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{{Short description|Mechanism of evolution by differential survival and reproduction of individuals}}{{For-multi|the Brad Laner album|Natural Selections|other uses}}
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[[File:Mutation and selection diagram (2).svg|thumb|A diagram demonstrating [[mutation]] and [[Selection (biology)|selection]]]]
{{Evolutionary biology}}

'''Natural selection''' is the differential survival and reproduction of individuals due to differences in [[phenotype]]. It is a key mechanism of [[evolution]], the change in the [[Heredity|heritable traits]] characteristic of a [[population<!--key concept in Darwinian biology-->]] over generations. [[Charles Darwin]] popularised the term "natural selection", contrasting it with [[selective breeding|artificial selection]], which is intentional, whereas natural selection is not.

[[Genetic diversity|Variation]] of traits, both [[Genotype|genotypic]] and phenotypic, exists within all populations of [[organism]]s. However, some traits are more likely to facilitate [[survival]] and [[reproductive success]]. Thus, these traits are passed {{not a typo|on to}} the next generation. These traits can also become more [[Allele frequency|common within a population]] if the environment that favours these traits remains fixed. If new traits become more favoured due to changes in a specific [[Ecological niche|niche]], [[microevolution]] occurs. If new traits become more favoured due to changes in the broader environment, [[macroevolution]] occurs. Sometimes, [[Speciation|new species can arise]] especially if these new traits are radically different from the traits possessed by their predecessors.

The likelihood of these traits being 'selected' and passed down are determined by many factors. Some are likely to be passed down because they [[Adaptation|adapt]] well to their environments. Others are passed down because these traits are actively preferred by mating partners, which is known as [[sexual selection]]. Female bodies also prefer traits that confer the lowest cost to their reproductive health, which is known as [[fecundity selection]].

Natural selection is a cornerstone of modern [[biology]]. The concept, published by Darwin and [[Alfred Russel Wallace]] in a [[On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection|joint presentation of papers in 1858]], was elaborated in Darwin's influential 1859 book ''[[On the Origin of Species|On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life]]''. He described natural selection as analogous to artificial selection, a process by which animals and plants with traits considered desirable by human breeders are systematically favoured for reproduction. The concept of natural selection originally developed in the absence of a valid theory of heredity; at the time of Darwin's writing, science had yet to develop modern theories of genetics. The union of traditional [[Darwinism|Darwinian evolution]] with subsequent discoveries in [[classical genetics]] formed the [[Modern synthesis (20th century)|modern synthesis of the mid-20th century]]. The addition of [[molecular genetics]] has led to [[evolutionary developmental biology]], which explains evolution at the molecular level. While genotypes can slowly change by random [[genetic drift]], natural selection remains the primary explanation for [[Adaptation|adaptive evolution]].

==Historical development==
{{Main|History of evolutionary thought}}

===Pre-Darwinian theories===
[[File:Aristotle Altemps Inv8575.jpg|thumb|upright|[[Aristotle]] considered whether different forms could have appeared, only the useful ones surviving.]]

Several philosophers of the [[classical era]], including [[Empedocles]]<ref>{{harvnb|Empedocles|1898|loc=[https://history.hanover.edu/texts/presoc/emp.html#book2 ''On Nature'', Book II]}}</ref> and his intellectual successor, the [[Roman Republic|Roman]] poet [[Lucretius]],<ref>{{harvnb|Lucretius|1916|loc=[http://classics.mit.edu/Carus/nature_things.5.v.html ''On the Nature of Things'', Book V]}}</ref> expressed the idea that nature produces a huge variety of creatures, randomly, and that only those creatures that manage to provide for themselves and reproduce successfully persist. Empedocles' idea that organisms arose entirely by the incidental workings of causes such as heat and cold was criticised by [[Aristotle]] in Book II of ''[[Physics (Aristotle)|Physics]]''.<ref>{{harvnb|Aristotle|loc=[http://classics.mit.edu/Aristotle/physics.2.ii.html ''Physics'', Book II, Chapters 4 and 8]}}</ref> He posited natural [[teleology]] in its place, and believed that form was achieved for a purpose, citing the regularity of heredity in species as proof.<ref>{{harvnb|Lear|1988|p=[https://books.google.com/books?id=hSAGlzPLq7gC&pg=PA38 38]}}</ref><ref name="henry">{{cite journal |last=Henry |first=Devin |date=September 2006 |title=Aristotle on the Mechanism of Inheritance |url=http://works.bepress.com/cgi/viewcontent.cgi?article=1010&context=devinhenry |journal=Journal of the History of Biology |volume=39 |issue=3 |pages=425–455 |doi=10.1007/s10739-005-3058-y|s2cid=85671523 }}</ref> Nevertheless, he accepted [[Aristotle's biology|in his biology]] that new types of animals, [[congenital disorder|monstrosities]] (τερας), can occur in very rare instances (''[[Generation of Animals]]'', Book IV).<ref>{{harvnb|Ariew|2002}}</ref> As quoted in Darwin's 1872 edition of ''[[The Origin of Species]]'', Aristotle considered whether different forms (e.g., of teeth) might have appeared accidentally, but only the useful forms survived:
{{Blockquote|So what hinders the different parts [of the body] from having this merely accidental relation in nature? as the teeth, for example, grow by necessity, the front ones sharp, adapted for dividing, and the grinders flat, and serviceable for masticating the food; since they were not made for the sake of this, but it was the result of accident. And in like manner as to the other parts in which there appears to exist an adaptation to an end. Wheresoever, therefore, all things together (that is all the parts of one whole) happened like as if they were made for the sake of something, these were preserved, having been appropriately constituted by an internal spontaneity, and whatsoever things were not thus constituted, perished, and still perish.|Aristotle|''Physics'', Book II, Chapter 8<ref>{{harvnb|Darwin|1872|p=[http://darwin-online.org.uk/content/frameset?itemID=F391&viewtype=text&pageseq=18 xiii]}}</ref>}}

But Aristotle rejected this possibility in the next paragraph, making clear that he is talking about <!--ontogeny --> the [[Developmental biology|development of animals as embryos]] with the phrase "either invariably or normally come about", not <!--phylogeny -->the origin of species:
{{Quote|...&nbsp;Yet it is impossible that this should be the true view. For teeth and all other natural things either invariably or normally come about in a given way; but of not one of the results of chance or spontaneity is this true. We do not ascribe to chance or mere coincidence the frequency of rain in winter, but frequent rain in summer we do; nor heat in the dog-days, but only if we have it in winter. If then, it is agreed that things are either the result of coincidence or for an end, and these cannot be the result of coincidence or spontaneity, it follows that they must be for an end; and that such things are all due to nature even the champions of the theory which is before us would agree. Therefore action for an end is present in things which come to be and are by nature.|Aristotle|''Physics'', Book II, Chapter 8<ref>{{harvnb|Aristotle|loc=[http://classics.mit.edu/Aristotle/physics.2.ii.html ''Physics'', Book II, Chapter 8]}}</ref>}}

The [[Struggle for existence#Historical development|struggle for existence]] was later described by the [[Islam]]ic writer [[Al-Jahiz]] in the 9th century, particularly in the context of top-down population regulation, but not in reference to individual variation or natural selection.<ref>{{cite journal |last=Zirkle |first=Conway |author-link=Conway Zirkle |date=25 April 1941 |title=Natural Selection before the 'Origin of Species' |journal=[[Proceedings of the American Philosophical Society]]|volume=84 |issue=1 |pages=71–123 |jstor=984852}}</ref><ref>{{harvnb|Agutter|Wheatley|2008|p=43}}</ref>

At the turn of the 16th century [[Leonardo da Vinci]] collected a set of fossils of ammonites as well as other biological material. He extensively reasoned in his writings that the shapes of animals are not given once and forever by the "upper power" but instead are generated in different forms naturally and then selected for reproduction by their compatibility with the environment.<ref>{{cite book|title=Leonardo, Codex C.|year=2016|publisher=Institut of France. Trans. Richter}}</ref>

The more recent classical arguments were reintroduced in the 18th century by [[Pierre Louis Maupertuis]]<ref>{{cite journal |last=Maupertuis |first=Pierre Louis |author-link=Pierre Louis Maupertuis |year=1746 |title=''Les Loix du mouvement et du repos déduites d'un principe metaphysique'' |trans-title=[[s:Translation:Derivation of the laws of motion and equilibrium from a metaphysical principle#I. Assessment of the Proofs of God's Existence that are Based on the Marvels of Nature|"Derivation of the laws of motion and equilibrium from a metaphysical principle"]] |language=fr |journal=Histoire de l'Académie Royale des Sciences et des Belles Lettres |location=Berlin |pages=267–294 |title-link=s:fr:Les Loix du mouvement et du repos déduites d'un principe metaphysique }}</ref> and others, including Darwin's grandfather, [[Erasmus Darwin]].

Until the early 19th century, the [[History of creationism#Renaissance to Darwin|prevailing view]] in [[Western world|Western societies]] was that differences between individuals of a species were uninteresting departures from their [[Theory of Forms|Platonic ideals]] (or [[wikt:typus|typus]]) of [[Baraminology|created kinds]]. However, the theory of [[uniformitarianism]] in geology promoted the idea that simple, weak forces could act continuously over long periods of time to produce radical changes in the Earth's landscape. The success of this theory raised awareness of the vast scale of [[Geologic time scale|geological time]] and made plausible the idea that tiny, virtually imperceptible changes in successive generations could produce consequences on the scale of differences between species.<ref name="Bowler 2003">{{cite book |last=Bowler |first=Peter J. |year=2003 |title=Evolution: The History of an Idea |edition=3rd |location=Berkeley, CA |publisher=[[University of California Press]] |isbn=978-0-520-23693-6 |oclc=43091892 |pages=[https://archive.org/details/evolutionhistory0000bowl_n7y8/page/129 129–134], [https://archive.org/details/evolutionhistory0000bowl_n7y8/page/158 158] |url=https://archive.org/details/evolutionhistory0000bowl_n7y8/page/129 }}</ref>

The early 19th-century zoologist [[Jean-Baptiste Lamarck]] suggested the [[inheritance of acquired characteristics]] as a mechanism for evolutionary change; adaptive traits acquired by an organism during its lifetime could be inherited by that organism's progeny, eventually causing [[transmutation of species]].<ref>{{harvnb|Lamarck|1809}}</ref> This theory, [[Lamarckism]], was an influence on the Soviet biologist [[Trofim Lysenko]]'s ill-fated antagonism to mainstream genetic theory as late as the mid-20th century.<ref name="Joravsky">{{cite journal |last=Joravsky |first=David |date=January 1959 |title=Soviet Marxism and Biology before Lysenko |journal=[[Journal of the History of Ideas]] |volume=20 |issue=1 |pages=85–104 |doi=10.2307/2707968 |jstor=2707968 }}</ref>

Between 1835 and 1837, the zoologist [[Edward Blyth#On natural selection|Edward Blyth]] worked on the area of variation, artificial selection, and how a similar process occurs in nature. Darwin acknowledged Blyth's ideas in the first chapter on variation of ''On the Origin of Species''.<ref>{{harvnb|Darwin|1859|p=[http://darwin-online.org.uk/content/frameset?pageseq=33&itemID=F373&viewtype=text 18]}}</ref>

===Darwin's theory===

{{Main|Inception of Darwin's theory|Development of Darwin's theory}}
{{Further|Coloration evidence for natural selection}}

[[File:Charles Darwin aged 51.jpg|thumb|Modern biology began in the nineteenth century with [[Charles Darwin]]'s work on [[evolution]] by natural selection.]]

In 1859, Charles Darwin set out his theory of evolution by natural selection as an explanation for [[adaptation]] and speciation. He defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".<ref>{{harvnb|Darwin|1859|p=[http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=side&pageseq=76 61]}}</ref> The concept was simple but powerful: individuals best adapted to their environments are more likely to survive and reproduce. As long as there is some variation between them and that variation is [[Heritability|heritable]], there will be an inevitable selection of individuals with the most advantageous variations. If the variations are heritable, then differential reproductive success leads to the evolution of particular populations of a species, and populations that evolve to be sufficiently different eventually become different species.<ref>{{harvnb|Darwin|1859|p=[http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=text&pageseq=20 5]}}</ref><ref>{{cite book |title=Strickberger's Evolution |edition=4th |author1=Hall, Brian K. |author2=Hallgrímsson, Benedikt |publisher=Jones and Bartlett |date=2008 |pages=4–6 |url=https://books.google.com/books?id=jrDD3cyA09kC&pg=PA4 |isbn=978-0-7637-0066-9 |oclc=796450355}}</ref>

[[File:Malthus 1826 vol 1 page 435 top Table England Population Growth 1780-1810.jpg|thumb|Part of [[Thomas Malthus]]'s table of [[population growth]] in England 1780–1810, from his ''[[An Essay on the Principle of Population|Essay on the Principle of Population]]'', 6th edition, 1826]]

