Editing Sexual selection

{{Short description|Mode of natural selection involving the choosing of and competition for mates}}
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{{Use British English|date=June 2015}}
[[File:Paradesia decora Keulemans.jpg|thumb|250px|Sexual selection creates colourful [[sexual dimorphism|differences between sexes]] in [[Goldie's bird-of-paradise]]. Male above; female below. Painting by [[John Gerrard Keulemans]].|alt=painting of male and female birds of paradise]]

'''Sexual selection''' is a mechanism of [[evolution]] in which members of one [[sex]] [[mate choice|choose mates]] of the other sex to [[mating|mate]] with (intersexual selection), and compete with members of the same sex for access to members of the opposite sex (intrasexual selection). These two forms of selection mean that some individuals have greater [[reproductive success]] than others within a [[population]], for example because they are more [[Animal sexual behaviour|attractive]] or prefer more attractive partners to produce [[offspring]]. Successful males benefit from frequent mating and monopolizing access to one or more fertile females. Females can maximise the return on the energy they invest in reproduction by selecting and mating with the best males.

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The concept was first articulated by [[Charles Darwin]] who wrote of a "second agency" other than [[natural selection]], in which competition between mate candidates could lead to speciation. The theory was given a mathematical basis by [[Ronald Fisher]] in the early 20th century. Sexual selection can lead males to extreme efforts to demonstrate their [[fitness (biology)|fitness]] to be chosen by females, producing [[sexual dimorphism]] in [[secondary sexual characteristic]]s, such as the ornate [[plumage]] of [[birds-of-paradise]] and [[peafowl]], or the antlers of [[deer]]. Depending on the species, these rules can be reversed. This is caused by a [[positive feedback]] mechanism known as a [[Fisherian runaway]], where the passing-on of the desire for a trait in one sex is as important as having the trait in the other sex in producing the runaway effect. Although the [[sexy son hypothesis]] indicates that females would prefer male offspring, [[Fisher's principle]] explains why the [[sex ratio]] is most often 1:1. 
Sexual selection is widely distributed in the animal kingdom, and is also found in [[Sexual selection in flowering plants|plants]] and [[Sexual selection in fungi|fungi]].

== 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"/>

== Modern theory ==

=== Reproductive success ===

{{Further|Bateman's principle}}

[[File:Irish Elk front.jpg|thumb|The enormous sexually selected antlers of the [[Irish elk]] might have helped it on its way to extinction.<ref name="Gould 1974"/>|alt=Photograph of a museum specimen of an Irish elk skull with large antlers]]

The [[reproductive success]] of an organism is measured by the number of [[offspring]] left behind, and by their quality or probable [[fitness (biology)|fitness]].<ref>{{cite journal |last=Orr |first=H. A. |title=Fitness and its role in evolutionary genetics |journal=Nature Reviews Genetics |volume=10 |issue=8 |pages=531–9 |date=August 2009 |pmid=19546856 |doi=10.1038/nrg2603 |pmc=2753274}}</ref><ref>{{cite book |last=Starr |first=Cecie |title=Biology: The Unity & Diversity of Life |year=2013 |publisher=Cengage Learning |page=281}}</ref><ref name="PHYS-20140129">{{cite news |last=Vogt |first=Yngve |title=Large testicles are linked to infidelity |url=http://phys.org/news/2014-01-large-testicles-linked-infidelity.html |date=January 29, 2014 |work=[[Phys.org]] |access-date=January 31, 2014 |url-status=live |archive-url=https://web.archive.org/web/20140131114327/http://phys.org/news/2014-01-large-testicles-linked-infidelity.html |archive-date=January 31, 2014 }}</ref> Sexual preference creates a tendency towards [[assortative mating]] or [[homogamy (biology)|homogamy]]. The general conditions of sexual discrimination appear to be (1) the acceptance of one mate precludes the effective acceptance of alternative mates, and (2) the rejection of an offer is followed by other offers, either certainly or at such high chance that the risk of non-occurrence is smaller than the chance advantage to be gained by selecting a mate. [[Bateman's principle]] states that the sex which invests the most in producing offspring becomes a limiting resource for which the other sex competes, illustrated by the greater [[parental investment|nutritional investment]] of an egg in a [[zygote]], and the limited capacity of females to reproduce; for example, in humans, a woman can only give birth every ten months, whereas a male can become a father numerous times in the same period.<ref>{{Cite journal |last=Bateman |first=Angus J. |author-link=Angus John Bateman |title=Intra-sexual selection in Drosophila |journal=Heredity |volume=2 |pages=349–368 |year=1948 |doi=10.1038/hdy.1948.21 |pmid=18103134 |issue=Pt. 3 |doi-access=free }}</ref> More recently, researchers have doubted whether Bateman was correct.<ref name="Newcomer">{{Cite journal |last1=Newcomer |first1=Scott D. |last2=Zeh |first2=Jeanne A. |last3=Zeh |first3=David W. |date=31 August 1999 |title=Genetic benefits enhance the reproductive success of polyandrous females |journal=Proceedings of the National Academy of Sciences |volume=96 |issue=18 |pages=10236–10241 |doi=10.1073/pnas.96.18.10236 |pmid=10468592 |pmc=17872 |bibcode=1999PNAS...9610236N |doi-access=free }}</ref>

