Sutton KA, Florman HM | title = In the beginning: lessons from fertilization in mice and worms | journal = Cell | volume = 114 | issue = 4 | pages = 401–4 | date = August 2003 | pmid = 12941269 | doi = 10.1016/s0092-8674(03)00648-2 | doi-access = free }}</ref> The zygote undergoes a rapid multiple rounds of mitotic cell period of cell divisions called [[cleavage (embryo)|cleavage]], which forms a ball of similar cells called a [[blastula]]. [[Gastrulation]] occurs, whereby morphogenetic movements convert the cell mass into a three [[germ layers]] that comprise the [[ectoderm]], [[mesoderm]] and [[endoderm]].

The end of gastrulation signals the beginning of [[organogenesis]], whereby the three [[germ layer]]s form the [[internal organ]]s of the organism.<ref name="gilbert2017">{{cite journal |last1=Gilbert|first1=S. F. |last2=Barresi|first2=M. J. F. |title=Developmental Biology, 11Th Edition 2016 |date=2017-05-01|journal=American Journal of Medical Genetics Part A | volume=173 | issue=5 | page=1430 | doi=10.1002/ajmg.a.38166 | issn = 1552-4833 }}</ref> The cells of each of the three germ layers undergo [[Cellular differentiation|differentiation]], a process where less-specialized cells become more-specialized through the expression of a specific set of genes. Cellular differentiation is influenced by extracellular signals such as growth factors that are exchanged to adjacent cells, which is called [[Juxtacrine signalling|juxtracrine]] signaling, or to neighboring cells over short distances, which is called [[paracrine signaling]].<ref>{{Cite journal|last=Edlund|first=Helena|date=July 2002|title=Organogenesis: Pancreatic organogenesis — developmental mechanisms and implications for therapy|journal=Nature Reviews Genetics|volume=3|issue=7|pages=524–532|doi=10.1038/nrg841|pmid=12094230|s2cid=2436869|issn=1471-0064}}</ref><ref>{{Cite journal|last=Rankin|first=Scott|date=2018|title=Timing is everything: Reiterative Wnt, BMP and RA signaling regulate developmental competence during endoderm organogenesis|journal=Developmental Biology|volume=434|issue=1|pages=121–132|via=NCBI|doi=10.1016/j.ydbio.2017.11.018|pmc=5785443|pmid=29217200}}</ref> Intracellular signals consist of a cell signaling itself ([[autocrine signaling]]), also play a role in organ formation. These signaling pathways allows for cell rearrangement and ensures that organs form at specific sites within the organism.<ref name="gilbert2017" /><ref>{{Cite journal|last1=Ader|first1=Marius|last2=Tanaka|first2=Elly M|title=Modeling human development in 3D culture|journal=Current Opinion in Cell Biology|volume=31|pages=23–28|doi=10.1016/j.ceb.2014.06.013|pmid=25033469|year=2014}}</ref>

===Immune system===
{{Further | Immunology}}
[[File:Primary immune response 1.png|thumb|right|Processes in the primary immune response]]
The [[immune system]] is a network of [[biological process]]es that detects and responds to a wide variety of [[pathogen]]s. Many species have two major subsystems of the immune system. The [[innate immune system]] provides a preconfigured response to broad groups of situations and stimuli. The [[adaptive immune system]] provides a tailored response to each stimulus by learning to recognize molecules it has previously encountered. Both use [[humoral immunity|molecules]] and [[cell-mediated immunity|cells]] to perform their functions.

Nearly all organisms have some kind of immune system. [[Bacteria]] have a rudimentary immune system in the form of [[enzyme]]s that protect against [[bacteriophage|virus]] infections. Other basic immune mechanisms evolved in ancient [[eukaryote|plants and animals]] and remain in their modern descendants. These mechanisms include [[phagocytosis]], [[antimicrobial peptides]] called [[defensin]]s, and the [[complement system]]. [[Jawed vertebrate]]s, including humans, have even more sophisticated defense mechanisms, including the ability to adapt to recognize pathogens more efficiently. Adaptive (or acquired) immunity creates an [[immunological memory]] leading to an enhanced response to subsequent encounters with that same pathogen. This process of acquired immunity is the basis of [[vaccination]].