Darwin's ideas were inspired by the observations that he had made on the [[second voyage of HMS Beagle|second voyage of HMS ''Beagle'']] (1831–1836), and by the work of a political economist, <!--the Reverend -->[[Thomas Robert Malthus]], who, in ''[[An Essay on the Principle of Population]]'' (1798), noted that population (if unchecked) [[exponential growth|increases exponentially]], whereas the food supply grows only [[linear function|arithmetically]]; thus, inevitable limitations of resources would have demographic implications, leading to a "struggle for existence".<ref>{{harvnb|Malthus|1798}}</ref> When Darwin read Malthus in 1838 he was already primed by his work as a [[Natural history|naturalist]] to appreciate the "struggle for existence" in nature. It struck him that as population outgrew resources, "favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The result of this would be the formation of new species."<ref name=auto120>{{harvnb|Darwin|1958|p=[http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F1497&pageseq=124 120]}}</ref> Darwin wrote:
{{Quote|If during the long course of ages and under varying conditions of life, organic beings vary at all in the several parts of their organisation, and I think this cannot be disputed; if there be, owing to the high geometrical powers of increase of each species, at some age, season, or year, a severe struggle for life, and this certainly cannot be disputed; then, considering the infinite complexity of the relations of all organic beings to each other and to their conditions of existence, causing an infinite diversity in structure, constitution, and habits, to be advantageous to them, I think it would be a most extraordinary fact if no variation ever had occurred useful to each being's own welfare, in the same way as so many variations have occurred useful to man. But if variations useful to any organic being do occur, assuredly individuals thus characterised will have the best chance of being preserved in the struggle for life; and from the strong principle of inheritance they will tend to produce offspring similarly characterised. This principle of preservation, I have called, for the sake of brevity, Natural Selection.|source=Darwin summarising natural selection in the fourth chapter of ''[[On the Origin of Species]]''<ref>{{harvnb|Darwin|1859|pp=[http://darwin-online.org.uk/content/frameset?pageseq=144&itemID=F373&viewtype=side 126–127]}}</ref>}}

Once he had this [[hypothesis]], Darwin was meticulous about gathering and refining evidence of [[consilience]] to meet standards of [[scientific method|methodology]] before making his [[scientific theory]] public.<ref name="Bowler 2003" /> He was in the process of writing his "big book" to present his research when the naturalist [[Alfred Russel Wallace]] independently conceived of the principle and described it in an essay he sent to Darwin to forward to [[Charles Lyell]]. Lyell and [[Joseph Dalton Hooker]] decided to present his essay together with unpublished writings that Darwin had sent to fellow naturalists, and ''[[On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection]]'' was read to the [[Linnean Society of London]] announcing co-discovery of the principle in July 1858.<ref>{{harvnb|Wallace|1871}}</ref> Darwin published a detailed account of his evidence and conclusions in ''[[On the Origin of Species]]'' in 1859. In later editions Darwin acknowledged that earlier writers—like [[William Charles Wells]] in 1813,<ref>{{harvnb|Darwin|1866|pp=[http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F385&pageseq=21 xiv–xv]}}</ref> and [[Patrick Matthew]] in 1831—had proposed similar basic ideas.<ref>{{harvnb|Darwin|1861|p=[http://darwin-online.org.uk/content/frameset?itemID=F381&viewtype=text&pageseq=20 xiii]}}</ref> However, they had not developed their ideas, or presented evidence to persuade others that the concept was useful.<ref name="Bowler 2003" />

[[File:LA2-NSRW-3-0536 cropped.jpg|thumb|upright|[[Charles Darwin]] noted that [[Pigeon fancying|pigeon fanciers]] had created many kinds of pigeon, such as [[Tumbler pigeon|Tumblers]] (1, 12), [[Fantail pigeon|Fantails]] (13), and [[Pouter pigeon|Pouters]] (14) by [[selective breeding]].]]

Darwin thought of natural selection by analogy to how farmers select crops or livestock for breeding, which he called "[[artificial selection]]"; in his early manuscripts he referred to a "Nature" which would do the selection. At the time, other mechanisms of evolution such as evolution by genetic drift were not yet explicitly formulated, and Darwin believed that selection was likely only part of the story: "I am convinced that Natural Selection has been the main but not exclusive means of modification."<ref>{{harvnb|Darwin|1859|p=[http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=side&pageseq=21 6]}}</ref> In a letter to Charles Lyell in September 1860, Darwin regretted the use of the term "Natural Selection", preferring the term "Natural Preservation".<ref>{{cite web |url=http://www.darwinproject.ac.uk/entry-2931 |title=Darwin, C. R. to Lyell, Charles |last=Darwin |first=Charles |author-link=Charles Darwin |date=28 September 1860 |website=[[Correspondence of Charles Darwin#Darwin Correspondence Project website|Darwin Correspondence Project]] |publisher=[[Cambridge University Library]] |location=Cambridge, UK |id=Letter 2931 |access-date=1 August 2015}}</ref>

For Darwin and his contemporaries, natural selection was in essence synonymous with evolution by natural selection. After the publication of ''On the Origin of Species'',<ref name="origin">{{harvnb|Darwin|1859}}</ref> educated people generally accepted that evolution had occurred in some form. However, natural selection remained controversial as a mechanism, partly because it was perceived to be too weak to explain the range of observed characteristics of living organisms, and partly because even supporters of evolution balked at its "unguided" and non-[[orthogenesis|progressive]] nature,<ref>{{harvnb|Eisley|1958}}</ref> a response that has been characterised as the single most significant impediment to the idea's acceptance.<ref>{{harvnb|Kuhn|1996}}</ref> However, some thinkers enthusiastically embraced natural selection; after reading Darwin, [[Herbert Spencer]] introduced the phrase ''[[survival of the fittest]]'', which became a popular summary of the theory.<ref name="sotf">{{cite web |url=http://www.darwinproject.ac.uk/entry-5145#mark-5145.f3 |title=Darwin, C. R. to Wallace, A. R., 5 July (1866) |website=Darwin Correspondence Project |publisher=Cambridge University Library |location=Cambridge, UK |id=Letter 5145 |access-date=12 January 2010}}</ref><ref>{{cite journal |last=Stucke |first=Maurice E. |date=Summer 2008 |title=Better Competition Advocacy |url=http://works.bepress.com/cgi/viewcontent.cgi?article=1000&context=maurice_stucke |journal=St. John's Law Review |location=Jamaica, NY |volume=82 |number=3 |pages=951–1036 |quote=This survival of the fittest, which I have here sought to express in mechanical terms, is that which Mr. Darwin has called 'natural selection, or the preservation of favoured races in the struggle for life.'}}—[[Herbert Spencer]], ''[https://archive.org/details/principlesbiolo05spengoog Principles of Biology]'' (1864), vol. 1, pp. 444–445</ref> The fifth edition of ''On the Origin of Species'' published in 1869 included Spencer's phrase as an alternative to natural selection, with credit given: "But the expression often used by Mr. Herbert Spencer of the Survival of the Fittest is more accurate, and is sometimes equally convenient."<ref>{{harvnb|Darwin|1872|p=[http://darwin-online.org.uk/content/frameset?itemID=F391&viewtype=text&pageseq=76 49].}}</ref> Although the phrase is still often used by non-biologists, modern biologists avoid it because it is [[Tautology (rhetoric)|tautological]] if "fittest" is read to mean "functionally superior" and is applied to individuals rather than considered as an averaged quantity over populations.<ref>{{cite journal |last1=Mills |first1=Susan K. |last2=Beatty |first2=John H. |year=1979 |title=The Propensity Interpretation of Fitness |url=https://mitpress.mit.edu/sites/default/files/titles/content/9780262195492_sch_0001.pdf |journal=[[Philosophy of Science (journal)|Philosophy of Science]] |volume=46 |issue=2 |pages=263–286 |doi=10.1086/288865 |citeseerx=10.1.1.332.697 |s2cid=38015862 |access-date=4 August 2015 |archive-url=https://web.archive.org/web/20151225093436/https://mitpress.mit.edu/sites/default/files/titles/content/9780262195492_sch_0001.pdf |archive-date=25 December 2015 |url-status=dead }}</ref>

===The modern synthesis===
{{Main|Modern synthesis (20th century)}}<!--of 1918-1932 approx -->

Natural selection relies crucially on the idea of heredity, but developed before the basic concepts of [[genetics]]. Although the [[Moravia]]n monk [[Gregor Mendel]], the father of modern genetics, was a contemporary of Darwin's, his work lay in obscurity, only being rediscovered in 1900.<ref>{{cite web |url=https://www.jic.ac.uk/germplas/PISUM/ZGS4F.HTM |title=Mendel's Peas |last=Ambrose |first=Mike |publisher=Germplasm Resources Unit, [[John Innes Centre]] |location=Norwich, UK |access-date=22 May 2015 |archive-url=https://web.archive.org/web/20160614210558/https://www.jic.ac.uk/germplas/PISUM/ZGS4F.HTM |archive-date=14 June 2016 |url-status=dead }}</ref> With the early 20th-century integration of evolution with [[Mendel's laws]] of inheritance, the so-called [[Modern synthesis (20th century)|modern synthesis]], scientists generally came to accept natural selection.<ref name=Huxley>{{cite book |last=Huxley |first=Julian |author-link=Julian S. Huxley |year=1929–1930 |chapter=The A B C of Genetics |title=The Science of Life |volume=2 |location=London |publisher=[[Amalgamated Press]] |oclc=3171056|title-link=The Science of Life }}</ref><ref>{{cite book |author=National Academy of Sciences |author-link=National Academy of Sciences |year=1999 |title=Science and Creationism: A View from the National Academy of Sciences |url=https://archive.org/details/sciencecreationi0000unse |edition=2nd |location=Washington, DC |publisher=National Academy Press |isbn=978-0-309-06406-4 |oclc=43803228 |url-access=registration }}</ref> The synthesis grew from advances in different fields. Ronald Fisher developed the required mathematical language and wrote ''[[The Genetical Theory of Natural Selection]]'' (1930).<ref name="fisher" /> [[J. B. S. Haldane]] introduced the concept of the "cost" of natural selection.<ref>{{harvnb|Haldane|1932}}</ref><ref>{{cite journal |last=Haldane |first=J. B. S. |author-link=J. B. S. Haldane |date=December 1957 |title=The Cost of Natural Selection |url=http://www.blackwellpublishing.com/ridley/classictexts/haldane2.pdf |journal=[[Journal of Genetics]] |volume=55 |issue=3 |pages=511–524 |doi=10.1007/BF02984069|s2cid=32233460 }}</ref>
[[Sewall Wright]] elucidated the nature of selection and adaptation.<ref>{{cite journal |last=Wright |first=Sewall |author-link=Sewall Wright |year=1932 |title=The roles of mutation, inbreeding, crossbreeding and selection in evolution |url=http://www.blackwellpublishing.com/ridley/classictexts/wright.asp |journal=Proceedings of the VI International Congress of Genetrics |volume=1 |pages=356–366}}</ref>
In his book ''[[Genetics and the Origin of Species]]'' (1937), [[Theodosius Dobzhansky]] established the idea that mutation, [[mutationism|once seen as a rival]] to selection, actually supplied the raw material for natural selection by creating genetic diversity.<ref>{{harvnb|Dobzhansky|1937}}</ref><ref>{{harvnb|Dobzhansky|1951}}</ref>

===A second synthesis===
[[File:Gap gene expression.svg|thumb|upright=0.8|[[Evolutionary developmental biology]] relates the evolution of [[Morphology (biology)|form]] to the precise pattern of gene activity, here [[gap gene]]s in the fruit fly, during embryonic development.<ref>{{cite book | first1=Sean B. |last1=Carroll |first2=Jennifer K. |last2=Grenier |first3=Scott D. |last3=Weatherbee |title=From DNA to Diversity: Molecular Genetics and the Evolution of Animal Design&nbsp;– Second Edition | publisher=Blackwell Publishing| year=2005 |isbn=978-1-4051-1950-4 |pages=66–67}}</ref>]]
{{Main|Evolutionary developmental biology#History}}

[[Ernst Mayr]] recognised the key importance of [[reproductive isolation]] for speciation in his ''[[Systematics and the Origin of Species]]'' (1942).<ref>{{harvnb|Mayr|1942}}</ref>
[[W. D. Hamilton]] conceived of [[kin selection]] in 1964.<ref name=Hamilton>{{Cite journal | last1=Hamilton | first1=W. | title=The genetical evolution of social behaviour | journal=Journal of Theoretical Biology | volume=7 | issue=1 | pages=1–52 | year=1964 | pmid=5875341 | doi=10.1016/0022-5193(64)90038-4| bibcode=1964JThBi...7....1H | s2cid=5310280 }}</ref> This synthesis cemented natural selection as the foundation of evolutionary theory, where it remains today. A second synthesis was brought about at the end of the 20th century by advances in [[molecular genetics]], creating the field of [[evolutionary developmental biology]] ("evo-devo"), which seeks to explain the evolution of [[Morphology (biology)|form]] in terms of the [[Gene regulatory network|genetic regulatory programs]] which control the development of the embryo at molecular level. Natural selection is here understood to act on embryonic development to change the morphology of the adult body.<ref name=Gilbert2003>{{cite journal |last1=Gilbert |first1=Scott F. |title=The morphogenesis of evolutionary developmental biology |journal=International Journal of Developmental Biology |date=2003 |volume=47 |issue=7–8 |pages=467–477 |pmid=14756322 |url=http://www.chd.ucsd.edu/_files/fall2008/Gilbert.2003.IJDB.pdf}}</ref><ref name=Gilbert1996>{{cite journal |last1=Gilbert |first1=S.F.|last2=Opitz |first2=J.M. |last3=Raff |first3=R.A. |title=Resynthesizing Evolutionary and Developmental Biology |journal=Developmental Biology |date=1996 |volume=173 |issue=2 |pages=357–372 |doi=10.1006/dbio.1996.0032 |pmid=8605997|doi-access=free }}</ref><ref name="Müller">{{cite journal |last1=Müller |first1=G.B. |title=Evo–devo: extending the evolutionary synthesis |journal=Nature Reviews Genetics |date=2007 |volume=8 |issue=12 |pages=943–949 |doi=10.1038/nrg2219 |pmid=17984972|s2cid=19264907 }}</ref><ref>{{cite book | first1=Sean B. |last1=Carroll |first2=Jennifer K. |last2=Grenier |first3=Scott D. |last3=Weatherbee |title=From DNA to Diversity: Molecular Genetics and the Evolution of Animal Design&nbsp;– Second Edition | publisher=Blackwell Publishing| year=2005 |isbn=978-1-4051-1950-4 |page=13}}</ref>

