Editing Sexual selection (section)

== History ==

=== Darwin ===

[[File:Darwin sexual caricature.gif|thumb |upright |Victorian cartoonists mocked Darwin's ideas about display in sexual selection. Here he is fascinated by the apparent [[steatopygia]] in the latest fashion.|alt=Victorian era cartoon of Darwin as a monkey looking at a woman in a bustle dress]]

{{Further|The Descent of Man, and Selection in Relation to Sex}}

Sexual selection was first proposed by Charles Darwin in ''[[On the Origin of Species]]'' (1859) and developed in ''[[The Descent of Man, and Selection in Relation to Sex]]'' (1871), as he felt that natural selection alone was unable to account for certain types of non-survival adaptations. He once wrote to a colleague that "The sight of a feather in a [[peahen|peacock]]'s tail, whenever I gaze at it, makes me sick!" His work divided sexual selection into male–male competition and female choice.<ref>{{cite journal |last=Darwin |first=Charles |author-link=Charles Darwin |title=On the Tendency of Species to form Varieties; and on the Perpetuation of Varieties and Species by Natural Means of Selection |journal=Journal of the Proceedings of the Linnean Society of London. Zoology |year=1858 |volume=3 |issue=9 |pages=46–50 |url=http://darwin-online.org.uk/converted/pdf/1858_species_F350.pdf |doi=10.1111/j.1096-3642.1858.tb02500.x |url-status=live |archive-url=https://web.archive.org/web/20121022104103/http://darwin-online.org.uk/converted/pdf/1858_species_F350.pdf |archive-date=22 October 2012 |doi-access=free }}</ref><ref name="Mendelson Safran 2021">{{cite journal |last1=Mendelson |first1=Tamra C. |last2=Safran |first2=Rebecca J. |title=Speciation by sexual selection: 20 years of progress |journal=[[Trends in Ecology & Evolution]] |date=2021 |volume=36 |issue=12 |pages=1153–1163 |doi=10.1016/j.tree.2021.09.004|pmid=34607719 |bibcode=2021TEcoE..36.1153M }}</ref>

{{blockquote|... depends, not on a struggle for existence, but on a struggle between the males for possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring.<ref name=origin>[[Charles Darwin|Darwin, Charles]] (1859). ''On the Origin of Species'' (1st edition). Chapter 4, p. 88. "And this leads me to say a few words on what I call Sexual Selection. This depends ..." {{cite web |url=http://darwin-online.org.uk/content/frameset?viewtype=side&itemID=F373&pageseq=12 |title=Archived copy |access-date=2011-05-22 |url-status=live |archive-url=https://web.archive.org/web/20111105031643/http://darwin-online.org.uk/content/frameset?viewtype=side&itemID=F373&pageseq=12 |archive-date=2011-11-05 }}</ref>}}

{{blockquote |... when the males and females of any animal have the same general habits ... but differ in structure, colour, or ornament, such differences have been mainly caused by sexual selection.<ref name=origin />}}

These views were to some extent opposed by [[Alfred Russel Wallace]], mostly after Darwin's death. He accepted that sexual selection could occur, but argued that it was a relatively weak form of selection. He argued that male–male competitions were forms of natural selection, but that the "drab" peahen's coloration is itself adaptive as [[camouflage]]. In his opinion, ascribing mate choice to females was attributing the ability to judge standards of beauty to animals (such as [[beetle]]s) far too cognitively undeveloped to be capable of [[aesthetic]] feeling.<ref name="Wallace">{{cite web |last=Wallace |first=Alfred Russel |author-link=Alfred Russel Wallace |title=Note on Sexual Selection (S459: 1892) |url=http://people.wku.edu/charles.smith/wallace/S459.htm |publisher=Charles Smith |access-date=13 January 2017 |date=1892 |url-status=live |archive-url=https://web.archive.org/web/20170217215250/http://people.wku.edu/charles.smith/wallace/S459.htm |archive-date=17 February 2017}}</ref>