===Animal behavior===
{{Further | Ethology}}
[[File:Reed_warbler_cuckoo.jpg|thumb|150px|left|Brood parasites, such as the cuckoo, provide a supernormal stimulus to the parenting species.]]
[[Behavior]]s play a central a role in animals' interaction with each other and with their environment.<ref name="hillisetal2014an">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = Animal behavior| title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 827–844 | isbn = 978-1464175121}}</ref> They are able to use their muscles to approach one another, [[Animal communication|vocalize]], seek shelter, and [[migration (ecology)|migrate]]. An animal's [[nervous system]] activates and coordinates its behaviors. [[Fixed action pattern]]s, for instance, are genetically determined and stereotyped behaviors that occur without learning.<ref name="hillisetal2014an" /><ref name="páez-rondón2018">{{Cite journal |last1=Páez-Rondón |first1=Oscar |last2=Aldana |first2=Elis |last3=Dickens |first3=Joseph |last4=Otálora-Luna |first4=Fernando |date=May 2018 |title=Ethological description of a fixed action pattern in a kissing bug (Triatominae): vision, gustation, proboscis extension and drinking of water and guava |journal=Journal of Ethology |volume=36 |issue=2 |pages=107–116 |doi=10.1007/s10164-018-0547-y |issn=0289-0771|doi-access=free }}</ref> These behaviors are under the control of the nervous system and can be quite elaborate.<ref name="hillisetal2014an" /> Examples include the pecking of [[kelp gull]] chicks at the red dot on their mother's beak. Other behaviors that have emerged as a result of [[natural selection]] include [[foraging]], [[mating]], and [[Altruism (biology)|altruism]].<ref name = "urry2017ay">{{cite book | last1 = Urry | first1 = Lisa | last2 = Cain | first2 = Michael | last3 = Wasserman | first3 =  Steven | last4 = Minorsky | first4 = Peter |  last5 = Reece | first5 = Jane | chapter = Animal behavior | title = Campbell Biology | publisher = Pearson | edition = 11th | date = 2017 | location = New York | pages = 1137–1161 | isbn = 978-0134093413}}</ref> In addition to evolved behavior, animals have evolved the ability to learn by modifying their behaviors as a result of early individual experiences.<ref name="hillisetal2014an" />

==Ecology==
{{Main|Ecology}}
[[Ecology]] is the study of the distribution and abundance of [[life]], the interaction between organisms and their [[natural environment|environment]].<ref>{{cite book |author1=Begon, M |title=Ecology: From individuals to ecosystems |author2=Townsend, CR |author3=Harper, JL |publisher=Blackwell |year=2006 |isbn=978-1-4051-1117-1 |edition=4th}}</ref> 

===Ecosystems===
{{Main|Ecosystem}}
[[File:Climate influence on terrestrial biome.svg|thumb|right | Terrestrial biomes are shaped by temperature and precipitation.]]
The [[community (ecology)|community]] of living ([[Biotic component|biotic]]) organisms in conjunction with the nonliving ([[Abiotic component|abiotic]]) components (e.g., water, light, radiation, temperature, [[humidity]], [[atmosphere]], [[acidity]], and soil) of their environment is called an [[ecosystem]].<ref name="habitats_of_the_world">{{cite book | title=Habitats of the world | year=2004 | url=https://books.google.com/books?id=U-_mlcy8rGgC&pg=PA238 | publisher=Marshall Cavendish | location=New York | isbn=978-0-7614-7523-1 | page=238 | access-date=2020-08-24 | archive-date=2021-04-15 | archive-url=https://web.archive.org/web/20210415113154/https://books.google.com/books?id=U-_mlcy8rGgC&pg=PA238 | url-status=live }}</ref><ref>Tansley (1934); Molles (1999), p. 482; Chapin ''et al.'' (2002), p. 380; Schulze ''et al.'' (2005); p. 400; Gurevitch ''et al.'' (2006), p. 522; Smith & Smith 2012, p. G-5</ref><ref name="hillisetal2014ao">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = The distribution of Earth's ecological systems | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 845–863 | isbn = 978-1464175121}}</ref> These biotic and abiotic components are linked together through nutrient cycles and energy flows.<ref name="Odum1971">{{cite book|title=Fundamentals of Ecology|url=https://archive.org/details/fundamentalsofec0000odum|url-access=registration|last=Odum|first=Eugene P|date=1971|publisher=Saunders|isbn=978-0-534-42066-6|edition=third|location=New York}}</ref> Energy from the sun enters the system through [[photosynthesis]] and is incorporated into plant tissue. By feeding on plants and on one another, animals play an important role in the movement of [[matter]] and [[energy]] through the system. They also influence the quantity of plant and [[Microbe|microbial]] [[Biomass (ecology)|biomass]] present. By breaking down dead [[organic matter]], [[decomposer]]s release [[carbon]] back to the atmosphere and facilitate [[nutrient cycling]] by converting nutrients stored in dead biomass back to a form that can be readily used by plants and other microbes.<ref name="chapinetal2002a">{{cite book | last1 = Chapin III | first1 = F. Stuart | last2 = Matson | first2 = Pamela A. | last3 = Mooney | first3 = Harold A. | chapter = The ecosystem concept | title = Principles of Terrestrial Ecosystem Ecology | date = 2002 | pages = 10 | publisher = Springer | location = New York | isbn = 978-0-387-95443-1}}</ref>