==Terminology==

The term ''natural selection'' is most often defined to operate on heritable traits, because these directly participate in evolution. However, natural selection is "blind" in the sense that changes in phenotype can give a reproductive advantage regardless of whether or not the trait is heritable. Following Darwin's primary usage, the term is used to refer both to the evolutionary consequence of blind selection and to its mechanisms.<ref name="origin" /><ref name="fisher">{{harvnb|Fisher|1930}}</ref><ref name="nomenclature1">{{harvnb|Williams|1966}}</ref><ref>{{harvnb|Endler|1986}}</ref> It is sometimes helpful to explicitly distinguish between selection's mechanisms and its effects; when this distinction is important, scientists define "(phenotypic) natural selection" specifically as "those mechanisms that contribute to the selection of individuals that reproduce", without regard to whether the basis of the selection is heritable.<ref name="nomenclature2">{{harvnb|Haldane|1954}}</ref><ref>{{cite journal |last1=Lande |first1=Russell |author-link1=Russell Lande |last2=Arnold |first2=Stevan J. |date=November 1983 |title=The Measurement of Selection on Correlated Characters |journal=[[Evolution (journal)|Evolution]] |volume=37 |issue=6 |pages=1210–1226 |doi=10.2307/2408842 |jstor=2408842|pmid=28556011 }}</ref><ref>{{harvnb|Futuyma|2005}}</ref> Traits that cause greater reproductive success of an organism are said to be ''selected for'', while those that reduce success are ''selected against''.<ref>{{harvnb|Sober|1993}}</ref>

==Mechanism==

===Heritable variation, differential reproduction===
[[File:Lichte en zwarte versie berkenspanner crop.jpg|thumb|upright=1.2|During the [[Industrial Revolution]], pollution killed many [[lichen]]s, leaving tree trunks dark. A [[industrial melanism|dark (melanic)]] morph of the [[peppered moth]] largely replaced the formerly usual light morph (both shown here). Since the moths are subject to [[predation]] by birds hunting by sight, the colour change offers better [[camouflage]] against the changed background, suggesting natural selection at work.]]
{{Main|Genetic variation}}

Natural variation occurs among the individuals of any population of organisms. Some differences may improve an individual's chances of surviving and reproducing such that its lifetime reproductive rate is increased, which means that it leaves more offspring. If the traits that give these individuals a reproductive advantage are also [[heritable]], that is, passed from parent to offspring, then there will be differential reproduction, that is, a slightly higher proportion of fast rabbits or efficient algae in the next generation. Even if the reproductive advantage is very slight, over many generations any advantageous heritable trait becomes dominant in the population. In this way the [[natural environment]] of an organism "selects for" traits that confer a reproductive advantage, causing evolutionary change, as Darwin described.<ref name=Michigan>{{cite web |title=Evolution and Natural Selection |url=http://www.globalchange.umich.edu/globalchange1/current/lectures/selection/selection.html |publisher=University of Michigan |access-date=9 November 2016 |date=10 October 2010}}</ref> This gives the appearance of purpose, but in natural selection there is no intentional choice.{{efn|In [[sexual selection]], a female animal making a choice of mate may be argued to be intending to get the best mate; there is no suggestion that she has any intention to improve the bloodline in the manner of an animal breeder.}} Artificial selection is [[Teleology|purposive]] where natural selection is not, though [[teleology in biology|biologists often use teleological language]] to describe it.<ref name=Stanford>{{cite web |title=Teleological Notions in Biology |url=http://plato.stanford.edu/entries/teleology-biology/ |website=Stanford Encyclopedia of Philosophy |access-date=28 July 2016 |date=18 May 2003}}</ref>

The [[peppered moth]] exists in both light and dark colours in Great Britain, but during the [[Industrial Revolution]], many of the trees on which the moths rested became blackened by [[soot]], giving the dark-coloured moths an advantage in hiding from predators. This gave dark-coloured moths a better chance of surviving to produce dark-coloured offspring, and in just fifty years from the first dark moth being caught, nearly all of the moths in industrial [[Manchester]] were dark. The balance was reversed by the effect of the [[Clean Air Act 1956]], and the dark moths became rare again, demonstrating the influence of natural selection on [[peppered moth evolution]].<!--<ref name="Peppered Moth">{{cite web |url=http://www.millerandlevine.com/km/evol/Moths/moths.html |title=The Peppered Moth – An Update |last=Miller |first=Kenneth R. |author-link=Kenneth R. Miller |date=August 1999 |website=millerandlevine.com |publisher=Miller And Levine Biology |access-date=9 November 2016}}</ref>--><ref>{{cite journal|last1=van't Hof |first1=Arjen E. |last2=Campagne |first2=Pascal |last3=Rigden |first3=Daniel J |display-authors=etal |title=The industrial melanism mutation in British peppered moths is a transposable element |journal=Nature |date=June 2016 |volume=534 |issue=7605 |pages=102–105 |doi=10.1038/nature17951 |pmid=27251284|bibcode=2016Natur.534..102H |s2cid=3989607 }}</ref> A recent study, using image analysis and avian vision models, shows that pale individuals more closely match lichen backgrounds than dark morphs and for the first time quantifies the [[camouflage]] of moths to [[predation]] risk.<ref name=Walton2018>{{cite journal |last1=Walton |first1=Olivia |last2=Stevens |first2=Martin |title=Avian vision models and field experiments determine the survival value of peppered moth camouflage |journal=Communications Biology |date=2018 |volume=1 |page=118 |doi=10.1038/s42003-018-0126-3 |pmid = 30271998|pmc=6123793 }}</ref>

===Fitness===

{{Main|Fitness (biology)}}

The concept of fitness is central to natural selection. In broad terms, individuals that are more "fit" have better potential for survival, as in the well-known phrase "[[survival of the fittest]]", but the precise meaning of the term is much more subtle. Modern evolutionary theory defines fitness not by how long an organism lives, but by how successful it is at reproducing. If an organism lives half as long as others of its species, but has twice as many offspring surviving to adulthood, its genes become more common in the adult population of the next generation. Though natural selection acts on individuals, the effects of chance mean that fitness can only really be defined "on average" for the individuals within a population. The fitness of a particular [[genotype]] corresponds to the average effect on all individuals with that genotype.<ref name=Orr2009>{{cite journal |last1=Orr |first1=H. Allen |title=Fitness and its role in evolutionary genetics |journal=Nat Rev Genet |date=August 2009 |volume=10 |issue=8 |pages=531–539 |doi=10.1038/nrg2603 |pmc=2753274 |pmid=19546856 }}</ref>
A distinction must be made between the concept of "survival of the fittest" and "improvement in fitness". "Survival of the fittest" does not give an "improvement in fitness", it only represents the removal of the less fit variants from a population. A mathematical example of "survival of the fittest" is given by Haldane in his paper "The Cost of Natural Selection".<ref>{{cite journal |title=The Cost of Natural Selection |last=Haldane |first=J. B. S. |author-link=J. B. S. Haldane |journal=Current Science |volume=63 |issue=9/10 |date=November 1992 |pages=612–625}}</ref> Haldane called this process "substitution" or more commonly in biology, this is called "fixation". This is correctly described by the differential survival and reproduction of individuals due to differences in phenotype. On the other hand, "improvement in fitness" is not dependent on the differential survival and reproduction of individuals due to differences in phenotype, it is dependent on the absolute survival of the particular variant. The probability of a beneficial mutation occurring on some member of a population depends on the total number of replications of that variant. The mathematics of "improvement in fitness was described by Kleinman.<ref>{{cite journal | last1=Kleinman | first1=A. | year=2014 | title=The basic science and mathematics of random mutation and natural selection | journal=Statistics in Medicine | volume=33 | issue=29 | pages=5074–5080 | doi=10.1002/sim.6307 | pmid=25244620 | doi-access=free }}</ref> An empirical example of "improvement in fitness" is given by the Kishony Mega-plate experiment.<ref>{{cite journal | pmc=5534434 | pmid=27609891 | doi=10.1126/science.aag0822 | volume=353 | issue=6304 | title=Spatiotemporal microbial evolution on antibiotic landscapes | year=2016 | journal=Science | pages=1147–51 | last1=Baym | first1=M. | last2=Lieberman | first2=T. D. | last3=Kelsic | first3=E. D. | last4=Chait | first4=R. | last5=Gross | first5=R. | last6=Yelin | first6=I. | last7=Kishony | first7=R. | bibcode=2016Sci...353.1147B}}</ref> In this experiment, "improvement in fitness" depends on the number of replications of the particular variant for a new variant to appear that is capable of growing in the next higher drug concentration region. Fixation or substitution is not required for this "improvement in fitness". On the other hand, "improvement in fitness" can occur in an environment where "survival of the fittest" is also acting. [[Richard Lenski]]'s classic [[E. coli long-term evolution experiment|''E. coli'' long-term evolution experiment]] is an example of adaptation in a competitive environment, ("improvement in fitness" during "survival of the fittest").<ref name="pmid18524956">{{cite journal |last2=Borland |first2=Christina Z. |last3=Lenski |first3=Richard E. |year=2008 |title=Historical contingency and the evolution of a key innovation in an experimental population of ''Escherichia coli'' |journal=Proceedings of the National Academy of Sciences |volume=105 |issue=23 |pages=7899–906 |bibcode=2008PNAS..105.7899B |doi=10.1073/pnas.0803151105 |jstor=25462703 |pmc=2430337 |pmid=18524956 |last1=Blount |first1=Zachary D.|doi-access=free }}</ref> The probability of a beneficial mutation occurring on some member of the lineage to give improved fitness is slowed by the competition. The variant which is a candidate for a beneficial mutation in this limited carrying capacity environment must first out-compete the "less fit" variants in order to accumulate the requisite number of replications for there to be a reasonable probability of that beneficial mutation occurring.<ref>{{Cite journal |title=Distribution of fixed beneficial mutations and the rate of adaptation in asexual populations |first1=B. H. |last1=Good |first2=I. M. |last2=Rouzine |first3=D. J. |last3=Balick |first4=O. |last4=Hallatschek |first5=M. M. |last5=Desai |date=27 February 2012 |journal=Proceedings of the National Academy of Sciences |volume=109 |issue=13 |pages=4950–4955 |doi=10.1073/pnas.1119910109|pmid=22371564 |pmc=3323973 |doi-access=free }}</ref>

===Competition===
{{Main|Competition (biology)}}

In biology, competition is an interaction between organisms in which the fitness of one is lowered by the presence of another. This may be because both rely on a [[Limiting factor|limited]] supply of a resource such as food, water, or [[Territory (animal)|territory]].<ref>{{harvnb|Begon|Townsend|Harper|1996}}</ref> Competition may be [[intraspecific competition|within]] or [[interspecific competition|between species]], and may be direct or indirect.<ref name="SahneyBentonFerry2010LinksDiversityVertebrates">{{cite journal |last1=Sahney |first1=Sarda |last2=Benton |first2=Michael J. |author-link2=Michael Benton |last3=Ferry |first3=Paul A. |date=23 August 2010 |title=Links between global taxonomic diversity, ecological diversity and the expansion of vertebrates on land |journal=[[Biology Letters]]|volume=6 |issue=4 |pages=544–547 |doi=10.1098/rsbl.2009.1024 |pmc=2936204 |pmid=20106856}}</ref> Species less suited to compete should [[competitive exclusion principle|in theory either adapt or die out]], since competition plays a powerful role in natural selection, but according to the "room to roam" theory it may be less important than expansion among larger [[clade]]s.<ref name="SahneyBentonFerry2010LinksDiversityVertebrates"/><ref name="Jardine2012">{{cite journal |last1=Jardine |first1=Phillip E. |last2=Janis |first2=Christine M. |last3=Sahney |first3=Sarda |last4=Benton |first4=Michael J. |date=1 December 2012 |title=Grit not grass: Concordant patterns of early origin of hypsodonty in Great Plains ungulates and Glires |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=365–366 |pages=1–10 |doi=10.1016/j.palaeo.2012.09.001|bibcode=2012PPP...365....1J }}</ref>

Competition is modelled by [[r/K selection theory|''r/K'' selection theory]], which is based on [[Robert MacArthur]] and [[E. O. Wilson]]'s work on [[Insular biogeography|island biogeography]].<ref>{{harvnb|MacArthur|Wilson|2001}}</ref> In this theory, selective pressures drive evolution in one of two stereotyped directions: ''r''- or ''K''-selection.<ref>{{cite journal |last=Pianka |first=Eric R. |author-link=Eric Pianka |date=November–December 1970 |title=On ''r''- and ''K''-Selection |journal=[[The American Naturalist]] |volume=104 |number=940 |pages=592–597 |doi=10.1086/282697 |jstor=2459020|bibcode=1970ANat..104..592P |s2cid=83933177 }}</ref> These terms, ''r'' and ''K'', can be illustrated in a [[Logistic function#In ecology: modeling population growth|logistic model]] of [[population dynamics]]:<ref name=Verhulst>{{cite journal |last=Verhulst |first=Pierre François |author-link=Pierre François Verhulst |year=1838 |title=''Notice sur la loi que la population suit dans son accroissement'' |url=https://archive.org/details/correspondancem02belggoog |language=fr |journal=Correspondance Mathématique et Physique |location=Brussels, Belgium |volume=10 |pages=113–121 |oclc=490225808}}</ref>

<math display="block">\frac{dN}{dt}=rN\left(1 - \frac{N}{K}\right) \qquad \!</math>
where ''r'' is the [[Population growth#Population growth rate|growth rate]] of the population (''N''), and ''K'' is the [[carrying capacity]] of its local environmental setting. Typically, ''r''-selected species exploit empty [[Ecological niche|niches]], and produce many offspring, each with a relatively low [[probability]] of surviving to adulthood. In contrast, ''K''-selected species are strong competitors in crowded niches, and [[Parental investment|invest]] more heavily in much fewer offspring, each with a relatively high probability of surviving to adulthood.<ref name=Verhulst/>

==Classification==

[[File:Genetic Distribution.svg|thumb|1: [[directional selection]]: a single extreme [[phenotype]] favoured.<br />2, [[stabilizing selection]]: intermediate favoured over extremes.<br />3: disruptive selection: extremes favoured over intermediate.<br />X-axis: [[phenotypic trait]]<br />Y-axis: number of organisms<br />Group A: original population<br />Group B: after selection]]

Natural selection can act on any heritable [[phenotypic trait]],<ref>{{harvnb|Zimmer|Emlen|2013}}</ref> and selective pressure can be produced by any aspect of the environment, including sexual selection and [[Competition (biology)|competition]] with members of the same or other species.<ref>{{harvnb|Miller|2000|p=8}}</ref><ref name="ArnqvistRowe2005">{{cite book |last1=Arnqvist |first1=Göran |last2=Rowe |first2=Locke |title=Sexual Conflict |url=https://books.google.com/books?id=JLfvwPqsHnMC&pg=PA15 |year=2005 |publisher=Princeton University Press |isbn=978-0-691-12218-2 |oclc=937342534 |pages=14–43}}</ref> However, this does not imply that natural selection is always directional and results in adaptive evolution; natural selection often results in the maintenance of the status quo by eliminating less fit variants.<ref name=Michigan/>

Selection can be classified in several different ways, such as by its effect on a trait, on genetic diversity, by the life cycle stage where it acts, by the unit of selection, or by the resource being competed for.