=== Honest signalling ===

{{Further|Signalling theory}}

The [[handicap principle]] of [[Amotz Zahavi]], [[Russell Lande]] and [[W. D. Hamilton]], holds that the male's survival until and through the age of reproduction with seemingly maladaptive traits is taken by the female as [[Signalling theory|a signal]] of his overall fitness. Such handicaps might prove he is either free of or resistant to [[disease]], or that he possesses more speed or a greater physical strength that is used to combat the troubles brought on by the exaggerated trait.<ref name="Zahavi 1975 pp. 205–214">{{cite journal |last=Zahavi |first=Amotz |author-link=Amotz Zahavi |title=Mate selection—A selection for a handicap |journal=Journal of Theoretical Biology |volume=53 |issue=1 |year=1975 |doi=10.1016/0022-5193(75)90111-3 |pages=205–214 |pmid=1195756 |bibcode=1975JThBi..53..205Z }}</ref><ref name="Zahavi 1977 pp. 603–605">{{cite journal |last=Zahavi |first=Amotz |author-link=Amotz Zahavi |title=The cost of honesty |journal=Journal of Theoretical Biology |volume=67 |issue=3 |year=1977 |issn=0022-5193 |doi=10.1016/0022-5193(77)90061-3 |pages=603–605|pmid=904334 |bibcode=1977JThBi..67..603Z }}</ref><ref name="Zahavi Zahavi 1997">{{cite book |last1=Zahavi |first1=Amotz |author1-link=Amotz Zahavi |last2=Zahavi |first2=Avishag |title=The handicap principle: a missing piece of Darwin's puzzle |publisher=Oxford University Press |location=New York |year=1997 |isbn=978-0-19-510035-8 |oclc=35360821 |url=http://eprints.soton.ac.uk/261475/1/10.1.1.40.3266.pdf}}</ref> Zahavi's work spurred a re-examination of the field and several new theories. In 1984, Hamilton and [[Marlene Zuk]] introduced the "Bright Male" hypothesis, suggesting that male elaborations might serve as a marker of health, by exaggerating the effects of disease and deficiency.<ref name="HamiltonZuk1982">{{cite journal |last1=Hamilton |first1=W. D. |author1-link=W. D. Hamilton |last2=Zuk |first2=M. |author2-link=Marlene Zuk |title=Heritable true fitness and bright birds: a role for parasites? |journal=Science |volume=218 |issue=4570 |year=1982 |pages=384–387 |doi=10.1126/science.7123238 |pmid=7123238 |bibcode=1982Sci...218..384H }}</ref>