The Earth's physical environment is shaped by [[solar energy]] and [[topography]].<ref name="hillisetal2014ao" /> The amount of solar energy input varies in space and time due to the spherical shape of the Earth and its axial [[Earth#Axial tilt and seasons|tilt]]. Variation in solar energy input drives [[weather]] and [[climate]] patterns. Weather is the day-to-day temperature and [[precipitation (meteorology)|precipitation]] activity, whereas climate is the long-term average of weather, typically averaged over a period of 30 years.<ref name="IPCC-2015">{{cite web |editor-last=Planton |editor-first=Serge |title=Annex III. Glossary: IPCC – Intergovernmental Panel on Climate Change |url=http://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_AnnexIII_FINAL.pdf |date=2013 |work=[[IPCC Fifth Assessment Report]] |page=1450 |access-date=25 July 2016 |archive-url=https://web.archive.org/web/20160524223615/http://www.ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_AnnexIII_FINAL.pdf |archive-date=2016-05-24 |url-status=dead }}</ref><ref name="NASA-20050201">{{cite web |last1=Shepherd |first1=Dr. J. Marshall |last2=Shindell |first2=Drew |last3=O'Carroll |first3=Cynthia M. |title=What's the Difference Between Weather and Climate? |url=http://www.nasa.gov/mission_pages/noaa-n/climate/climate_weather.html |date=1 February 2005 |work=[[NASA]] |access-date=13 November 2015 |archive-date=22 September 2020 |archive-url=https://web.archive.org/web/20200922095736/https://www.nasa.gov/mission_pages/noaa-n/climate/climate_weather.html/ |url-status=live }}</ref> Variation in topography also produces environmental heterogeneity. On the [[Windward and leeward|windward]] side of a mountain, for example, air rises and cools, with water changing from gaseous to liquid or solid form, resulting in [[precipitation]] such as rain or snow.<ref name="hillisetal2014ao" /> As a result, wet environments allow for lush vegetation to grow. In contrast, conditions tend to be dry on the leeward side of a mountain due to the lack of precipitation as air descends and warms, and moisture remains as water vapor in the atmosphere. [[Temperature]] and precipitation are the main factors that shape terrestrial [[biome]]s.

===Populations===
{{Further | Population ecology}}
[[File:Logistic_Carrying_Capacity.svg|thumb|right|Reaching carrying capacity through a logistic growth curve]]
A [[population]] is the number of [[organism]]s of the same [[species]] that occupy an [[geographical area|area]] and [[Sexual reproduction|reproduce]] from generation to generation.<ref name="hillisetal2014ap">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4 = Price | first4 = Mary V. | chapter = Populations| title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 864–897 | isbn = 978-1464175121}}</ref><ref name = "urry2017ba">{{cite book | last1 = Urry | first1 = Lisa | last2 = Cain | first2 = Michael | last3 = Wasserman | first3 =  Steven | last4 = Minorsky | first4 = Peter |  last5 = Reece | first5 = Jane | chapter = Population ecology | title = Campbell Biology | publisher = Pearson | edition = 11th | date = 2017 | location = New York | pages = 1188–1211 | isbn = 978-0134093413}}</ref><ref>{{cite web |title=Population |url=http://www.biology-online.org/dictionary/Population |publisher=Biology Online |access-date=5 December 2012 |archive-date=13 April 2019 |archive-url=https://web.archive.org/web/20190413145351/https://www.biology-online.org/dictionary/Population |url-status=live }}</ref><ref>{{cite web |title=Definition of population (biology) |url=http://oxforddictionaries.com/definition/english/population?q=population |work=Oxford Dictionaries |publisher=Oxford University Press |access-date=5 December 2012 |quote=a community of animals, plants, or humans among whose members interbreeding occurs |archive-date=4 March 2016 |archive-url=https://web.archive.org/web/20160304104019/http://www.oxforddictionaries.com/definition/english/population?q=population |url-status=dead }}</ref><ref>{{cite book | last=Hartl | first=Daniel | title=Principles of Population Genetics | publisher=[[Sinauer Associates]] | year=2007 | isbn=978-0-87893-308-2 | page=45}}</ref> Its abundance can be measured using [[population density]], which is the number of individuals per unit area (e.g., land or tree) or volume (e.g., sea or air).<ref name="hillisetal2014ap" /> Given that it is usually impractical to count every individual within a large population to determine its size, [[population size]] can be estimated by multiplying population density by the area or volume. [[Population growth]] during short-term intervals can be determined using the [[Population growth#Population growth rate|population growth rate equation]], which takes into consideration [[Birth rate|birth]], [[Death Rate|death]], and [[immigration rate]]s. In the longer term, the [[exponential growth]] of a population tends to slow down as it reaches its [[carrying capacity]], which can be modeled using the [[Logistic function|logistic equation]].<ref name = "urry2017ba"/> The carrying capacity of an [[Natural environment|environment]] is the maximum population size of a [[species]] that can be sustained by that specific environment, given the food, [[habitat]], [[Drinking water|water]], and other [[resource]]s that are available.<ref name=":32">{{Cite journal|date=2018-01-01|title=The flexible application of carrying capacity in ecology|journal=Global Ecology and Conservation|language=en|volume=13|pages=e00365|doi=10.1016/j.gecco.2017.e00365|issn=2351-9894|doi-access=free|last1=Chapman|first1=Eric J.|last2=Byron|first2=Carrie J.}}</ref> The carrying capacity of a population can be affected by changing environmental conditions such as changes in the availability resources and the cost of maintaining them. In [[human population]]s, new [[technology|technologies]] such as the [[Green revolution]] have helped increase the Earth's carrying capacity for humans over time, which has stymied the attempted predictions of impending population decline, the famous of which was by [[Thomas Robert Malthus|Thomas Malthus]] in the 18th century.<ref name="hillisetal2014ap" />