[[File:Tribolium castaneum87-300.jpg|thumb|Sexual selection protected [[flour beetle]]s from extinction in a ten-year experiment.<ref name="popben"/>|alt=Photograph of flour beetles]]

Darwin's ideas on sexual selection were met with scepticism by his contemporaries and not considered of great importance, until in the 1930s biologists decided to include sexual selection as a mode of natural selection.<ref>{{cite book |last=Miller |first=G. F. |year=2000 |title=The Mating Mind: How sexual choice shaped the evolution of human nature |publisher=Heinemann |location=London |isbn=978-0-434-00741-7 |page=24}}</ref> Only in the 21st century have they become more important in [[biology]]; the theory is now seen as generally applicable and analogous to natural selection.<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> A ten-year study, experimentally varying sexual selection on [[flour beetle]]s with other factors held constant, showed that sexual selection protected even an [[Inbreeding|inbred]] population against extinction.<ref name="popben">[http://phys.org/news/2015-05-population-benefits-sexual-males.html Population benefits of sexual selection explain the existence of males phys.org May 18, 2015 Report] on a study by the [[University of East Anglia]] {{webarchive |url=https://web.archive.org/web/20150821000158/http://phys.org/news/2015-05-population-benefits-sexual-males.html |date=August 21, 2015 }}</ref>

=== Fisherian runaway ===

{{Main|Fisherian runaway}}

[[Ronald Fisher]], the [[England|English]] [[statistician]] and [[evolutionary biologist]], developed his ideas about sexual selection in his 1930 book ''[[The Genetical Theory of Natural Selection]]''. These include the [[sexy son hypothesis]], which might suggest a preference for male offspring, and [[Fisher's principle]], which explains why the sex ratio is usually close to 1:1. The [[Fisherian runaway]] describes how sexual selection accelerates the preference for a specific ornament, causing the preferred trait and female preference for it to increase together in a [[positive feedback]] runaway cycle.<ref name="Fisher 1930">[[Ronald Fisher|Fisher, R. A.]] (1930) ''[[The Genetical Theory of Natural Selection]]''. Oxford University Press, {{ISBN |0-19-850440-3}}, {{usurped|1=[https://web.archive.org/web/20070927201928/http://www.memeoid.net/books/GeneticalTheoryOfNS-Chapter6.pdf Chapter 6]}}.</ref> He remarked that:<ref name=Blind/>

{{blockquote |... plumage development in the male, and sexual preference for such developments in the female, must thus advance together, and so long as the process is unchecked by severe counterselection, will advance with ever-increasing speed. In the total absence of such checks, it is easy to see that the speed of development will be proportional to the development already attained, which will therefore increase with time [[exponential growth|exponentially]], or in [[geometric progression]]. —''Ronald Fisher, 1930''<ref name="Fisher 1930"/>}}

[[File:Euplectes progne male South Africa cropped.jpg|thumb|Male [[long-tailed widowbird]] |alt=Photograph of a bird with an exceptionally long tail ]]

This causes a dramatic increase in both the male's conspicuous feature and in female preference for it, resulting in marked [[sexual dimorphism]], until practical physical constraints halt further exaggeration. A [[positive feedback]] loop is created, producing extravagant physical structures in the non-limiting sex. A classic example of female choice and potential runaway selection is the [[long-tailed widowbird]]. While males have long tails that are selected for by female choice, female tastes in tail length are still more extreme with females being attracted to tails longer than those that naturally occur.<ref name="Andersson 1994">{{cite book |last=Andersson |first=M. |year=1994 |pages=115–117 |title=Sexual Selection |publisher=Princeton University Press |isbn=0-691-00057-3}}</ref> Fisher understood that female preference for long tails may be passed on genetically, in conjunction with genes for the long tail itself. Long-tailed widowbird offspring of both sexes inherit both sets of genes, with females [[Gene expression|expressing]] their genetic preference for long tails, and males showing off the coveted long tail itself.<ref name=Blind/>