=== Male intrasexual competition ===

[[File:Susa group, mountain gorilla.jpg|thumb|upright=0.8|Male [[mountain gorilla]], a species with very large males<ref>{{cite book |last1=Williamson |first1=E. A. |last2=Butynski |first2=T. M. |year=2013 |title=Mammals of Africa |volume=2. Primates |editor1=Butynski, T. M. |editor2=Kingdon, J. |editor3=Kalina, J. |isbn=9781408189962 |location=London, New Delhi, New York, Sydney |publisher=Bloomsbury |pages=45–53 |chapter=''Gorilla beringei'' eastern gorilla |chapter-url=https://books.google.com/books?id=B_07noCPc4kC&pg=RA1-PA45}}</ref>|alt=Photograph of a large male gorilla]]

{{Main|Male intrasexual competition}}

Male–male competition occurs when two males of the same species compete for the opportunity to mate with a female. Sexually dimorphic traits, size, sex ratio,<ref name="Weir 2012">{{Cite journal |last=Weir |first=Laura K. |date=2012-11-22 |title=Male–male competition and alternative male mating tactics influence female behavior and fertility in Japanese medaka (Oryzias latipes) |journal=Behavioral Ecology and Sociobiology |volume=67 |issue=2 |pages=193–203 |doi=10.1007/s00265-012-1438-9 |s2cid=15410498 }}</ref> and the social situation<ref name="Proctor-2012">{{Cite journal |last1=Proctor |first1=D. S. |last2=Moore |first2=A. J. |last3=Miller |first3=C. W. |date=2012-03-09 |title=The form of sexual selection arising from male–male competition depends on the presence of females in the social environment |journal=Journal of Evolutionary Biology |volume=25 |issue=5 |pages=803–812 |doi=10.1111/j.1420-9101.2012.02485.x |pmid=22404372 |s2cid=594384 }}</ref> may all play a role in the effects male–male competition has on the reproductive success of a male and the mate choice of a female. Larger males tend to win male–male conflicts.<ref>{{Cite journal |last=Otronen |first=Merja |date=1984-08-01 |title=Male contests for territories and females in the fly Dryomyza Anilis |journal=Animal Behaviour |volume=32 |issue=3 |pages=891–898 |doi=10.1016/S0003-3472(84)80167-0 |s2cid=53188298 }}</ref> Males take many risks in such conflicts, so the value of the resource must be large enough to justify those risks.<ref name="Nelson-Flower 2015">{{Cite journal |last1=Nelson-Flower |first1=Martha J. |last2=Ridley |first2=Amanda R. |date=2015-09-24 |title=Male–male competition is not costly to dominant males in a cooperatively breeding bird |journal=Behavioral Ecology and Sociobiology |volume=69 |issue=12 |pages=1997–2004 |doi=10.1007/s00265-015-2011-0 |bibcode=2015BEcoS..69.1997N |s2cid=15032582 |issn=0340-5443}}</ref><ref name="Luo 2016">{{Cite journal |last1=Luo |first1=Zhenhua |last2=Li |first2=Chenliang |last3=Wang |first3=Hui |last4=Shen |first4=Hang |last5=Zhao |first5=Mian |last6=Gu |first6=Qi |last7=Liao |first7=Chunlin |last8=Gu |first8=Zhirong |last9=Wu |first9=Hua |display-authors=3 |date=2016-02-23 |title=Male–male competition drives sexual selection and group spawning in the Omei treefrog, Rhacophorus omeimontis |journal=Behavioral Ecology and Sociobiology |volume=70 |issue=4 |pages=593–605 |doi=10.1007/s00265-016-2078-2 |bibcode=2016BEcoS..70..593L |s2cid=13912038 |issn=0340-5443}}</ref> [[Winner and loser effects]] further influence male behaviour.<ref name="Zeng 2018">{{Cite journal |last1=Zeng |first1=Yang |last2=Zhou |first2=Feng-Hao |last3=Zhu |first3=Dao-Hong |date=2018-06-26 |title=Fight outcome briefly affects the reproductive fitness of male crickets |journal=Scientific Reports |volume=8 |issue=1 |pages=9695 |doi=10.1038/s41598-018-27866-4  |pmc=6018733 |pmid=29946077 |bibcode=2018NatSR...8.9695Z }}</ref> Male–male competition may also affect a female's ability to select the best mates, and therefore decrease the likelihood of successful reproduction.<ref name="Cayuela 2016">{{Cite journal |last1=Cayuela |first1=Hugo |last2=Lengagne |first2=Thierry |last3=Kaufmann |first3=Bernard |last4=Joly |first4=Pierre |last5=Léna |first5=Jean-Paul |date=2016-06-24 |title=Larval competition risk shapes male–male competition and mating behavior in an anuran |journal=Behavioral Ecology |volume=27 |issue=6 |pages=arw100 |doi=10.1093/beheco/arw100 |doi-access=free }}</ref>