===Communities===
{{Main|Community (ecology)}}
[[File:TrophicWeb.jpg|thumb|370px | A (a) trophic pyramid and a (b) simplified food web. The trophic pyramid represents the biomass at each level.<ref name="Odum05">{{cite book | last1=Odum | first1=E. P. | last2=Barrett | first2=G. W. | title=Fundamentals of Ecology | edition=5th | publisher=Brooks/Cole, a part of Cengage Learning | year=2005 | isbn=978-0-534-42066-6 | url=http://www.cengage.com/aushed/instructor.do?disciplinenumber=1041&product_isbn=9780534420666&courseid=BI03&codeid=2BF6&subTab=&mainTab=About_the_Book&mailFlag=true&topicName= | url-status=dead | archive-url=https://web.archive.org/web/20110820163059/http://www.cengage.com/aushed/instructor.do?disciplinenumber=1041&product_isbn=9780534420666&courseid=BI03&codeid=2BF6&subTab=&mainTab=About_the_Book&mailFlag=true&topicName= | archive-date=2011-08-20 }}</ref>]]
A community is a group of [[population]]s of two or more different [[species]] occupying the same geographical area at the same time. A [[biological interaction]] is the effect that a pair of [[organism]]s living together in a community have on each other. They can be either of the same species (intraspecific interactions), or of different species (interspecific interactions). These effects may be short-term, like [[pollination]] and [[predation]], or long-term; both often strongly influence the [[evolution]] of the species involved. A long-term interaction is called a [[symbiosis]]. Symbioses range from [[Mutualism (biology)|mutualism]], beneficial to both partners, to [[competition (biology)|competition]], harmful to both partners.<ref>{{cite journal |last1=Wootton |first1=JT |last2=Emmerson |first2=M |title=Measurement of Interaction Strength in Nature |journal=[[Annual Review of Ecology, Evolution, and Systematics]] |volume=36|pages=419–44|year=2005|jstor=30033811 |doi=10.1146/annurev.ecolsys.36.091704.175535}}</ref>

Every species participates as a consumer, resource, or both in [[consumer–resource interactions]], which form the core of [[food chain]]s or [[food web]]s.<ref name="hillisetal2014aq">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = Ecological and evolutionary consequences within and among species | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 882–897| isbn = 978-1464175121}}</ref> There are different [[trophic level]]s within any food web, with the lowest level being the primary producers (or [[autotroph]]s) such as plants and algae that convert energy and inorganic material into [[organic compound]]s, which can then be used by the rest of the community.<ref name=bryantfrigaard/><ref>{{cite book | author=Smith, AL |title=Oxford dictionary of biochemistry and molecular biology |publisher=Oxford University Press |location=Oxford [Oxfordshire] |year=1997 |page=508 | isbn=978-0-19-854768-6 | quote=Photosynthesis – the synthesis by organisms of organic chemical compounds, esp. carbohydrates, from carbon dioxide using energy obtained from light rather than the oxidation of chemical compounds.}}</ref><ref>{{cite journal | last=Edwards | first=Katrina | title=Microbiology of a Sediment Pond and the Underlying Young, Cold, Hydrologically Active Ridge Flank | journal=Woods Hole Oceanographic Institution }}</ref> At the next level are the [[heterotroph]]s, which are the species that obtain energy by breaking apart organic compounds from other organisms.<ref name="hillisetal2014aq" /> Heterotrophs that consume plants are primary consumers (or [[herbivore]]s) whereas heterotrophs that consume herbivores are secondary consumers (or [[carnivore]]s). And those that eat secondary consumers are tertiary consumers and so on. [[Omnivore|Omnivorous]] heterotrophs are able to consume at multiple levels. Finally, there are [[decomposer]]s that feed on the waste products or dead bodies of organisms.<ref name="hillisetal2014aq" />

On average, the total amount of energy incorporated into the [[biomass]] of a trophic level per unit of time is about one-tenth of the energy of the trophic level that it consumes. Waste and dead material used by decomposers as well as heat lost from metabolism make up the other ninety percent of energy that is not consumed by the next trophic level.<ref name="hillisetal2014ar">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = Ecological communities | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 898–915| isbn = 978-1464175121}}</ref>