===By effect on a trait===

Selection has different effects on traits. [[Stabilizing selection]] acts to hold a trait at a stable optimum, and in the simplest case all deviations from this optimum are selectively disadvantageous. [[Directional selection]] favours extreme values of a trait. The uncommon [[disruptive selection]] also acts during transition periods when the current mode is sub-optimal, but alters the trait in more than one direction. In particular, if the trait is quantitative and [[univariate]] then both higher and lower trait levels are favoured. Disruptive selection can be a precursor to [[speciation]].<ref name=Michigan/>

===By effect on genetic diversity===

Alternatively, selection can be divided according to its effect on [[genetic diversity]]. [[Negative selection (natural selection)|Purifying or negative selection]] acts to remove genetic variation from the population (and is opposed by [[Mutation#By inheritance|''de novo'' mutation]], which introduces new variation.<ref>{{harvnb|Lemey|Salemi|Vandamme|2009}}</ref><ref>{{cite web |url=http://www.nature.com/scitable/topicpage/Negative-Selection-1136 |title=Negative Selection |last=Loewe |first=Laurence |year=2008 |work=Nature Education |publisher=[[Nature Publishing Group]] |location=Cambridge, MA |oclc=310450541}}</ref> In contrast, [[balancing selection]] acts to maintain genetic variation in a population, even in the absence of ''de novo'' mutation, by negative [[frequency-dependent selection]]. One mechanism for this is [[heterozygote advantage]], where individuals with two different alleles have a selective advantage over individuals with just one allele. The polymorphism at the human [[ABO blood group]] locus has been explained in this way.<ref>{{cite journal |last1=Villanea |first1=Fernando A. |last2=Safi |first2=Kristin N. |last3=Busch |first3=Jeremiah W. |title=A General Model of Negative Frequency Dependent Selection Explains Global Patterns of Human ABO Polymorphism |journal=PLOS ONE |date=May 2015 |volume=10 |issue=5 |pages=e0125003 |doi=10.1371/journal.pone.0125003 |pmid=25946124 |pmc=4422588|bibcode=2015PLoSO..1025003V |doi-access=free }}</ref>

[[File:Life cycle of a sexually reproducing organism.svg|thumb|upright=1.1|Different types of selection act at each [[Biological life cycle|life cycle stage]] of a sexually reproducing organism.<ref name=Christiansen1984/>]]

===By life cycle stage===

Another option is to classify selection by the [[Biological life cycle|life cycle]] stage at which it acts. Some biologists recognise just two types: [[Natural selection#Types of selection|viability (or survival) selection]], which acts to increase an organism's probability of survival, and fecundity (or fertility or reproductive) selection, which acts to increase the rate of reproduction, given survival. Others split the life cycle into further components of selection. Thus viability and survival selection may be defined separately and respectively as acting to improve the probability of survival before and after reproductive age is reached, while fecundity selection may be split into additional sub-components including sexual selection, gametic selection, acting on [[gamete]] survival, and compatibility selection, acting on [[zygote]] formation.<ref name=Christiansen1984>{{harvnb|Christiansen|1984|pp=65–79}}</ref>

===By unit of selection===

Selection can also be classified by the level or [[unit of selection]]. Individual selection acts on the individual, in the sense that adaptations are "for" the benefit of the individual, and result from selection among individuals. [[Gene selection]] acts directly at the level of the gene. In [[kin selection]] and [[intragenomic conflict]], gene-level selection provides a more apt explanation of the underlying process. [[Group selection]], if it occurs, acts on groups of organisms, on the assumption that groups replicate and mutate in an analogous way to genes and individuals. There is an ongoing debate over the degree to which group selection occurs in nature.<ref>{{cite journal |author=Wade, Michael J. |display-authors=etal |title=Multilevel and kin selection in a connected world |journal=Nature |date=2010 |volume=463 |issue=7283 |pages=E8–E9 | doi=10.1038/nature08809 |pmid=20164866 |pmc=3151728|bibcode=2010Natur.463....8W }}</ref>

===By resource being competed for===

[[File:Pavo cristatus in Barbados Wildlife Reserve 12.jpg|thumb|upright=1.2|right|The [[Peafowl|peacock]]'s elaborate plumage is mentioned by Darwin as an example of [[sexual selection]],<ref name=DarwinSexualSelection>Darwin, Charles (1859). On the Origin of Species (1st edition). Chapter 4, page 88. "And this leads me to say a few words on what I call Sexual Selection. This depends ..." http://darwin-online.org.uk/content/frameset?viewtype=side&itemID=F373&pageseq=12</ref> and is a classic example of [[Fisherian runaway]],<ref name=Greenfield/> driven to its conspicuous size and [[animal coloration|coloration]] through [[mate choice]] by females over many generations.]]
{{Further|Sexual selection}}

Finally, selection can be classified according to the [[Resource (biology)|resource]] being competed for. Sexual selection results from competition for mates. Sexual selection typically proceeds via fecundity selection, sometimes at the expense of viability. [[Ecological selection]] is natural selection via any means other than sexual selection, such as kin selection, competition, and [[Infanticide (zoology)|infanticide]]. Following Darwin, natural selection is sometimes defined as ecological selection, in which case sexual selection is considered a separate mechanism.<ref>{{harvnb|Mayr|2006}}</ref>

Sexual selection as first articulated by Darwin (using the example of the [[Peafowl|peacock]]'s tail)<ref name=DarwinSexualSelection/> refers specifically to competition for mates,<ref>{{harvnb|Andersson|1994}}</ref> which can be ''intrasexual'', between individuals of the same sex, that is male–male competition, or ''intersexual'', where one gender [[mate choice|chooses mates]], most often with males displaying and females choosing.<ref name="Hosken2011">{{cite journal |last1=Hosken |first1=David J. |last2=House |first2=Clarissa M. |title=Sexual Selection |journal=Current Biology |date=January 2011 |doi=10.1016/j.cub.2010.11.053 |pmid=21256434 |volume=21 |issue=2 |pages=R62–R65|s2cid=18470445 |doi-access=free |bibcode=2011CBio...21..R62H }}</ref> However, in some species, mate choice is primarily by males, as in some fishes of the family [[Syngnathidae]].<ref name="Eens">{{cite journal |last1=Eens |first1=Marcel |last2=Pinxten |first2=Rianne |date=5 October 2000 |title=Sex-role reversal in vertebrates: behavioural and endocrinological accounts |journal=Behavioural Processes |volume=51 |issue=1–3 |pages=135–147 |doi=10.1016/S0376-6357(00)00124-8 |pmid=11074317|s2cid=20732874 }}</ref><ref name="Barlow">{{cite journal |last=Barlow |first=George W. |date=March 2005 |title=How Do We Decide that a Species is Sex-Role Reversed? |journal=[[The Quarterly Review of Biology]] |volume=80 |issue=1 |pages=28–35 |doi=10.1086/431022 |pmid=15884733|s2cid=44774132 }}</ref>

<!--[[File:Alligator Pipefish 2.jpg|upright=1.2|thumb|[[Alligator pipefish]] males select females.]]-->
Phenotypic traits can be [[signalling theory|displayed]] in one sex and desired in the other sex, causing a [[positive feedback]] loop called a [[Fisherian runaway]], for example, the extravagant plumage of some male birds such as the peacock.<ref name=Greenfield>{{cite journal |author1=Greenfield, M.D. |author2=Alem, S. |author3=Limousin, D. |author4=Bailey, N.W. |title=The dilemma of Fisherian sexual selection: Mate choice for indirect benefits despite rarity and overall weakness of trait-preference genetic correlation |journal=Evolution |date=2014 |volume=68 |issue=12 |pages=3524–3536 |doi=10.1111/evo.12542 |pmid=25308282 |s2cid=2619084 |doi-access=free }}</ref> An alternate theory proposed by the same [[Ronald Fisher]] in 1930 is the [[sexy son hypothesis]], that mothers want promiscuous sons to give them large numbers of grandchildren and so choose promiscuous fathers for their children. Aggression between members of the same sex is sometimes associated with very distinctive features, such as the antlers of [[Deer|stags]], which are used in combat with other stags. More generally, intrasexual selection is often associated with [[sexual dimorphism]], including differences in body size between males and females of a species.<ref name="Hosken2011"/>

==Arms races==
[[File:Antibiotic resistance.svg|thumb|upright|Selection in action: [[Antimicrobial resistance|resistance to antibiotics]] grows through the survival of individuals less affected by the antibiotic. Their offspring inherit the resistance.]]
{{Further|Antimicrobial resistance}}

Natural selection is seen in action in the development of [[Antimicrobial resistance|antibiotic resistance]] in [[microorganism]]s. Since the discovery of [[penicillin]] in 1928, [[antibiotics]] have been used to fight bacterial diseases. The widespread misuse of antibiotics has selected for microbial resistance to antibiotics in clinical use, to the point that the [[Methicillin-resistant Staphylococcus aureus|methicillin-resistant ''Staphylococcus aureus'']] (MRSA) has been described as a "superbug" because of the threat it poses to health and its relative invulnerability to existing drugs.<ref>{{cite news |last1=Harvey |first1=Fiona |author-link=Fiona Harvey |last2=Carson |first2=Mary |last3=O'Kane |first3=Maggie |last4=Wasley |first4=Andrew |date=18 June 2015 |title=MRSA superbug found in supermarket pork raises alarm over farming risks |url=https://www.theguardian.com/society/2015/jun/18/mrsa-superbug-in-supermarket-pork-raises-alarm-farming-risks |work=[[The Guardian]]}}</ref> Response strategies typically include the use of different, stronger antibiotics; however, new [[strain (biology)|strains]] of MRSA have recently emerged that are resistant even to these drugs.<ref name=Schito_2006>{{cite journal |last=Schito |first=Gian C. |date=March 2006 |title=The importance of the development of antibiotic resistance in ''Staphylococcus aureus'' |journal=Clinical Microbiology and Infection |volume=12 |issue=Suppl s1 |pages=3–8 |pmid=16445718 |doi=10.1111/j.1469-0691.2006.01343.x|doi-access=free }}</ref> This is an [[evolutionary arms race]], in which bacteria develop strains less susceptible to antibiotics, while medical researchers attempt to develop new antibiotics that can kill them. A similar situation occurs with [[pesticide resistance]] in plants and insects. Arms races are not necessarily induced by man; a well-documented example involves the spread of a gene in the butterfly ''[[Hypolimnas bolina]]'' suppressing male-killing activity by ''[[Wolbachia]]'' bacteria parasites on the island of [[Samoa]], where the spread of the gene is known to have occurred over a period of just five years.<ref>{{cite journal |last1=Charlat |first1=Sylvain |last2=Hornett |first2=Emily A. |last3=Fullard |first3=James H. |last4=Davies |first4=Neil |last5=Roderick |first5=George K. |last6=Wedell |first6=Nina |last7=Hurst |first7=Gregory D.D. |display-authors=3 |date=13 July 2007 |title=Extraordinary Flux in Sex Ratio |journal=[[Science (journal)|Science]] |volume=317 |issue=5835 |page=214 |doi=10.1126/science.1143369 |pmid=17626876|bibcode=2007Sci...317..214C |s2cid=45723069 }}</ref><ref>{{cite journal |last1=Moran |first1=Gregory J. |display-authors=etal |title=Methicillin-Resistant S. Aureus Infections among Patients in the Emergency Department |journal=New England Journal of Medicine |date=2006 |volume=355 |issue=7 |pages=666–674 |doi=10.1056/NEJMoa055356 |pmid=16914702 |doi-access=free }}</ref>