=== Multiple models ===

{{Further|Sexy son hypothesis|Sexual conflict|Mate choice}}

More recently, the field has grown to include other areas of study, not all of which fit Darwin's definition of sexual selection. A "bewildering"<ref name="Kokko Jennions 2006"/> range of models variously attempt to relate sexual selection not only to the fundamental<ref name="Kokko Jennions 2006"/> questions of [[anisogamy]] and parental roles, but also to mechanisms such as [[sex ratio]]s – governed by [[Fisher's principle]],<ref name="Hamilton 1967">{{cite journal |last=Hamilton |first=W. D. |author-link=W. D. Hamilton |year=1967 |title=Extraordinary sex ratios |journal=Science |volume=156 |issue=3774 |pages=477–488 |bibcode=1967Sci...156..477H |pmid=6021675 |doi=10.1126/science.156.3774.477}}</ref> parental care, [[Sexy son hypothesis|investing in sexy sons]], [[sexual conflict]], and the "most-debated effect",<ref name="Kokko Jennions 2006"/> namely [[mate choice]].<ref name="Kokko Jennions 2006">{{cite journal |last1=Kokko |first1=Hanna |last2=Jennions |first2=Michael D. |last3=Brooks |first3=Robert |title=Unifying and Testing Models of Sexual Selection |journal=Annual Review of Ecology, Evolution, and Systematics |volume=37 |issue=1 |year=2006 |pages=43–66 |doi=10.1146/annurev.ecolsys.37.091305.110259 |url=https://www.researchgate.net/profile/Michael_Jennions/publication/261950187_Unifying_and_Testing_Models_of_Sexual_Selection/links/550775830cf2d7a281257def.pdf<!--not a copyvio as it's the author, and not redundant to the DOI either--> |hdl=1885/22652 |hdl-access=free }}</ref> Elaborated characteristics that might seem costly, like the tail of the Montezuma swordfish (''[[Xiphophorus montezumae]]''), do not always have an energetics, performance or even survival cost; this may be because "compensatory traits" have evolved in concert with the sexually selected traits.<ref name="Oufiero 2015">{{cite web |last=Oufiero |first=Christopher E. |title=Sexual Selection, Costs, and Compensation |date=May 2015<!--updated, presumably--> |publisher=University of California Riverside |url=http://idea.ucr.edu/documents/flash/sexual_selection_costs/story.htm |archive-url=https://web.archive.org/web/20140606234023/http://idea.ucr.edu/documents/flash/sexual_selection_costs/story.htm |archive-date=6 June 2014}}</ref>

=== Toolkit of natural selection ===

[[File:Protarchaeopteryx 4713.JPG|thumb|''[[Protarchaeopteryx]]'' was flightless, but had feathers, perhaps used in courtship, that [[Exaptation|pre-adapted]] it for flight.|alt=Artist's reconstruction of a proto-bird fossil as if it used its small wings in courtship display]]

Sexual selection may explain how characteristics such as feathers had survival value at an early stage in their evolution. The earliest proto-birds such as ''[[Protarchaeopteryx]]'' had well-developed feathers but could not fly. The feathers may have served as insulation, helping females incubate their eggs, but if proto-bird courtship combined displays of forelimb feathers with energetic jumps, then the [[Origin of avian flight|transition to flight]] could have been relatively smooth.<ref name="Clarke 2013">{{cite journal |last=Clarke |first=J. |title=Feathers Before Flight |journal=Science| volume=340 | issue=6133 |date=9 May 2013 |doi=10.1126/science.1235463 |pages=690–692 |pmid=23661746 |bibcode=2013Sci...340..690C |s2cid=31802107 }}</ref>