===Biosphere===
{{Main|Biosphere}}
[[File:Carbon cycle.jpg|thumb|250px|right| [[Carbon cycle#Fast and slow cycles|Fast carbon cycle]] showing the movement of carbon between land, atmosphere, and oceans in billions of tons per year. Yellow numbers are natural fluxes, red are human contributions, white are stored carbon. Effects of the [[Carbon cycle#Fast and slow cycles|slow carbon cycle]], such as volcanic and tectonic activity, are not included.<ref name="nasacc">{{cite web|last1=Riebeek|first1=Holli|title=The Carbon Cycle|url=http://earthobservatory.nasa.gov/Features/CarbonCycle/?src=eoa-features|website=Earth Observatory|publisher=NASA|access-date=5 April 2018|date=16 June 2011|archive-url=https://web.archive.org/web/20160305010126/http://earthobservatory.nasa.gov/Features/CarbonCycle/?src=eoa-features|archive-date=5 March 2016|url-status=live|df=dmy-all}}</ref>]]
In the global ecosystem (or biosphere), [[matter]] exist as different interacting compartments, which can be biotic or abiotic as well as accessible or inaccessible, depending on their forms and locations.<ref name="hillisetal2014as">{{cite book | last1 = Hillis | first1 = David M. | last2 = Sadava | first2 = David | last3 = Hill | first3 = Richard W. | last4= Price | first4 = Mary V. | chapter = The distribution of Earth's ecological systems | title = Principles of Life | publisher = Sinauer Associates | edition = 2nd | date = 2014 | location = Sunderland, Mass. | pages = 916–934 | isbn = 978-1464175121}}</ref> For example, matter from terrestrial autotrophs are both biotic and accessible to other organisms whereas the matter in rocks and minerals are abiotic and inaccessible. A [[biogeochemical cycle]] is a pathway by which specific [[chemical element|element]]s of matter are turned over or moved through the biotic ([[biosphere]]) and the abiotic ([[lithosphere]], [[atmosphere]], and [[hydrosphere]]) compartments of [[Earth]]. There are biogeochemical cycles for [[nitrogen cycle|nitrogen]], [[carbon cycle|carbon]], and [[water cycle|water]]. In some cycles there are ''reservoirs'' where a substance remains or is [[:Wiktionary:sequestered|sequestered]] for a long period of time.

[[Climate change]] includes both global warming driven by human-induced emissions of [[greenhouse gas]]es and the resulting large-scale shifts in weather patterns. Though there have been [[Climate variability and change|previous periods of climatic change]], since the mid-20th century humans have had an unprecedented impact on Earth's climate system and caused change on a global scale.<ref>{{Harvnb|IPCC AR5 WG1 Summary for Policymakers|2013|p=4|ps=: Warming of the climate system is unequivocal, and since the 1950s many of the observed changes are unprecedented over decades to millennia. The atmosphere and ocean have warmed, the amounts of snow and ice have diminished, sea level has risen, and the concentrations of greenhouse gases have increased}}; {{harvnb|IPCC SR15 Ch1|2018|p=54|ps=: Abundant empirical evidence of the unprecedented rate and global scale of impact of human influence on the Earth System (Steffen et al., 2016; Waters et al., 2016) has led many scientists to call for an acknowledgment that the Earth has entered a new geological epoch: the [[Anthropocene]].}}</ref> The largest driver of warming is the [[Greenhouse gas emissions|emission of greenhouse gases]], of which more than 90% are [[carbon dioxide]] and [[methane]].<ref>{{harvnb|EPA|2020|ps=: Carbon dioxide (76%), Methane (16%), Nitrous Oxide (6%).}}</ref> [[Fossil fuel]] burning ([[coal]], [[petroleum|oil]], and [[natural gas]]) for [[World energy consumption|energy consumption]] is the main source of these emissions, with additional contributions from agriculture, deforestation, and [[Industrial processes#Chemical processes by main basic material|manufacturing]].<ref>{{harvnb|EPA|2020|ps=: Carbon dioxide enters the atmosphere through burning fossil fuels (coal, natural gas, and oil), solid waste, trees and other biological materials, and also as a result of certain chemical reactions (e.g., manufacture of cement). Fossil fuel use is the primary source of {{CO2}}. {{CO2}} can also be emitted from direct human-induced impacts on forestry and other land use, such as through deforestation, land clearing for agriculture, and degradation of soils. Methane is emitted during the production and transport of coal, natural gas, and oil. [[Methane emissions]] also result from livestock and other agricultural practices and by the decay of organic waste in municipal solid waste landfills.}}</ref> Temperature rise is accelerated or tempered by [[Climate change feedback|climate feedbacks]], such as loss of [[Albedo|sunlight-reflecting snow and ice cover]], increased [[water vapor]] (a greenhouse gas itself), and changes to [[Carbon sink|land and ocean carbon sinks]].