{{clear|left}}

==Evolution by means of natural selection==
{{Main|Evolution|Darwinism}}

A prerequisite for natural selection to result in adaptive evolution, novel traits and speciation is the presence of heritable genetic variation that results in fitness differences. Genetic variation is the result of mutations, [[genetic recombination]]s and alterations in the [[karyotype]] (the number, shape, size and internal arrangement of the [[chromosome]]s). Any of these changes might have an effect that is highly advantageous or highly disadvantageous, but large effects are rare. In the past, most changes in the genetic material were considered neutral or close to neutral because they occurred in [[noncoding DNA]] or resulted in a [[synonymous substitution]]. However, many mutations in [[non-coding DNA]] have deleterious effects.<ref name="NCFitnessEffects">{{cite journal |last1=Kryukov |first1=Gregory V. |last2=Schmidt |first2=Steffen |last3=Sunyaev |first3=Shamil |date=1 August 2005 |title=Small fitness effect of mutations in highly conserved non-coding regions |journal=[[Human Molecular Genetics]] |volume=14 |issue=15 |pages=2221–2229 |doi=10.1093/hmg/ddi226 |pmid=15994173|doi-access=free }}</ref><ref name="NCFitnessEffects2">{{cite journal |last1=Bejerano |first1=Gill |last2=Pheasant |first2=Michael |last3=Makunin |first3=Igor |last4=Stephen |first4=Stuart |last5=Kent |first5=W. James |last6=Mattick |first6=John S. |last7=Haussler |first7=David |display-authors=3 |date=28 May 2004 |title=Ultraconserved Elements in the Human Genome |journal=Science |volume=304 |issue=5675 |pages=1321–1325 |doi=10.1126/science.1098119 |pmid=15131266|url=http://www.bx.psu.edu/~ross/ComparGeno/BejeranoUCEsSci.pdf |bibcode=2004Sci...304.1321B |citeseerx=10.1.1.380.9305 |s2cid=2790337 }}</ref> Although both mutation rates and average fitness effects of mutations are dependent on the organism, a majority of mutations in humans are slightly deleterious.<ref name="Eyre-Walker">{{cite journal |last1=Eyre-Walker |first1=Adam |last2=Woolfit |first2=Megan |last3=Phelps |first3=Ted |date=June 2006 |title=The Distribution of Fitness Effects of New Deleterious Amino Acid Mutations in Humans |journal=[[Genetics (journal)|Genetics]] |volume=173 |issue=2 |pages=891–900 |doi=10.1534/genetics.106.057570 |pmc=1526495 |pmid=16547091}}</ref>

Some mutations occur in [[evo-devo gene toolkit|"toolkit" or regulatory genes]]. Changes in these often have large effects on the phenotype of the individual because they regulate the function of many other genes. Most, but not all, mutations in regulatory genes result in non-viable embryos. Some nonlethal regulatory mutations occur in [[Homeobox#Hox genes|HOX genes]] in humans, which can result in a [[cervical rib]]<ref>{{cite journal |last=Galis |first=Frietson |date=April 1999 |title=Why do almost all mammals have seven cervical vertebrae? Developmental constraints, ''Hox'' genes, and cancer |journal=[[Journal of Experimental Zoology]]|volume=285 |issue=1 |pages=19–26 |doi=10.1002/(SICI)1097-010X(19990415)285:1<19::AID-JEZ3>3.0.CO;2-Z |pmid=10327647|bibcode=1999JEZ...285...19G }}</ref> or [[polydactyly]], an increase in the number of fingers or toes.<ref>{{cite journal |last1=Zákány |first1=József |last2=Fromental-Ramain |first2=Catherine |last3=Warot |first3=Xavier |last4=Duboule |first4=Denis |author-link4=Denis Duboule |date=9 December 1997 |title=Regulation of number and size of digits by posterior ''Hox'' genes: A dose-dependent mechanism with potential evolutionary implications |journal=[[Proceedings of the National Academy of Sciences of the United States of America]] |volume=94 |issue=25 |pages=13695–13700 |doi=10.1073/pnas.94.25.13695  |pmc=28368 |pmid=9391088|bibcode=1997PNAS...9413695Z |doi-access=free }}</ref> When such mutations result in a higher fitness, natural selection favours these phenotypes and the novel trait spreads in the population.
Established traits are not immutable; traits that have high fitness in one environmental context may be much less fit if environmental conditions change. In the absence of natural selection to preserve such a trait, it becomes more variable and deteriorate over time, possibly resulting in a [[Vestigiality|vestigial]] manifestation of the trait, also called [[evolutionary baggage]]. In many circumstances, the apparently vestigial structure may retain a limited functionality, or may be co-opted for other advantageous traits in a phenomenon known as [[Exaptation|preadaptation]]. A famous example of a vestigial structure, the eye of the [[Spalax|blind mole-rat]], is believed to retain function in [[Photoperiodism|photoperiod]] perception.<ref name="Sanyal">{{cite journal |last1=Sanyal |first1=Somes |last2=Jansen |first2=Harry G. |last3=de Grip |first3=Willem J. |last4=Nevo |first4=Eviatar |author-link4=Eviatar Nevo |last5=de Jong |first5=Welfried W. |display-authors=4 |date=July 1990 |title=The Eye of the Blind Mole Rat, ''Spalax ehrenbergi''. Rudiment with Hidden Function? |url=http://iovs.arvojournals.org/article.aspx?articleid=2160417 |journal=[[Investigative Ophthalmology & Visual Science]]|volume=31 |issue=7 |pages=1398–1404 |pmid=2142147}}</ref>

===Speciation===
{{Main|Speciation}}
Speciation requires a degree of [[reproductive isolation]]—that is, a reduction in gene flow. However, it is intrinsic to the concept of a [[species]] that [[hybrid (biology)|hybrids]] are selected against, opposing the evolution of reproductive isolation, a problem that was recognised by Darwin. The problem does not occur in [[allopatric]] speciation with geographically separated populations, which can diverge with different sets of mutations. [[E. B. Poulton]] realized in 1903 that reproductive isolation could evolve through divergence, if each lineage acquired a different, incompatible allele of the same gene. Selection against the heterozygote would then directly create reproductive isolation, leading to the [[Bateson–Dobzhansky–Muller model]], further elaborated by [[H. Allen Orr]]<ref>{{cite journal | last1=Orr | first1=H.A. |author-link=H. Allen Orr| title=Dobzhansky, Bateson, and the Genetics of Speciation | journal=Genetics | volume=144 | issue=4 | pages=1331–1335 | year=1996 | doi=10.1093/genetics/144.4.1331 | pmc=1207686 | pmid=8978022 }}</ref> and [[Sergey Gavrilets]].<ref>{{citation |last=Gavrilets |first=S. |title=Fitness Landscapes and the Origin of Species |year=2004 |publisher=Princeton University Press |isbn=978-0-691-11983-0}}</ref> With [[Reinforcement (speciation)|reinforcement]], however, natural selection can favour an increase in pre-zygotic isolation, influencing the process of speciation directly.<ref name="Modes and Mechanisms of Speciation">{{cite journal |title=Modes and Mechanisms of Speciation |author1=Schuler, Hannes |author2=Hood, Glen R. |author3=Egan, Scott P. |author4=Feder, Jeffrey L. | journal=Reviews in Cell Biology and Molecular Medicine | year=2016 | volume=2 | issue=3 | pages=60–93 }}</ref>

==Genetic basis==

===Genotype and phenotype===
{{Main|Genotype–phenotype distinction}}

Natural selection acts on an organism's phenotype, or physical characteristics. Phenotype is determined by an organism's genetic make-up (genotype) and the environment in which the organism lives. When different organisms in a population possess different versions of a gene for a certain trait, each of these versions is known as an [[allele]]. It is this genetic variation that underlies differences in phenotype. An example is the [[ABO]] blood type [[antigen]]s in humans, where three alleles govern the phenotype.<ref>{{cite web |url=http://omim.org/entry/110300 |title=ABO Glycosyltransferase; ABO |author1=McKusick, Victor A. |author2=Gross, Matthew B. |date=18 November 2014 |work=Online Mendelian Inheritance in Man |publisher=National Library of Medicine |access-date=7 November 2016}}</ref>

Some traits are governed by only a single gene, but most traits are influenced by the interactions of many genes. A variation in one of the many genes that contributes to a trait may have only a small effect on the phenotype; together, these genes can produce a continuum of possible phenotypic values.<ref>{{harvnb|Falconer|Mackay|1996}}</ref>

===Directionality of selection===<!-- This section is linked from [[Race and intelligence]] -->
{{Main|Directional selection}}

When some component of a trait is heritable, selection alters the frequencies of the different alleles, or variants of the gene that produces the variants of the trait. Selection can be divided into three classes, on the basis of its effect on allele frequencies: [[directional selection|directional]], [[stabilizing selection|stabilizing]], and [[disruptive selection]].<ref name="Rice">{{harvnb|Rice|2004|loc=See especially chapters 5 and 6 for a quantitative treatment}}</ref> Directional selection occurs when an allele has a greater fitness than others, so that it increases in frequency, gaining an increasing share in the population. This process can continue until the allele is [[fixation (population genetics)|fixed]] and the entire population shares the fitter phenotype.<ref>{{cite journal |author1=Rieseberg, L.H. |author2=Widmer, A. |author3=Arntz, A.M. |author4=Burke, J.M. |date=2002 |title=Directional selection is the primary cause of phenotypic diversification |journal=PNAS |volume=99 |issue=19 |pages=12242–12245 |doi=10.1073/pnas.192360899 |pmid=12221290 |pmc=129429|bibcode=2002PNAS...9912242R |doi-access=free }}</ref> Far more common is stabilizing selection, which lowers the frequency of alleles that have a deleterious effect on the phenotype—that is, produce organisms of lower fitness. This process can continue until the allele is eliminated from the population. Stabilizing selection [[Conserved sequence|conserves]] functional genetic features, such as [[protein biosynthesis|protein-coding genes]] or [[regulatory sequence]]s, over time by selective pressure against deleterious variants.<ref>{{cite journal |vauthors=Charlesworth B, Lande R, Slatkin M |date=1982 |title=A neo-Darwinian commentary on macroevolution |journal=Evolution |volume=36 |issue=3 |doi=10.1111/j.1558-5646.1982.tb05068.x |pmid=28568049 |pages=474–498|jstor=2408095 |s2cid=27361293 |doi-access=free }}</ref> Disruptive (or diversifying) selection is selection favouring extreme trait values over intermediate trait values. Disruptive selection may cause [[sympatric speciation]] through [[niche partitioning]].

Some forms of [[balancing selection]] do not result in fixation, but maintain an allele at intermediate frequencies in a population. This can occur in [[diploid]] species (with pairs of chromosomes) when [[Zygosity#Heterozygous|heterozygous]] individuals (with just one copy of the allele) have a higher fitness than homozygous individuals (with two copies). This is called heterozygote advantage or over-dominance, of which the best-known example is the resistance to malaria in humans heterozygous for [[sickle-cell anaemia]]. Maintenance of allelic variation can also occur through [[disruptive selection|disruptive or diversifying selection]], which favours genotypes that depart from the average in either direction (that is, the opposite of over-dominance), and can result in a [[Multimodal distribution|bimodal distribution]] of trait values. Finally, balancing selection can occur through frequency-dependent selection, where the fitness of one particular phenotype depends on the distribution of other phenotypes in the population. The principles of [[game theory]] have been applied to understand the fitness distributions in these situations, particularly in the study of kin selection and the evolution of [[reciprocal altruism]].<ref name="Hamilton"/><ref name="Trivers">{{cite journal |last=Trivers |first=Robert L. |author-link=Robert Trivers |date=March 1971 |title=The Evolution of Reciprocal Altruism |journal=The Quarterly Review of Biology |volume=46 |issue=1 |pages=35–57 |doi=10.1086/406755 |jstor=2822435|s2cid=19027999 }}</ref>

===Selection, genetic variation, and drift===
{{Main|Genetic variation|Genetic drift}}

A portion of all genetic variation is functionally neutral, producing no phenotypic effect or significant difference in fitness. [[Motoo Kimura]]'s [[neutral theory of molecular evolution]] by [[genetic drift]] proposes that this variation accounts for a large fraction of observed genetic diversity.<ref name=Kimura>{{cite book |author=Kimura, Motoo |author-link=Motoo Kimura |date=1983 |title=The neutral theory of molecular evolution |publisher=Cambridge University Press |isbn=978-0-521-23109-1 |oclc=8776549}}</ref> Neutral events can radically reduce genetic variation through [[population bottleneck]]s.<ref>{{cite encyclopedia |editor-last=Robinson |editor-first=Richard |encyclopedia=Genetics |title=Population Bottleneck |url=https://archive.org/details/genetics0000unse |year=2003 |publisher=Macmillan Reference US |volume=3 |isbn=978-0-02-865609-0 |oclc=3373856121 |url-access=registration }}</ref> which among other things can cause the [[founder effect]] in initially small new populations.<ref name=Campbell1996>{{cite book |last=Campbell |first=Neil A. |author-link=Neil Campbell (scientist) |year=1996 |title=Biology |url=https://archive.org/details/biologycamp00camp |url-access=registration |edition=4th |publisher=[[Benjamin Cummings]] |isbn=978-0-8053-1940-8 |oclc=3138680061 |page=[https://archive.org/details/biologycamp00camp/page/423 423]}}</ref> When genetic variation does not result in differences in fitness, selection cannot directly affect the frequency of such variation. As a result, the genetic variation at those sites is higher than at sites where variation does influence fitness.<ref name=Rice/> However, after a period with no new mutations, the genetic variation at these sites is eliminated due to genetic drift. Natural selection reduces genetic variation by eliminating maladapted individuals, and consequently the mutations that caused the maladaptation. At the same time, new mutations occur, resulting in a [[mutation–selection balance]]. The exact outcome of the two processes depends both on the rate at which new mutations occur and on the strength of the natural selection, which is a function of how unfavourable the mutation proves to be.<ref name=Lynch>{{cite journal |last1=Lynch |first1=Michael |title=Evolution of the mutation rate |journal=Trends in Genetics |date=August 2010 |volume=26 |issue=8 |pages=345–352 |doi=10.1016/j.tig.2010.05.003 |pmid=20594608 |pmc=2910838}}</ref>