Sexual selection may sometimes generate features that help cause a species' extinction, as has historically been suggested for the giant antlers of the [[Irish elk]] (''Megaloceros giganteus'') that became extinct in [[Holocene]]<ref>{{Cite journal |last1=van der Plicht |first1=J. |last2=Molodin |first2=V. I. |last3=Kuzmin |first3=Y. V. |last4=Vasiliev |first4=S. K. |last5=Postnov |first5=A. V. |last6=Slavinsky |first6=V. S. |date=15 April 2015 |title=New Holocene refugia of giant deer (Megaloceros giganteus Blum.) in Siberia: updated extinction patterns |url=https://www.sciencedirect.com/science/article/pii/S027737911500075X |journal=Quaternary Science Reviews |volume=114 |pages=182–188 |doi=10.1016/j.quascirev.2015.02.013 |bibcode=2015QSRv..114..182V |issn=0277-3791}}</ref> Eurasia<ref name="Gould 1974">{{cite journal |last=Gould |first=Stephen Jay |author-link=Stephen Jay Gould |year=1974 |title=Origin and Function of 'Bizarre' Structures – Antler Size and Skull Size in 'Irish Elk', Megaloceros giganteus |journal=Evolution |volume=28 |issue=2 |pages=191–220 |doi=10.2307/2407322 |pmid=28563271 |jstor=2407322 }}</ref> (although climate-induced habitat deterioration and anthropogenic pressure are now considered more likely causes).<ref>{{Cite journal |last1=Lister |first1=Adrian M. |last2=Stuart |first2=Anthony J. |date=2019-01-01 |title=The extinction of the giant deer Megaloceros giganteus (Blumenbach): New radiocarbon evidence |url=https://www.sciencedirect.com/science/article/pii/S1040618219300333 |journal=Quaternary International |series=SI: Quaternary International 500 |volume=500 |pages=185–203 |doi=10.1016/j.quaint.2019.03.025 |bibcode=2019QuInt.500..185L |issn=1040-6182}}</ref> It may, however, also do the opposite, driving species divergence—sometimes through elaborate changes in [[sex organs|genitalia]]<ref>{{Cite journal |last=Eberhard |first=William G. |date=24 March 2009 |title=Evolution of genitalia: theories, evidence, and new directions |url=https://repository.si.edu/bitstream/handle/10088/9845/stri_Eberhard_2010.pdf |journal=Genetica |volume=138 |issue=1 |pages=5–18 |doi=10.1007/s10709-009-9358-y |pmid=19308664 |s2cid=1409845 }}</ref>—such that new species emerge.<ref>Hosken, David J.; Stockley, Paula. "[http://www.sexologia.ulusofona.pt/biblio/Indice_files/Sexual%20selection%20and%20genital%20evolution.pdf Sexual selection and genital evolution] {{webarchive |url=https://web.archive.org/web/20171012045147/http://www.sexologia.ulusofona.pt/biblio/Indice_files/Sexual%20selection%20and%20genital%20evolution.pdf |date=12 October 2017}}." ''Trends in Ecology & Evolution'' 19.2 (2004): 87–93.</ref><ref>Arnqvist, Göran. "[http://heart.sdsu.edu/~website/biology_307/pdfs/genitalia.pdf Comparative evidence for the evolution of genitalia by sexual selection] {{webarchive |url=https://web.archive.org/web/20120127135826/http://heart.sdsu.edu/~website/Biology_307/pdfs/genitalia.pdf |date=27 January 2012}}." Nature 393.6687 (1998): 784.</ref>   Sexual selection often interacts with natural selection to drive [[speciation]].<ref>Maan, M.E.; Seehausen, O. "Ecology, sexual selection and speciation". ''Ecology Letters'', 2011 Jun; 14(6). pp. 591-602. doi: 10.1111/j.1461-0248.2011.01606.x. PMID 21375683.</ref>