===Conservation===
{{Main|Conservation biology}}
Conservation biology is the study of the conservation of [[Earth]]'s [[biodiversity]] with the aim of protecting [[species]], their [[habitats]], and [[ecosystems]] from excessive rates of [[extinction]] and the erosion of biotic interactions.<ref name="SahneyBenton2008RecoveryFromProfoundExtinction">{{cite journal |last1=Sahney |first1=S. |last2=Benton |first2=M. J |title=Recovery from the most profound mass extinction of all time |journal=Proceedings of the Royal Society B: Biological Sciences |volume=275 |issue=1636 |pages=759–65 |year=2008 |pmid=18198148 |pmc=2596898 |doi=10.1098/rspb.2007.1370 }}</ref><ref name="ConsBiol80">{{cite book |author1=Soulé, Michael E. |author2=Wilcox, Bruce A. |title=Conservation biology: an evolutionary-ecological perspective |publisher=Sinauer Associates |location=Sunderland, Mass. |year=1980 |isbn=978-0-87893-800-1 }}</ref><ref>{{cite journal |last1=Soulé |first1=Michael E. |title=What is Conservation Biology? |journal=BioScience |volume=35 |issue=11 |pages=727–34 |year=1986 |url=http://www.michaelsoule.com/resource_files/85/85_resource_file1.pdf |doi=10.2307/1310054 |jstor=1310054 |publisher=American Institute of Biological Sciences |access-date=2021-05-15 |archive-date=2019-04-12 |archive-url=https://web.archive.org/web/20190412085412/http://www.michaelsoule.com/resource_files/85/85_resource_file1.pdf |url-status=dead }}</ref> It is concerned with factors that influence the maintenance, loss, and restoration of biodiversity and the science of sustaining evolutionary processes that engender [[genetics|genetic]], [[population]], [[species]], and ecosystem diversity.<ref name="Hunter96">{{cite book |author=Hunter, Malcolm L. |title=Fundamentals of conservation biology |publisher=Blackwell Science |location=Oxford |year=1996 |isbn=978-0-86542-371-8 |url=https://archive.org/details/fundamentalsofco00hunt }}</ref><ref name="Groom06">{{cite book |author1=Meffe, Gary K. |author2=Martha J. Groom |title=Principles of conservation biology |publisher=Sinauer Associates |location=Sunderland, Mass. |year=2006 |isbn=978-0-87893-518-5 |edition=3rd}}</ref><ref name="Dyke08">{{cite book |last=Van Dyke |first=Fred |date=2008 |title=Conservation biology: foundations, concepts, applications |location=New York |publisher=[[Springer-Verlag]] |edition=2nd |isbn=9781402068904 |oclc=232001738 |doi=10.1007/978-1-4020-6891-1 |url=https://books.google.com/books?id=Evh1UD3ZYWcC |access-date=2021-05-15 |archive-date=2020-07-27 |archive-url=https://web.archive.org/web/20200727115147/https://books.google.com/books?id=Evh1UD3ZYWcC |url-status=live }}</ref><ref name="SahneyBentonFerry2010LinksDiversityVertebrates">{{cite journal |last1=Sahney |first1=S. |last2=Benton |first2=M. J. |last3=Ferry |first3=P. A. |title=Links between global taxonomic diversity, ecological diversity and the expansion of vertebrates on land |journal=Biology Letters |volume=6 |issue=4 |pages=544–7 |year=2010 |pmid=20106856 |pmc=2936204 |doi=10.1098/rsbl.2009.1024 }}</ref> The concern stems from estimates suggesting that up to 50% of all species on the planet will disappear within the next 50 years,<ref name="Koh">{{cite journal |last1=Koh |first1=Lian Pin |last2=Dunn |first2=Robert R. |last3=Sodhi |first3=Navjot S. |last4=Colwell |first4=Robert K. |last5=Proctor |first5=Heather C. |last6=Smith |first6=Vincent S. |title=Species coextinctions and the biodiversity crisis |journal=Science |volume=305 |issue=5690 |pages=1632–4 |year=2004 |pmid=15361627 |doi=10.1126/science.1101101 |bibcode=2004Sci...305.1632K |s2cid=30713492 }}</ref> which has contributed to poverty, starvation, and will reset the course of evolution on this planet.<ref>Millennium Ecosystem Assessment (2005). ''Ecosystems and Human Well-being: Biodiversity Synthesis.'' World Resources Institute, Washington, D.C.[http://www.millenniumassessment.org/documents/document.354.aspx.pdf] {{Webarchive|url=https://web.archive.org/web/20191014033601/http://www.millenniumassessment.org/documents/document.354.aspx.pdf |date=2019-10-14 }}</ref><ref name="Jackson">{{cite journal |last1=Jackson |first1=J. B. C. |title=Ecological extinction and evolution in the brave new ocean |journal=Proceedings of the National Academy of Sciences |volume=105 |issue=Suppl 1 |pages=11458–65 |year=2008 |pmid=18695220 |pmc=2556419 |doi=10.1073/pnas.0802812105  |bibcode=2008PNAS..10511458J |doi-access=free }}</ref> [[Biodiversity]] affects the functioning of ecosystems, which provide a variety of [[Ecosystem services|services]] upon which people depend.

Conservation biologists research and educate on the trends of [[biodiversity loss]], species [[extinction]]s, and the negative effect these are having on our capabilities to [[Sustainability|sustain]] the well-being of human society. Organizations and citizens are responding to the [[Holocene extinction|current biodiversity crisis]] through conservation action plans that direct research, monitoring, and education programs that engage concerns at local through global scales.<ref name="Soule86">{{cite book|last = Soule| first= Michael E. | title=Conservation Biology: The Science of Scarcity and Diversity| year = 1986 | publisher = Sinauer Associates| page = 584 | isbn=978-0-87893-795-0}}</ref><ref name="Hunter96"/><ref name="Groom06" /><ref name="Dyke08"/>

== See also ==
{{div col|colwidth=20em}}
* [[Biology in fiction]]
* [[Glossary of biology]]
* [[List of biological websites]]
* [[List of biologists]]
* [[List of biology journals]]
* [[List of biology topics]]
* [[List of life sciences]]
* [[List of omics topics in biology]]
* [[National Association of Biology Teachers]]
* [[Outline of biology]]
* Periodic table of life sciences in [[Tinbergen's four questions]]
* [[Reproduction]]
* [[Science tourism]]
* [[:Category:Biology terminology|Terminology of biology]]
{{div col end}}{{Notelist}}

