Editing Natural selection (section)

==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 |url-access=subscription }}</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 seated crop.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>