== Sex role reversal ==

Sex role reversal (SRR) was first referred to as the phenomenon of females within a given species competing for mate access, rather than males fulfilling that role.<ref name="Fritzsche et al"/> Later, SRR was redefined to include cases where males in a species have higher parental investment. The concept's most notable mention was in Darwin’s The Descent of Man and Selection in Relation to Sex (1871). He brought attention to females having to undergo selection for access to male mating partners instead of the assumed natural order of males undergoing intra/intersexual competition of the time.<ref name="Eens et al."/>

From this point, several key studies were conducted on populations where females sought males, following Darwin’s description of the bird Barred buttonquail(Turnix suscitator).<ref name="Eens et al."/> This included studies of species that consistently subverted expected sex hierarchy norms, such as studies of the Pipefish family(Syngnathinae) or Seahorse family(Hippocampus).<ref name="Vincent et al"/> Females of these species are generally larger, more colorful, and more aggressive than males.<ref name="Eens et al."/> Most studies that succeeded after Darwin’s notes focused on this supposed reversal of animal behavior and attempted to understand what caused this female dominance.<ref name="Gwynne"/> Most studies during the first half of the 20th century believed it to be a result of unbalanced sex ratios, i.e, many more females than males of a population, and for this hierarchy to be unchanging.<ref name="Fritzsche et al"/> Keynote studies on reproduction of fruit flies(Drosophila) in the 1960s, however, illustrated sexual diversity based on environmental factors such as food availability as well as sex ratios.<ref name="Merrell"/> What was also distinct about the studies was that they demonstrated how quickly the sexes' passive and dominant roles could change given the ecological conditions.<ref name="Merrell"/>

The redefinition and wide use of SRR as a tool in animal behavior came from the 1970s. Darwin’s mention of sex role reversal resulting from a much larger ratio of females to males was later picked up by researchers Stephen T. Emlen and Lewis W. Oring, who reworked the initial concept to a concrete definition. As opposed to females, SRR became redefined by males taking on the bulk of parental investment of offspring.<ref name="Fritzsche et al"/> Researchers were attempting to exclude human-biased projections onto animal behavior. SRR showing parental investment as opposed to “masculine” sexual behaviors performed by females was an attempt to exclude human bias from animal observation. The highest consistency of males taking on higher parental investment than females was most noted in fish, bird, or amphibian species.<ref name="Fritzsche et al"/> Biologists’ new understanding of sexual selection came from observing mate selection based upon resource availability. 
Resource and mating trade-offs exist for any sexually reproducing organism. For example, male Sandpipers(Actitis macularia) are generally responsible for caring for the nest and protecting eggs, a task seen in other bird species to be shared with or exclusive to female members.<ref name="Gwynne"/> Thus, other mating opportunities for male sandpipers are a tradeoff for offspring care, making them unavailable for breeding. Generally, the sex that has to produce eggs and care for offspring in the zygote stage will divest resources from itself in the post-zygotic stage. This means that during this period, they would be unable to engage in other mating opportunities, providing a barrier to mating rates. Instances of the zygote carrying sex (i.e, the female) devoting less time to post-zygote parenting do present many questions to population behaviour.<ref name="Gwynne"/>

After Emlen and Oring's publications,<ref name="Emlen et al"/> questions remained on what environmental conditions led to SRR. A smaller ratio of males to females is the most widely accepted reason for SRR. Populations that consistently have a higher number of females to males have been noted for distinct dimorphism, as well as males undertaking higher parental investment in particular species. Animals exhibiting monogamous mating behaviour very rarely have traits of SRR. The most common mating type to take on SSR is polygamous species, in which dominant individuals have access to many mating partners, and conspecifics are driven away. Examples of polygamy coinciding with SRR are consistent with the Gulf pipefish(Syngnathus scovelli).<ref name="Vincent et al"/> Pipefish have been studied for decades due to observations of males being the choosier sex of mating partners, and females selected based on markers of higher quality, such as sex and the presence of secondary traits. Dominant females had numerous male mates, while other females were driven away. The study found that the same line of strong sexual dimorphism in polygamous species was carried to SRR examples, with females being the ones facing sexual selection as opposed to males.<ref name="Gwynne"/>