== References ==
{{Reflist}}


== Further reading ==
{{Further | Bibliography of biology}}
{{refbegin|30em}}
* {{cite book |last=Benton |first=Michael J. |author-link=Michael Benton |year=1997 |title=Vertebrate Palaeontology |edition=2nd |location=London |publisher=[[Chapman & Hall]] |isbn=978-0-412-73800-5 |oclc=37378512|title-link=Vertebrate Palaeontology (Benton)}}
* {{cite book |editor-last=Darwin |editor-first=Francis |editor-link=Francis Darwin |year=1909 |title=The foundations of The origin of species, a sketch written in 1842 |url=http://darwin-online.org.uk/converted/pdf/1909_Foundations_F1555.pdf |location=Cambridge |publisher=Printed at the University Press |lccn=61057537 |oclc=1184581 |access-date=27 November 2014 |archive-date=4 March 2016 |archive-url=https://web.archive.org/web/20160304111606/http://darwin-online.org.uk/converted/pdf/1909_Foundations_F1555.pdf |url-status=live }}
* {{cite book |last1=Hall |first1=Brian K. |last2=Hallgrímsson |first2=Benedikt |title=Strickberger's Evolution |date=6 December 2007 |publisher=Jones & Bartlett Publishers |isbn=978-1-4496-4722-3 |url=https://books.google.com/books?id=jrDD3cyA09kC |language=en}}
* {{cite book|vauthors=Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P|title=Molecular Biology of the Cell|url=https://archive.org/details/molecularbiolog000wils|url-access=registration|edition=4th|publisher=Garland|year=2002|isbn=978-0-8153-3218-3|oclc=145080076}}
* {{cite book |vauthors=Begon M, Townsend CR, Harper JL| title=Ecology: From Individuals to Ecosystems|edition=4th|publisher=Blackwell Publishing Limited|year=2005|isbn=978-1-4051-1117-1|oclc=57639896| title-link=Ecology: From Individuals to Ecosystems}}
* {{cite book|last=Campbell|first=Neil| name-list-style=vanc |author-link=Neil Campbell (scientist)|title=Biology|edition=7th|publisher=Benjamin-Cummings Publishing Company|year=2004|isbn=978-0-8053-7146-8|oclc=71890442}}
* {{cite book|last=Colinvaux|first=Paul|name-list-style=vanc|author-link=Paul Colinvaux|title=Why Big Fierce Animals are Rare: An Ecologist's Perspective|edition=reissue|publisher=Princeton University Press|year=1979|isbn=978-0-691-02364-9|oclc=10081738|url=https://archive.org/details/whybigfierceanim00paul}}
* {{cite book|last=Mayr|first=Ernst|title=The Growth of Biological Thought: Diversity, Evolution, and Inheritance|url=https://books.google.com/books?id=pHThtE2R0UQC|year=1982|publisher=Harvard University Press|isbn=978-0-674-36446-2|access-date=2015-06-27|archive-date=2015-10-03|archive-url=https://web.archive.org/web/20151003080726/https://books.google.com/books?id=pHThtE2R0UQC|url-status=live}}
* {{cite book|last=Hoagland|first=Mahlon| name-list-style=vanc |title=The Way Life Works|edition=reprint|publisher=Jones and Bartlett Publishers inc|year=2001|isbn=978-0-7637-1688-2|oclc=223090105}}
* {{cite book|last=Janovy|first=John |title=On Becoming a Biologist|edition=2nd|publisher=Bison Books|year=2004|isbn=978-0-8032-7620-8|oclc=55138571}}
* {{cite book|last=Johnson|first=George B.|author-link=Johnson George B.|title=Biology, Visualizing Life|publisher=Holt, Rinehart, and Winston|year=2005|isbn=978-0-03-016723-2|oclc=36306648|url=https://archive.org/details/holtbiologyvisua00john}}
* {{cite book|last1=Tobin | first1=Allan | last2=Dusheck | first2=Jennie |title=Asking About Life|edition=3rd|publisher=Wadsworth|location=Belmont, Calif. |year=2005|isbn=978-0-534-40653-0}}
* {{cite book |ref={{harvid|IPCC AR5 WG1|2013}}<!-- ipcc:20200215 -->
 |author=IPCC |author-link=IPCC
 |year=2013
 |title=Climate Change 2013: The Physical Science Basis
 |series=Contribution of Working Group I to the [[IPCC Fifth Assessment Report|Fifth Assessment Report]] of the Intergovernmental Panel on Climate Change
 |display-editors=4
 |editor1-first=T. F. |editor1-last=Stocker
 |editor2-first=D. |editor2-last=Qin
 |editor3-first=G.-K. |editor3-last=Plattner
 |editor4-first=M. |editor4-last=Tignor
 |editor5-first=S. K. |editor5-last=Allen
 |editor6-first=J. |editor6-last=Boschung
 |editor7-first=A. |editor7-last=Nauels
 |editor8-first=Y. |editor8-last=Xia
 |editor9-first=V. |editor9-last=Bex
 |editor10-first=P. M. |editor10-last=Midgley
 |publisher=Cambridge University Press
 |place=Cambridge, UK & New York
 |isbn=978-1-107-05799-9 <!-- ISBN in printed source is incorrect. -->
 |url=http://www.climatechange2013.org/images/report/WG1AR5_ALL_FINAL.pdf  <!-- Same file, new url per IPCC. -->
}}.
** {{cite book |ref={{harvid|IPCC AR5 WG1 Summary for Policymakers|2013}}
 |chapter=Summary for Policymakers
 |chapter-url=https://ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_SPM_FINAL.pdf
 |year=2013
 |author=IPCC |author-link=IPCC
 |title={{Harvnb|IPCC AR5 WG1|2013}}
}}
* {{cite book |ref={{harvid|IPCC SR15|2018}} <!-- ipcc:20200312 -->
 |author=IPCC |author-link=IPCC
 |year=2018
 |title=Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty
 |display-editors=4
 |editor-first1=V. |editor-last1=Masson-Delmotte
 |editor-first2=P. |editor-last2=Zhai
 |editor-first3=H.-O. |editor-last3=Pörtner
 |editor-first4=D. |editor-last4=Roberts
 |editor-first5=J. |editor-last5=Skea
 |editor-first6=P. R. |editor-last6=Shukla
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 |editor-first9=C. |editor-last9=Péan
 |editor-first10=R. |editor-last10=Pidcock
 |editor-first11=S. |editor-last11=Connors
 |editor-first12=J. B. R. |editor-last12=Matthews
 |editor-first13=Y. |editor-last13=Chen
 |editor-first14=X. |editor-last14=Zhou
 |editor-first15=M. I. |editor-last15=Gomis
 |editor-first16=E. |editor-last16=Lonnoy
 |editor-first17=T. |editor-last17=Maycock
 |editor-first18=M. |editor-last18=Tignor
 |editor-first19=T. |editor-last19=Waterfeld
 |publisher=Intergovernmental Panel on Climate Change
 |isbn=<!-- not issued? -->
 |url=https://www.ipcc.ch/site/assets/uploads/sites/2/2019/06/SR15_Full_Report_High_Res.pdf
}} [https://www.ipcc.ch/sr15/ Global Warming of 1.5 ºC —].
** {{cite book |ref={{harvid|IPCC SR15 Ch1|2018}} <!-- ipcc:20200312 -->
 |year=2018
 |chapter=Chapter 1: Framing and Context
 |chapter-url=https://www.ipcc.ch/site/assets/uploads/sites/2/2019/05/SR15_Chapter1_High_Res.pdf
 |display-authors=4
 |first1=M. R. |last1=Allen
 |first2=O. P. |last2=Dube
 |first3=W. |last3=Solecki
 |first4=F. |last4=Aragón-Durand
 |first5=W. |last5=Cramer
 |first6=S. |last6=Humphreys
 |first7=M. |last7=Kainuma
 |first8=J. |last8=Kala
 |first9=N. |last9=Mahowald
 |first10=Y. |last10=Mulugetta
 |first11=R. |last11=Perez
 |first12=M. |last12=Wairiu
 |first13=K. |last13=Zickfeld
 |title={{Harvnb|IPCC SR15|2018}}
 |pages=49–91
}}
* {{cite web |ref={{harvid|EPA|2020}}
|url=https://www.epa.gov/ghgemissions/overview-greenhouse-gases
 |title=Overview of Greenhouse Gases
 |author=US EPA
 |date=15 September 2020
|access-date=15 September 2020
}}
{{refend}}