[[Richard Dawkins]] presents a non-mathematical explanation of the runaway sexual selection process in his book ''[[The Blind Watchmaker]]''.<ref name=Blind>{{cite book |last=Dawkins |first=Richard |author-link=Richard Dawkins |title=The Blind Watchmaker: Why the Evidence of Evolution Reveals a Universe Without Design |url=https://books.google.com/books?id=sPpaZnZMDG0C |year=1996 |publisher=Norton |isbn=978-0-393-31570-7 |pages=Chapter 8, Explosions and Spirals}}</ref> Females that prefer long tailed males tend to have mothers that chose long-tailed fathers. As a result, they carry both sets of genes in their bodies. That is, genes for long tails and for preferring long tails become [[Linkage disequilibrium|linked]]. The taste for long tails and tail length itself may therefore become correlated, tending to increase together. The more tails lengthen, the more long tails are desired. Any slight initial imbalance between taste and tails may set off an explosion in tail lengths. Fisher wrote that:

{{blockquote|The exponential element, which is the kernel of the thing, arises from the rate of change in hen taste being proportional to the absolute average degree of taste. —''Ronald Fisher, 1932''<ref>[[Ronald Fisher]] in a letter to [[Charles Galton Darwin]], 22 November 1932, cited in Fisher, R. A., Bennett, J. H. 1999. ''The genetical theory of natural selection: A complete variorum edition'', Oxford University Press, Oxford, p. 308</ref>}}

[[File:Peacock Flying.jpg|thumb|upright=1.35<!--for low-aspect-ratio image-->|The peacock tail in flight, the proposed classic example of a [[Fisherian runaway]]|alt=Photograph of a flying peacock ]]

The female widowbird chooses to mate with the most attractive long-tailed male so that her progeny, if male, will themselves be attractive to females of the next generation—thereby fathering many offspring that carry the female's genes. Since the rate of change in preference is proportional to the average taste amongst females, and as females desire to secure the services of the most sexually attractive males, an additive effect is created that, if unchecked, can yield exponential increases in a given taste and in the corresponding desired sexual attribute.<ref name=Blind/>

{{blockquote|It is important to notice that the conditions of relative stability brought about by these or other means, will be far longer duration than the process in which the ornaments are evolved. In most existing species the runaway process must have been already checked, and we should expect that the more extraordinary developments of sexual plumage are not due like most characters to a long and even course of evolutionary progress, but to sudden spurts of change. —''Ronald Fisher, 1930''<ref name="Fisher 1930"/>}}

Since Fisher's initial conceptual model of the 'runaway' process, [[Russell Lande]] and Peter O'Donald have provided detailed mathematical proofs that define the circumstances under which runaway sexual selection can take place.<ref>{{cite journal | last1=Lande | first1=Russell |author-link=Russell Lande | year=1981 | title=Models of speciation by sexual selection on polygenic traits | journal=PNAS | volume=78 | issue=6 | pages=3721–3725 | doi=10.1073/pnas.78.6.3721 | pmid=16593036 | pmc=319643 | bibcode=1981PNAS...78.3721L | doi-access=free }}</ref><ref>{{cite book |last=O'Donald |first=Peter |year=1980 |title=Genetic Models of Sexual Selection |publisher=[[Cambridge University Press]] |isbn=9780521225335 }}</ref> Alongside this, biologists have extended Darwin's formulation; Malte Andersson's widely accepted<ref name="Kokko Jennions 2014">{{cite journal | last1=Kokko | first1=H. | last2=Jennions | first2=M. D. | title=The Relationship between Sexual Selection and Sexual Conflict | journal=Cold Spring Harbor Perspectives in Biology | publisher=Cold Spring Harbor Laboratory | volume=6 | issue=9 | date=18 July 2014 | issn=1943-0264 | doi=10.1101/cshperspect.a017517 | pages=a017517| pmid=25038050 | pmc=4142970 }}</ref> 1994 definition is that "sexual selection is the differences in reproduction that arise from variation among individuals in traits that affect success in competition over mates and fertilizations".<ref name="Andersson 1994"/><ref name="Kokko Jennions 2014"/> Despite some practical challenges for biologists, the concept of sexual selection is "straightforward".<ref name="Kokko Jennions 2014"/>