Other factors, like resource availability, may affect populations switching towards or away from SRR. Recently, however, biologists studying numerous different insect species have found that nest availability could be an enormous factor. The relative availability of nests for females to lay eggs was a determining factor in whether or not parental investment by males and female sexual dominance became present in the Broad-winged katydid(Microcentrum rhombifolium). When nests are less frequent in an environment, females undergo intrasexual competition to gain access to males, and males generally take on more parenting responsibilities. Co-parenting becomes much more common for populations of katydids in a habitat that allows for more nests, and female aggression decreases.<ref name="Gwynne"/>

Researchers now understand that the dynamics between male and female counterparts are more complex than “female dominant” and “male dominant.” The field is studying the adaptation of different sexual roles as a result of fluid factors such as sex ratio or resource investment.

== In different taxa ==

Sexual selection is widely distributed among the [[eukaryote]]s, occurring in plants, fungi, and animals. Since Darwin's pioneering observations on humans, it has been studied intensively among the insects, spiders, amphibians, scaled reptiles, birds, and mammals, revealing many distinctive behaviours and physical adaptations.<ref name="Nieuwenhuis 2012"/>

=== In mammals ===
{{Main|Sexual selection in humans|Sexual selection in mammals}}

Darwin conjectured that [[Heritability|heritable]] traits such as beards, hairlessness, and [[steatopygia]] in different human populations are results of [[sexual selection in humans]].<ref>{{cite book |last=Darwin |first=Charles |author-link=Charles Darwin |title=The Descent of Man and Selection in Relation to Sex |year=1882 |location=London |publisher=John Murray |page=578<!--and much of the book--> |url=http://darwin-online.org.uk/content/frameset?itemID=F955&viewtype=text&pageseq=1}}</ref> Humans are sexually dimorphic; females select males using factors including voice pitch, facial shape, muscularity, and height.<ref>{{cite book |last=Buss |first=David |date=2019 |edition=Sixth |title=Evolutionary Psychology: The New Science of the Mind |chapter=Women's Long-Term Mating Strategies |url=https://books.google.com/books?id=Sn6JDwAAQBAJ |publisher=Routledge |isbn=9780429590061 }}</ref><ref name="Feinberg Jones 2006">{{cite journal |last1=Feinberg |first1=D. R. |last2=Jones |first2=B. C. |last3=Law Smith |first3=M. J. |last4=Moore |first4=F. R. |last5=DeBruine |first5=L. M. |last6=Cornwell |first6=R. E. |last7=Hillier |first7=S. G. |last8=Perrett |first8=D. I. |display-authors=3 |date=1 February 2006 |title=Menstrual cycle, trait estrogen level, and masculinity preferences in the human voice |journal=Hormones and Behavior |volume=49 |issue=2 |pages=215–222 |doi=10.1016/j.yhbeh.2005.07.004 |pmid=16055126 |s2cid=14884832}}</ref>

Among the many instances of sexual selection in mammals is extreme sexual dimorphism, with males as much as six times heavier than females, and male fighting for dominance among [[elephant seal]]s. Dominant males establish large [[Harem (zoology)|harem]]s of several dozen females; unsuccessful males may attempt to copulate with a harem male's females if the dominant male is inattentive. This forces the harem male to defend his territory continuously, not feeding for as much as three months.<ref>{{cite encyclopedia |title=Earless Seals |encyclopedia=Encyclopedia of Marine Mammals |edition=2nd |editor-last=Perrin |editor-first=William F. |editor2-last=Würsig |editor2-first=Bernd |editor3-last=Thewissen |editor3-first=J. G. M. |publisher=Academic Press |location=Burlington, Massachusetts |isbn=978-0-12-373553-9 |url=https://books.google.com/books?id=2rkHQpToi9sC&q=elephant+seal+greatest+sexual+dimorphism&pg=PA23 |page=346 |year=2008 }}</ref><ref name="McCann 1981">{{cite journal |last=McCann |first=T. S. |year=1981 |title=Aggression and sexual activity of male Southern elephant seals, ''Mirounga leonina'' |journal=Journal of Zoology |volume=195 |issue=3 |pages=295–310 |doi=10.1111/j.1469-7998.1981.tb03467.x }}</ref>