[[Genetic linkage]] occurs when the [[locus (genetics)|loci]] of two alleles are close on a chromosome. During the formation of gametes, recombination reshuffles the alleles. The chance that such a reshuffle occurs between two alleles is inversely related to the distance between them. [[Selective sweep]]s occur when an allele becomes more common in a population as a result of positive selection. As the prevalence of one allele increases, closely linked alleles can also become more common by "[[genetic hitchhiking]]", whether they are neutral or even slightly deleterious. A strong selective sweep results in a region of the genome where the positively selected [[haplotype]] (the allele and its neighbours) are in essence the only ones that exist in the population. Selective sweeps can be detected by measuring [[linkage disequilibrium]], or whether a given haplotype is overrepresented in the population. Since a selective sweep also results in selection of neighbouring alleles, the presence of a block of strong linkage disequilibrium might indicate a 'recent' selective sweep near the centre of the block.<ref name=MaynardSmithHaigh>{{Cite journal |last1=Smith |first1=John Maynard |author-link1=John Maynard Smith |last2=Haigh|first2=John |date=1974 |title=The hitch-hiking effect of a favourable gene |journal=Genetics Research |volume=23 |issue=1 |pages=23–35 |doi=10.1017/S0016672300014634 |pmid=4407212|doi-access=free }}</ref>

[[Background selection]] is the opposite of a selective sweep. If a specific site experiences strong and persistent purifying selection, linked variation tends to be weeded out along with it, producing a region in the genome of low overall variability. Because background selection is a result of deleterious new mutations, which can occur randomly in any haplotype, it does not produce clear blocks of linkage disequilibrium, although with low recombination it can still lead to slightly negative linkage disequilibrium overall.<ref name="Keightley & Otto 2006">{{cite journal |last1=Keightley |first1=Peter D. |author-link1=Peter Keightley |last2=Otto |first2=Sarah P. |author-link2=Sarah Otto |date=7 September 2006 |title=Interference among deleterious mutations favours sex and recombination in finite populations |journal=[[Nature (journal)|Nature]] |volume=443 |issue=7107 |pages=89–92 |doi=10.1038/nature05049  |pmid=16957730|bibcode=2006Natur.443...89K |s2cid=4422532 }}</ref>

==Impact==
{{Main|Universal Darwinism}}

Darwin's ideas, along with those of [[Adam Smith]] and [[Karl Marx]], had a profound influence on 19th century thought, including his radical claim that "elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner" evolved from the simplest forms of life by a few simple principles.<ref>{{harvnb|Darwin|1859|p=[http://darwin-online.org.uk/content/frameset?pageseq=507&itemID=F373&viewtype=side 489]}}</ref> This inspired some of Darwin's most ardent supporters—and provoked the strongest opposition. Natural selection had the power, according to [[Stephen Jay Gould]], to "dethrone some of the deepest and most traditional comforts of Western thought", such as the belief that humans have a special place in the world.<ref>{{cite journal |last=Gould |first=Stephen Jay |author-link=Stephen Jay Gould |date=12 June 1997 |title=Darwinian Fundamentalism |url=http://www.nybooks.com/articles/archives/1997/jun/12/darwinian-fundamentalism/ |journal=[[The New York Review of Books]] |volume=44 |issue=10}}</ref>

In the words of the philosopher [[Daniel Dennett]], "Darwin's dangerous idea" of evolution by natural selection is a "universal acid," which cannot be kept restricted to any vessel or container, as it soon leaks out, working its way into ever-wider surroundings.<ref>{{harvnb|Dennett|1995}}</ref> Thus, in the last decades, the concept of natural selection has spread from [[evolutionary biology]] to other disciplines, including [[evolutionary computation]], [[quantum Darwinism]], [[evolutionary economics]], [[evolutionary epistemology]], [[evolutionary psychology]], and [[Lee Smolin#Cosmological natural selection|cosmological natural selection]]. This unlimited applicability has been called [[universal Darwinism]].<ref>{{cite book |author=von Sydow, M. |date=2012 |url=http://www.univerlag.uni-goettingen.de/content/list.php?cat=subject&show=Biologie&details=isbn-978-3-86395-006-4 |title=From Darwinian Metaphysics towards Understanding the Evolution of Evolutionary Mechanisms. A Historical and Philosophical Analysis of Gene-Darwinism and Universal Darwinism |publisher=Universitätsverlag Göttingen |isbn=978-3-86395-006-4 |oclc=1088022023}}</ref>

===Origin of life===
{{Main|Abiogenesis}}

How life originated from inorganic matter remains an unresolved problem in biology. One prominent hypothesis is that life first appeared [[RNA world|in the form of short self-replicating RNA]] polymers.<ref>{{cite journal |last1=Eigen |first1=Manfred |author-link1=Manfred Eigen |last2= Gardiner |first2=William |last3=Schuster |author-link3= Peter Schuster |first3=Peter |last4= Winkler-Oswatitsch |first4=Ruthild |display-authors=3 |date= April 1981 |title= The Origin of Genetic Information |journal=[[Scientific American]] |volume= 244 |issue=4 |pages= 88–92, 96, ''et passim'' |doi=10.1038/scientificamerican0481-88|pmid=6164094|bibcode=1981SciAm.244d..88E }}</ref> On this view, life may have come into existence when [[RNA]] chains first experienced the basic conditions, as conceived by Charles Darwin, for natural selection to operate. These conditions are: heritability, [[Genetic variability|variation of type]], and competition for limited resources. The fitness of an early [[Abiogenesis#RNA synthesis and replication|RNA replicator]] would likely have been a function of adaptive capacities that were intrinsic (i.e., determined by the [[Nucleic acid sequence|nucleotide sequence]]) and the availability of resources.<ref name="Bernstein">{{cite journal |last1=Bernstein |first1=Harris |last2=Byerly |first2=Henry C. |last3=Hopf |first3=Frederick A. |last4=Michod |first4=Richard A. |last5=Vemulapalli |first5=G. Krishna |display-authors=3 |date=June 1983 |title=The Darwinian Dynamic |journal=The Quarterly Review of Biology |volume=58 |number=2 |pages=185–207 |doi=10.1086/413216 |jstor=2828805|s2cid=83956410 }}</ref><ref name="Michod">{{harvnb|Michod|1999}}</ref> The three primary adaptive capacities could logically have been: (1) the capacity to replicate with moderate fidelity (giving rise to both heritability and variation of type), (2) the capacity to avoid decay, and (3) the capacity to acquire and process resources.<ref name="Bernstein" /><ref name="Michod" /> These capacities would have been determined initially by the folded configurations (including those configurations with [[ribozyme]] activity) of the RNA replicators that, in turn, would have been encoded in their individual nucleotide sequences.<ref>{{cite journal |last=Orgel |first=Leslie E. |author-link=Leslie Orgel |year=1987 |title=Evolution of the Genetic Apparatus: A Review |journal=Cold Spring Harbor Symposia on Quantitative Biology |volume=52 |pages=9–16 |doi=10.1101/sqb.1987.052.01.004 |pmid=2456886}}</ref>

===Cell and molecular biology===

In 1881, the embryologist [[Wilhelm Roux]] published ''Der Kampf der Theile im Organismus'' (''The Struggle of Parts in the Organism'') in which he suggested that the development of an organism results from a Darwinian competition between the parts of the embryo, occurring at all levels, from molecules to organs.<ref>{{harvnb|Roux|1881}}</ref> In recent years, a modern version of this theory has been proposed by [[:fr:Jean-Jacques Kupiec|Jean-Jacques Kupiec]]. According to this cellular Darwinism, [[stochastic|random variation]] at the molecular level generates diversity in cell types whereas cell interactions impose a characteristic order on the developing embryo.<ref>{{cite web |url=http://www.scitopics.com/Cellular_Darwinism_stochastic_gene_expression_in_cell_differentiation_and_embryo_development.html |title=Cellular Darwinism (stochastic gene expression in cell differentiation and embryo development) |last=Kupiec |first=Jean-Jacques |author-link=:fr:Jean-Jacques Kupiec |date=3 May 2010 |website=SciTopics |archive-url=https://web.archive.org/web/20100804050452/http://www.scitopics.com/Cellular_Darwinism_stochastic_gene_expression_in_cell_differentiation_and_embryo_development.html |archive-date=4 August 2010 |access-date=11 August 2015}}</ref>

===Social and psychological theory===
{{Main|Evolutionary psychology}}
The social implications of the theory of evolution by natural selection also became the source of continuing controversy. [[Friedrich Engels]], a German [[Political philosophy|political philosopher]] and co-originator of the ideology of [[communism]], wrote in 1872 that "Darwin did not know what a bitter satire he wrote on mankind, and especially on his countrymen, when he showed that free competition, the struggle for existence, which the economists celebrate as the highest historical achievement, is the normal state of the ''animal kingdom''<!--author's emphasis-->."<ref>{{harvnb|Engels|1964}}</ref> Herbert Spencer and the eugenics advocate [[Francis Galton]]'s interpretation of natural selection as necessarily progressive, leading to supposed advances in intelligence and civilisation, became a justification for [[colonialism]], [[eugenics]], and [[social Darwinism]]. For example, in 1940, [[Konrad Lorenz]], in writings that he subsequently disowned, used the theory as a justification for policies of the [[Nazi]] state. He wrote "...&nbsp;selection for toughness, heroism, and social utility&nbsp;... must be accomplished by some human institution, if mankind, in default of selective factors, is not to be ruined by domestication-induced degeneracy. The racial idea as the basis of our state has already accomplished much in this respect."<ref>{{cite journal |last=Eisenberg |first=Leon |author-link=Leon Eisenberg |date=September 2005 |title=Which image for Lorenz? |journal=[[American Journal of Psychiatry]] |type=Letter to the editor |volume=162 |issue=9 |page=1760 |doi=10.1176/appi.ajp.162.9.1760 |pmid=16135651}} Eisenberg quoting translation of ''Durch Domestikation verursachte Störungen arteigenen Verhaltens'' (1940, p. 2) by [[Konrad Lorenz]].</ref> Others have developed ideas that human societies and culture [[Sociocultural evolution|evolve]] by mechanisms analogous to those that apply to evolution of species.<ref>{{harvnb|Wilson|2002}}</ref>

More recently, work among anthropologists and psychologists has led to the development of [[sociobiology]] and later of evolutionary psychology, a field that attempts to explain features of [[Psychology|human psychology]] in terms of adaptation to the ancestral environment. The most prominent example of evolutionary psychology, notably advanced in the early work of [[Noam Chomsky]] and later by [[Steven Pinker]], is the hypothesis that the human brain has adapted to [[language acquisition|acquire]] the [[grammar|grammatical]] rules of [[natural language]].<ref name="Pinker">{{harvnb|Pinker|1995}}</ref> Other aspects of human behaviour and social structures, from specific cultural norms such as [[Westermarck effect#Westermarck effect|incest avoidance]] to broader patterns such as [[gender role]]s, have been hypothesised to have similar origins as adaptations to the early environment in which modern humans evolved. By analogy to the action of natural selection on genes, the concept of [[meme]]s—"units of cultural transmission," or culture's equivalents of genes undergoing selection and recombination—has arisen, first described in this form by [[Richard Dawkins]] in 1976<ref>{{harvnb|Dawkins|1976|p=192}}</ref> and subsequently expanded upon by philosophers such as [[Daniel Dennett]] as explanations for complex cultural activities, including human [[consciousness]].<ref>{{harvnb|Dennett|1991}}</ref>

===Information and systems theory===
In 1922, [[Alfred J. Lotka]] proposed that natural selection might be understood as a physical principle that could be described in terms of the use of energy by a system,<ref>{{cite journal |last=Lotka |first=Alfred J. |author-link=Alfred J. Lotka |date=June 1922 |title=Contribution to the energetics of evolution |journal=PNAS |volume=8 |issue=6 |pages=147–151 |doi=10.1073/pnas.8.6.147 |pmc=1085052 |pmid=16576642|bibcode=1922PNAS....8..147L |doi-access=free }}</ref><ref>{{cite journal |last=Lotka |first=Alfred J. |date=June 1922 |title=Natural selection as a physical principle |journal=PNAS |volume=8 |issue=6 |pages=151–154 |doi=10.1073/pnas.8.6.151 |pmc=1085053 |pmid=16576643|bibcode=1922PNAS....8..151L |doi-access=free }}</ref> a concept later developed by [[Howard T. Odum]] as the [[maximum power principle]] in [[thermodynamics]], whereby evolutionary systems with selective advantage maximise the rate of useful energy transformation.<ref>{{cite book |author=Odum, H. T. |author-link=Howard T. Odum |date=1995 |title=Self-Organization and Maximum Empower |editor=Hall, C. A. S. |publisher=Colorado University Press}}</ref>

The principles of natural selection have inspired a variety of computational techniques, such as "soft" [[artificial life]], that simulate selective processes and can be highly efficient in 'adapting' entities to an environment defined by a specified [[fitness function]].<ref>{{harvnb|Kauffman|1993}}</ref> For example, a class of heuristic [[Mathematical optimization|optimisation]] algorithms known as [[genetic algorithm]]s, pioneered by [[John Henry Holland]] in the 1970s and expanded upon by [[David E. Goldberg]],<ref>{{harvnb|Goldberg|1989}}</ref> identify optimal solutions by simulated reproduction and mutation of a population of solutions defined by an initial [[probability distribution]].<ref>{{harvnb|Mitchell|1996}}</ref> Such algorithms are particularly useful when applied to problems whose [[energy landscape]] is very rough or has many local minima.<ref>{{cite web |title=Genetic Algorithms |url=http://www.pharmacologicalsciences.us/genetic-algorithms/scoring-functions.html |website=Pharmacological Sciences |date=7 November 2016 |access-date=7 November 2016}}</ref>