== External links ==
{{Sister project links}}
* {{curlie|Science/Biology}}
* [http://phylonames.org/code/ OSU's Phylocode]
* [https://www.biologyonline.com/dictionary Biology Online – Wiki Dictionary]
* [https://ocw.mit.edu/courses/biology/7-012-introduction-to-biology-fall-2004/ MIT video lecture series on biology]
* [https://www.onezoom.org/ OneZoom Tree of Life]

'''Journal links'''
* [https://journals.plos.org/plosbiology/ PLOS Biology] A peer-reviewed, open-access journal published by the [[Public Library of Science]]
* [http://www.cell.com/current-biology/ ''Current Biology'']: General journal publishing [[original research]] from all areas of biology
* ''[https://royalsocietypublishing.org/journal/rsbl Biology Letters]'': A [[Impact factor|high-impact]] [[Royal Society]] journal publishing [[peer-review]]ed biology papers of general interest
* ''[https://www.science.org/collections Science]'': Internationally renowned [[American Association for the Advancement of Science|AAAS]] science journal – see sections of the life sciences
* ''[https://www.ijbs.com/ International Journal of Biological Sciences]'': A biological journal publishing significant peer-reviewed scientific papers
* ''[https://www.press.jhu.edu/journals/perspectives-biology-and-medicine Perspectives in Biology and Medicine]'': An [[interdisciplinarity|interdisciplinary]] [[scholarly method|scholarly]] journal publishing [[essay]]s of broad relevance

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{{二次利用|date=4 December 2022}}
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