===In fiction===
{{Main|Evolution in fiction}}

Darwinian evolution by natural selection is pervasive in literature, whether taken optimistically in terms of how humanity may evolve towards perfection, or pessimistically in terms of the dire consequences of the interaction of human nature and the struggle for survival. Among major responses is [[Samuel Butler (novelist)|Samuel Butler]]'s 1872 pessimistic ''[[Erewhon]]'' ("nowhere", written mostly backwards). In 1893 [[H. G. Wells]] imagined "[[The Man of the Year Million]]", transformed by natural selection into a being with a huge head and eyes, and shrunken body.<ref name=SFE>{{cite encyclopedia |last1=Stableford |first1=Brian M. |last2=Langford |first2=David R. |title=Evolution |url=http://www.sf-encyclopedia.com/entry/evolution |encyclopedia=The Encyclopedia of Science Fiction |publisher=Gollancz |access-date=24 July 2018 |date=5 July 2018}}</ref>

==Notes==
{{Notelist}}

==References==
{{Reflist}}

===Sources===
{{Refbegin|30em}}
* {{cite book |last1=Agutter |first1=Paul S. |last2=Wheatley |first2=Denys N. |year=2008 |title=Thinking about Life: The History and Philosophy of Biology and Other Sciences |location=Dordrecht, the Netherlands; London |publisher=[[Springer Science+Business Media]] |isbn=978-1-4020-8865-0 |lccn=2008933269 |oclc=304561132 }}
* {{cite book |last=Andersson |first=Malte |year=1994 |title=Sexual Selection |series=Monographs in Behavior and Ecology |location=Princeton, NJ |publisher=[[Princeton University Press]] |isbn=978-0-691-00057-2 |lccn=93033276 |oclc=28891551 }}
* {{cite book |last=Ariew |first=André |year=2002 |chapter=Platonic and Aristotelian Roots of Teleological Arguments |chapter-url=http://web.missouri.edu/~ariewa/Teleology.pdf |editor1-last=Ariew |editor1-first=André |editor2-last=Cummins |editor2-first=Robert |editor3-last=Perlman |editor3-first=Mark |title=Functions: New Essays in the Philosophy of Psychology and Biology |location=Oxford; New York |publisher=[[Oxford University Press]] |isbn=978-0-19-824103-4 |lccn=2002020184 |oclc=48965141 |archive-url=https://web.archive.org/web/20090219021804/http://web.missouri.edu/~ariewa/Teleology.pdf |archive-date=19 February 2009 }}
* {{cite book |author=Aristotle |author-link=Aristotle |title=Physics |others=Translated by R.P. Hardie and R.K. Gaye |publisher=The Internet Classics Archive |oclc=54350394 |title-link=Physics (Aristotle) }}
* {{cite book |last1=Begon |first1=Michael |last2=Townsend |first2=Colin R. |last3=Harper |first3=John L. |author-link3=John L. Harper |year=1996 |title=Ecology: Individuals, Populations and Communities |edition=3rd |location=Oxford; Cambridge, MA |publisher=[[Wiley-Blackwell|Blackwell Science]] |isbn=978-0-632-03801-5 |lccn=95024627 |oclc=32893848 |title-link=Ecology: From Individuals to Ecosystems }}
* {{cite book |last=Christiansen |first=Freddy B. |year=1984 |chapter=The Definition and Measurement of Fitness |editor-last=Shorrocks |editor-first=Bryan |title=Evolutionary Ecology: The 23rd Symposium of the British Ecological Society, Leeds, 1982 |series=Symposium of the [[British Ecological Society]] |volume=23 |location=Oxford; Boston |publisher=[[Wiley-Blackwell#Blackwell Publishing history|Blackwell Scientific Publications]] |isbn=978-0-632-01189-6 |lccn=85106855 |oclc=12586581 |chapter-url=https://archive.org/details/evolutionaryecol0000brit }} Modified from Christiansen by adding survival selection in the reproductive phase.
* {{cite book |last=Darwin |first=Charles |author-link=Charles Darwin |year=1859 |title=On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life |edition=1st |location=London |publisher=[[John Murray (publishing house)|John Murray]] |lccn=06017473 |oclc=741260650 |title-link=On the Origin of Species }} The book is available from [http://darwin-online.org.uk/content/frameset?pageseq=1&itemID=F373&viewtype=side The Complete Work of Charles Darwin Online]. Retrieved 2015-07-23.
* {{cite book |last=Darwin |first=Charles |year=1861 |title=On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life |url=http://darwin-online.org.uk/content/frameset?pageseq=1&itemID=F381&viewtype=side |edition=3rd |location=London |publisher=John Murray |lccn=04001284 |oclc=550913 }}
* {{cite book |last=Darwin |first=Charles |year=1866 |title=On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life |url=http://darwin-online.org.uk/content/frameset?itemID=F385&viewtype=text&pageseq=1 |edition=4th |location=London |publisher=John Murray |lccn=|oclc=}}
* {{cite book |last=Darwin |first=Charles |year=1872 |title=The Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life |url=http://darwin-online.org.uk/content/frameset?pageseq=1&itemID=F391&viewtype=side |edition=6th |location=London |publisher=John Murray |oclc=1185571 }}
* {{cite book |last=Darwin |first=Charles |year=1958 |editor-last=Barlow |editor-first=Nora |title=The Autobiography of Charles Darwin, 1809–1882: With original omissions restored; Edited and with Appendix and Notes by his grand-daughter, Nora Barlow |url=http://darwin-online.org.uk/content/frameset?itemID=F1497&viewtype=side&pageseq=1 |location=London |publisher=[[William Collins, Sons|Collins]] |lccn=93017940 |oclc=869541868 }}
* {{cite book |last=Dawkins |first=Richard |author-link=Richard Dawkins |year=1976 |title=The Selfish Gene |location=New York |publisher=Oxford University Press |isbn=978-0-19-857519-1 |lccn=76029168 |oclc=2681149 |title-link=The Selfish Gene }}
* {{cite book |last=Dennett |first=Daniel C. |author-link=Daniel Dennett |year=1991 |title=Consciousness Explained |edition=1st |location=Boston, MA |publisher=[[Little, Brown and Company]] |isbn=978-0-316-18065-8 |lccn=91015614 |oclc=23648691 |title-link=Consciousness Explained }}
* {{cite book |last=Dennett |first=Daniel C. |year=1995 |title=Darwin's Dangerous Idea: Evolution and the Meanings of Life |location=New York |publisher=[[Simon & Schuster]] |isbn=978-0-684-80290-9 |lccn=94049158 |oclc=31867409 |title-link=Darwin's Dangerous Idea }}
* {{cite book |last=Dobzhansky |first=Theodosius |author-link=Theodosius Dobzhansky |year=1937 |title=Genetics and the Origin of Species |series=[[Columbia University Biological Series]] |location=New York |publisher=[[Columbia University Press]] |lccn=37033383 |oclc=766405 |title-link=Genetics and the Origin of Species }}
** {{cite book |last=Dobzhansky |first=Theodosius |year=1951 |title=Genetics and the Origin of Species |url=https://archive.org/details/in.ernet.dli.2015.547790 |series=Columbia University Biological Series |edition=3rd revised |location=New York |publisher=Columbia University Press |lccn=51014816 |oclc=295774 |author-mask=2 }}
* {{cite book |last=Eisley |first=Loren |author-link=Loren Eiseley |year=1958 |title=Darwin's Century: Evolution and the Men Who Discovered It |edition=1st |location=Garden City, NY |publisher=[[Doubleday (publisher)|Doubleday]] |lccn=58006638 |oclc=168989 }}
* {{cite book |author=Empedocles |author-link=Empedocles |year=1898 |chapter=Empedokles |editor-last=Fairbanks |editor-first=Arthur |editor-link=Arthur Fairbanks |title=The First Philosophers of Greece |chapter-url=https://archive.org/details/cu31924029013162 |others=Translation by Arthur Fairbanks |location=London |publisher=Kegan Paul, Trench, Trübner & Co. Ltd. |lccn=03031810 |oclc=1376248 }} {{Internet Archive|id=cu31924029013162|name=The First Philosophers of Greece}}.
* {{cite book |last=Endler |first=John A. |author-link=John Endler |year=1986 |title=Natural Selection in the Wild |location=Princeton, NJ |publisher=Princeton University Press |isbn=978-0-691-08386-5 |lccn=85042683 |oclc=12262762 }}
* {{cite book |last=Engels |first=Friedrich |author-link=Friedrich Engels |year=1964 |orig-year=1883 |title=Dialectics of Nature |edition=3rd rev. |others=1939 preface by [[J.B.S. Haldane]] |location=Moscow, USSR |publisher=[[Progress Publishers]] |lccn=66044448 |oclc=807047245 |title-link=Dialectics of Nature }} The book is available from the [https://www.marxists.org/archive/marx/works/1883/don/index.htm Marxist Internet Archive].
* {{cite book |last1=Falconer |first1=Douglas S. |author-link1=Douglas Scott Falconer |last2=Mackay |first2=Trudy F.C. |year=1996 |title=Introduction to Quantitative Genetics |edition=4th |location=Harlow, England |publisher=[[Longman]] |isbn=978-0-582-24302-6 |oclc=824656731 |url=https://archive.org/details/introductiontoqu00falc }}
* {{cite book |last=Fisher |first=Ronald Aylmer |author-link=Ronald Fisher |year=1930 |title=The Genetical Theory of Natural Selection |location=Oxford |publisher=[[Oxford University Press#Clarendon Press|The Clarendon Press]] |lccn=30029177 |oclc=493745635 |title-link=The Genetical Theory of Natural Selection }}
* {{cite book |last=Futuyma |first=Douglas J. |author-link=Douglas J. Futuyma |year=2005 |title=Evolution |location=Sunderland, MA |publisher=[[Sinauer Associates]] |isbn=978-0-87893-187-3 |lccn=2004029808 |oclc=57311264 |url=https://archive.org/details/evolution0000futu }}
* {{cite book |last=Goldberg |first=David E. |author-link=David E. Goldberg |year=1989 |title=Genetic Algorithms in Search, Optimization and Machine Learning |location=Reading, MA |publisher=[[Addison-Wesley|Addison-Wesley Publishing Company]] |isbn=978-0-201-15767-3 |lccn=88006276 |oclc=17674450 }}
* {{cite book |last=Haldane |first=J B.S. |author-link=J. B. S. Haldane |year=1932 |title=The Causes of Evolution |location=London; New York |publisher=[[Longman|Longmans, Green & Co.]] |lccn=32033284 |oclc=5006266 |title-link=The Causes of Evolution }} "This book is based on a series of lectures delivered in January 1931 at the Prifysgol Cymru, Aberystwyth, and entitled 'A re-examination of Darwinism'."
* {{cite book |last=Haldane |first=J.B.S. |author-link=J. B. S. Haldane |date=1954 |chapter=The Measurement of Natural Selection |chapter-url=http://wellcomelibrary.org/player/b18033878#?asi=0&ai=494&z=-0.6189%2C-0.0449%2C2.8968%2C1.6356 |editor1-last=Montalenti |editor1-first=Giuseppe |editor2-last=Chiarugi |editor2-first=A. |title=Atti del IX Congresso Internazionale di Genetica, Bellagio (Como) 24–31 agosto 1953 |trans-title=Proceedings of the 9th International Congress of Genetics |series=Caryologia |volume=6 (1953/54) Suppl. |location=Florence, Italy |publisher=[[University of Florence]] |pages=480–487 |oclc=9069245 }}
* {{cite book |last=Kauffman |first=Stuart |author-link=Stuart Kauffman |year=1993 |title=The Origins of Order: Self-Organisation and Selection in Evolution |location=New York |publisher=Oxford University Press |isbn=978-0-19-507951-7 |lccn=91011148 |oclc=23253930 }}
* {{cite book |last=Lamarck |first=Jean-Baptiste |author-link=Jean-Baptiste Lamarck |year=1809 |title=Philosophie Zoologique |location=Paris |publisher=Dentu et L'Auteur |oclc=2210044 |title-link=Philosophie Zoologique }} {{Internet Archive|id=philosophiezool06unkngoog|name=Philosophie zoologique (1809)}}.
* {{cite book |last=Lear |first=Jonathan |author-link=Jonathan Lear |year=1988 |title=Aristotle: The Desire to Understand |url=https://archive.org/details/aristotledesiret0000lear |url-access=registration |location=Cambridge, UK; New York |publisher=[[Cambridge University Press]] |isbn=978-0-521-34762-4 |lccn=87020284 |oclc=16352317 }}
* {{cite book |last=Kuhn |first=Thomas S. |author-link=Thomas Kuhn |year=1996 |title=The Structure of Scientific Revolutions |edition=3rd |location=Chicago, IL |publisher=University of Chicago Press |isbn=978-0-226-45808-3 |lccn=96013195 |oclc=34548541 }}
* {{cite book |editor1-last=Lemey |editor1-first=Philippe |editor2-last=Salemi |editor2-first=Marco |editor3-last=Vandamme |editor3-first=Anne-Mieke |year=2009 |title=The Phylogenetic Handbook: A Practical Approach to Phylogenetic Analysis and Hypothesis Testing |edition=2nd |location=Cambridge, UK; New York |publisher=Cambridge University Press |isbn=978-0-521-73071-6 |lccn=2009464132 |oclc=295002266 }}
* {{cite book |author=Lucretius |author-link=Lucretius |year=1916 |chapter=Book V |editor-last=Leonard |editor-first=William Ellery |editor-link=William Ellery Leonard |title=De rerum natura |others=Translated by William Ellery Leonard |location=Medford/Somerville, MA |publisher=[[Tufts University]] |oclc=33233743 |title-link=De rerum natura }}
* {{cite book |last1=MacArthur |first1=Robert H. |author-link1=Robert MacArthur |last2=Wilson |first2=Edward O. |author-link2=E. O. Wilson |year=2001 |orig-year=Originally published 1967 |title=The Theory of Island Biogeography |others=New preface by Edward O. Wilson |series=Princeton Landmarks in Biology |location=Princeton, NJ |publisher=Princeton University Press |isbn=978-0-691-08836-5 |lccn=00051495 |oclc=45202069 |title-link=The Theory of Island Biogeography }}
* {{cite book |last=Malthus |first=Thomas Robert |author-link=Thomas Robert Malthus |year=1798 |title=An Essay on the Principle of Population, As It Affects the Future Improvement of Society: with Remarks on the Speculations of Mr. Godwin, M. Condorcet, and Other Writers |edition=1st |location=London |publisher=J. Johnson |lccn=46038215 |oclc=65344349 |title-link=An Essay on the Principle of Population }} The book is available [http://www.faculty.rsu.edu/users/f/felwell/www/Theorists/Malthus/Essay.htm#112 here] {{Webarchive|url=https://web.archive.org/web/20210715132543/http://www.faculty.rsu.edu/users/f/felwell/www/Theorists/Malthus/Essay.htm#112 |date=15 July 2021 }} from Frank Elwell, [[Rogers State University]].
* {{cite book |last=Mayr |first=Ernst |author-link=Ernst Mayr |year=1942 |title=Systematics and the Origin of Species from the Viewpoint of a Zoologist |series=Columbia Biological Series |volume=13 |location=New York |publisher=Columbia University Press |lccn=43001098 |oclc=766053 |title-link=Systematics and the Origin of Species }}
* {{cite book |last=Mayr |first=Ernst |year=2006 |orig-year=Originally published 1972; Chicago, IL: Aldine Publishing Co. |chapter=Sexual Selection and Natural Selection |editor-last=Campbell |editor-first=Bernard G. |title=Sexual Selection and the Descent of Man: The Darwinian Pivot |location=New Brunswick, NJ |publisher=[[Transaction Publishers|AldineTransaction]] |isbn=978-0-202-30845-6 |lccn=2005046652 |oclc=62857839 }}
* {{cite book |last=Michod |first=Richard A. |year=1999 |title=Darwinian Dynamics: Evolutionary Transitions in Fitness and Individuality |location=Princeton, NJ |publisher=Princeton University Press |isbn=978-0-691-02699-2 |lccn=98004166 |oclc=38948118 |url=https://archive.org/details/darwiniandynamic00mich }}
* {{cite book |last=Miller |first=Geoffrey |author-link=Geoffrey Miller (psychologist) |year=2000 |title=The Mating Mind: How Sexual Choice Shaped the Evolution of Human Nature |edition=1st |location=New York |publisher=[[Doubleday (publisher)|Doubleday]] |isbn=978-0-385-49516-5 |lccn=00022673 |oclc=43648482 |url=https://archive.org/details/matingmind00geof }}
* {{cite book |last=Mitchell |first=Melanie |author-link=Melanie Mitchell |year=1996 |title=An Introduction to Genetic Algorithms |series=Complex Adaptive Systems |location=Cambridge, MA |publisher=[[MIT Press]] |isbn=978-0-262-13316-6 |lccn=95024489 |oclc=42854439 |url=https://archive.org/details/introductiontoge00mitc }}
* {{cite book |last=Pinker |first=Steven |author-link=Steven Pinker |year=1995 |orig-year=Originally published 1994; New York: [[William Morrow and Company]] |title=The Language Instinct: How the Mind Creates Language |edition=1st [[Harper Perennial]] |location=New York |publisher=Harper Perennial |isbn=978-0-06-097651-4 |lccn=94039138 |oclc=670524593 |title-link=The Language Instinct }}
* {{cite book |last=Rice |first=Sean H. |year=2004 |title=Evolutionary Theory: Mathematical and Conceptual Foundations |location=Sunderland, MA |publisher=Sinauer Associates |isbn=978-0-87893-702-8 |lccn=2004008054 |oclc=54988554 }}
* {{cite book |last=Roux |first=Wilhelm |author-link=Wilhelm Roux |year=1881 |title=Der Kampf der Theile im Organismus |url=https://archive.org/details/bub_gb_66lBAAAAYAAJ |location=Leipzig |publisher=[[Wilhelm Engelmann]] |oclc=8200805 }} {{Internet Archive|id=derkampfdertheil00roux|name=Der Kampf der Theile im Organismus}} Retrieved 2015-08-11.
* {{cite book |last=Sober |first=Elliott |author-link=Elliott Sober |year=1993 |orig-year=Originally published 1984; Cambridge, MA: MIT Press |title=The Nature of Selection: Evolutionary Theory in Philosophical Focus |location=Chicago, IL |publisher=[[University of Chicago Press]] |isbn=978-0-226-76748-2 |lccn=93010367 |oclc=896826726 }}
* {{cite book |last=Wallace |first=Alfred Russel |author-link=Alfred Russel Wallace |year=1871 |orig-year=Originally published 1870 |title=Contributions to the Theory of Natural Selection. A Series of Essays |url=http://quod.lib.umich.edu/cgi/t/text/text-idx?c=moa&idno=AJP5195.0001.001&view=toc |edition=2nd, with corrections and additions |location=New York |publisher=[[Macmillan Publishers|Macmillan & Co.]] |lccn=agr04000394 |oclc=809350209 }}
* {{cite book |last=Williams |first=George C. |author-link=George C. Williams (biologist) |year=1966 |title=Adaptation and Natural Selection: A Critique of Some Current Evolutionary Thought |series=Princeton Science Library |location=Princeton, NJ |publisher=Princeton University Press |isbn=978-0-691-02615-2 |lccn=65017164 |oclc=35230452 |title-link=Adaptation and Natural Selection }}
* {{cite book |last=Wilson |first=David Sloan |author-link=David Sloan Wilson |year=2002 |title=Darwin's Cathedral: Evolution, Religion, and the Nature of Society |location=Chicago, IL |publisher=University of Chicago Press |isbn=978-0-691-02615-2 |lccn=2002017375 |oclc=48777441 }}
* {{cite book |last1=Zimmer |first1=Carl |author-link1=Carl Zimmer |last2=Emlen |first2=Douglas J. |author-link2=Douglas Emlen |year=2013 |title=Evolution: Making Sense of Life |edition=1st |location=Greenwood Village, CO |publisher=Roberts and Company Publishers |isbn=978-1-936221-17-2 |lccn=2012025118 |oclc=767565909 }}
{{Refend}}

==Further reading==
* '''For technical audiences'''
** {{cite book |last=Bell |first=Graham |author-link=Graham Bell (biologist) |year=2008 |title=Selection: The Mechanism of Evolution |edition=2nd |location=Oxford; New York |publisher=[[Oxford University Press]] |isbn=978-0-19-856972-5 |lccn=2007039692 |oclc=170034792|ref=none}}
** {{cite book |last=Johnson |first=Clifford |year=1976 |title=Introduction to Natural Selection |location=Baltimore, MD |publisher=University Park Press |isbn=978-0-8391-0936-5 |lccn=76008175 |oclc=2091640 |url=https://archive.org/details/introductiontona00john |ref=none}}
** {{cite book |last=Gould |first=Stephen Jay |year=2002 |author-link=Stephen Jay Gould |title=The Structure of Evolutionary Theory |location=Cambridge, MA |publisher=[[Harvard University Press|Belknap Press of Harvard University Press]] |isbn=978-0-674-00613-3 |lccn=2001043556 |oclc=47869352 |title-link=The Structure of Evolutionary Theory |ref=none}}
** {{cite book |last=Maynard Smith |first=John |author-link=John Maynard Smith |year=1993 |orig-year=Originally published 1958; Harmondsworth, England: [[Penguin Books]] |title=The Theory of Evolution |edition=Canto |location=Cambridge, New York |publisher=[[Cambridge University Press]] |isbn=978-0-521-45128-4 |lccn=93020358 |oclc=27676642|title-link=The Theory of Evolution |ref=none}}
** {{cite journal |last=Popper |first=Karl |author-link=Karl Popper |date=December 1978 |title=Natural Selection and the Emergence of Mind |url=http://www.informationphilosopher.com/solutions/philosophers/popper/natural_selection_and_the_emergence_of_mind.html |journal=[[Dialectica]] |volume=32 |issue=3–4 |pages=339–355 |doi=10.1111/j.1746-8361.1978.tb01321.x |issn=0012-2017|ref=none}}
** {{cite journal |last1=Sammut-Bonnici |first1=Tanya |last2=Wensley |first2=Robin |date=September 2002 |title=Darwinism, probability and complexity: Market-based organizational transformation and change explained through the theories of evolution |journal=[[International Journal of Management Reviews]] |volume=4 |issue=3 |pages=291–315 |doi=10.1111/1468-2370.00088 |issn=1460-8545|url=http://wrap.warwick.ac.uk/57024/1/WRAP_Sammut-Bonnici_httpwrap%20warwick%20ac%20uk57024%20%282%29.pdf |ref=none}}
** {{cite book |editor-last=Sober |editor-first=Elliott |editor-link=Elliott Sober |year=1994 |title=Conceptual Issues in Evolutionary Biology |edition=2nd |location=Cambridge, MA |publisher=[[MIT Press]] |isbn=978-0-262-69162-8 |lccn=93008199 |oclc=28150417 |url=https://archive.org/details/conceptualissues0000unse |ref=none}}
** {{cite book |last=Williams |first=George C. |author-link=George C. Williams (biologist) |year=1992 |title=Natural Selection: Domains, Levels, and Challenges |series=Oxford Series in Ecology and Evolution |location=New York |publisher=Oxford University Press |isbn=978-0-19-506933-4 |lccn=91038938 |oclc=228136567 |ref=none}}
* '''For general audiences'''
** {{cite book |last=Dawkins |first=Richard |author-link=Richard Dawkins |year=1996 |title=Climbing Mount Improbable |url=https://archive.org/details/climbingmountimp0000dawk |url-access=registration |edition=1st American |location=New York |publisher=W.W. Norton & Company |isbn=978-0-393-03930-6 |lccn=34633422 |oclc=34633422 |ref=none}}
** {{cite book |last=Gould |first=Stephen Jay |year=1977 |title=Ever Since Darwin: Reflections in Natural History |edition=1st |location=New York |publisher=W.W. Norton & Company |isbn=978-0-393-06425-4 |lccn=77022504 |oclc=3090189|title-link=Ever Since Darwin |ref=none}}
** {{cite book |last=Jones |first=Steve |author-link=Steve Jones (biologist) |year=2000 |title=Darwin's Ghost: The Origin of Species Updated |edition=1st |location=New York |publisher=[[Random House]] |isbn=978-0-375-50103-6 |lccn=99053246 |oclc=42690131 |title-link=Almost Like a Whale |ref=none}}
** {{cite journal |last=Lewontin |first=Richard C. |author-link=Richard Lewontin |date=September 1978 |title=Adaptation |journal=[[Scientific American]] |volume=239 |issue=3 |pages=212–230 |doi=10.1038/scientificamerican0978-212 |issn=0036-8733 |pmid=705323|bibcode=1978SciAm.239c.212L |ref=none}}
** {{cite book |last=Mayr |first=Ernst |author-link=Ernst Mayr |year=2002 |orig-year=Originally published 2001; New York: [[Basic Books]] |title=What Evolution Is |series=Science Masters |location=London |publisher=[[Weidenfeld & Nicolson]] |isbn=978-0-297-60741-0 |lccn=2001036562 |oclc=248107061 |ref=none}}
** {{cite book |last=Weiner |first=Jonathan |author-link=Jonathan Weiner |year=1994 |title=The Beak of the Finch: A Story of Evolution in Our Time |edition=1st |location=New York |publisher=[[Alfred A. Knopf|Knopf]] |isbn=978-0-679-40003-5 |lccn=93036755 |oclc=29029572 |title-link=The Beak of the Finch |ref=none}}
* '''Historical'''
** {{cite book |last=Kohn |first=Marek |author-link=Marek Kohn |year=2004 |title=A Reason for Everything: Natural Selection and the English Imagination |location=London |publisher=[[Faber and Faber]] |isbn=978-0-571-22392-3 |lccn=2005360890 |oclc=57200626 |ref=none}}
** {{cite journal |last=Zirkle |first=Conway |author-link=Conway Zirkle |date=25 April 1941 |title=Natural Selection before the 'Origin of Species' |journal=[[Proceedings of the American Philosophical Society]] |volume=84 |issue=1 |pages=71–123 |jstor=984852 |issn=0003-049X|ref=none}}

==External links==
{{Wikiquote}}
* {{cite web |url=http://literature.org/authors/darwin-charles/the-origin-of-species/chapter-04.html |archive-url=https://web.archive.org/web/20010225025716/http://www.literature.org/authors/darwin-charles/the-origin-of-species/chapter-04.html |url-status=dead |archive-date=25 February 2001 |title=On the Origin of Species |last=Darwin |first=Charles |author-link=Charles Darwin |ref=none}}&nbsp;– Chapter 4, Natural Selection

{{Evolution}}
{{Evolutionary psychology}}
{{Population genetics}}
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[[Category:Natural selection| ]]
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[[Category:Sexual selection]]