Also seen in mammals is sex-role reversal, as in the highly social [[meerkat]]s, where a large female is dominant within a pack, and female–female competition is observed. The dominant female produces most of the offspring; the subordinate females are nonbreeding, providing [[Altruism (biology)|altruistic]] care to the young.<ref>{{cite journal |last1=Clutton-Brock |first1=T. H. |last2=Hodge |first2=S. J. |last3=Spong |first3=G.  |year=2006 |title=Intrasexual competition and sexual selection in cooperative mammals |url=https://scholar.sun.ac.za/handle/10019.1/12319 |journal=Nature |volume=444 |issue=7122 |pages=1065–8 |doi=10.1038/nature05386 |pmid=17183322 |bibcode=2006Natur.444.1065C |s2cid=4397323 }}</ref><ref name=coop>{{cite journal |last1=Clutton-Brock |first1=T. H. |last2=Russell |first2=A. F. |last3=Sharpe |first3=L. L. |title=Behavioural tactics of breeders in cooperative meerkats |journal=Animal Behaviour |date=2004 |volume=68 |issue=5 |pages=1029–1040 |doi=10.1016/j.anbehav.2003.10.024 |s2cid=53175143}}</ref>{{Clear}}

=== In arthropods ===
{{Main|Sexual selection in spiders|Sexual selection in insects}}

Sexual selection occurs in a wide range of [[spider]] species, both before and after copulation.<ref name="Eberhard 2009">{{cite journal |last=Eberhard |first=William G. |title=Postcopulatory sexual selection: Darwin's omission and its consequences |journal=Proceedings of the National Academy of Sciences |volume=106 |issue=supplement 1 |date=16 June 2009 |doi=10.1073/pnas.0901217106 |pages=10025–10032|pmid=19528642 |pmc=2702800 |doi-access=free }}</ref> Post-copulatory sexual selection involves sperm competition and cryptic female choice.  Sperm competition occurs where the sperm of more than one male competes to fertilise the egg of the female. Cryptic female choice involves the expelling of a male's sperm during or after copulations.<ref name="Peretti Eberhard 2010">{{cite journal |last1=Peretti |first1=A. V. |last2=Eberhard |first2=W. G. |title=Cryptic female choice via sperm dumping favours male copulatory courtship in a spider |journal=Journal of Evolutionary Biology |volume=23 |issue=2 |year=2010 |doi=10.1111/j.1420-9101.2009.01900.x |pages=271–281|pmid=20487130 |s2cid=9110472 |doi-access=free }}</ref>

Many forms of sexual selection exist among the insects. Parental care is often provided by female insects, as in bees, but male parental care is found in [[Belostomatidae|belostomatid]] water bugs, where the male, after fertilizing the eggs, allows the female to glue her eggs onto his back. He broods them until the [[Nymph (biology)|nymph]]s hatch 2–4 weeks later. The eggs are large and reduce the ability of the male to fertilise other females and catch prey, and increases its predation risk.<ref name="Gilbert Manica 2015">{{cite journal |last1=Gilbert |first1=James D. J. |last2=Manica |first2=Andrea |title=The evolution of parental care in insects: A test of current hypotheses |journal=Evolution |volume=69 |issue=5 |date=30 April 2015 |doi=10.1111/evo.12656 |pages=1255–1270|pmid=25825047 |pmc=4529740 |s2cid=17791711 }}</ref>

Among the [[Firefly|fireflies]] (Lampyrid beetles), males fly in darkness and emit a species-specific pattern of light flashes, which are answered by perching receptive females. The colour and temporal variation of the flashes contribute to success in attracting females.<ref name="Lewis Cratsley 2008">{{cite journal |last1=Lewis |first1=Sara M. |last2=Cratsley |first2=Christopher K. |s2cid=16360536 |date=January 2008 |title=Flash Signal Evolution, Mate Choice, and Predation in Fireflies |journal=[[Annual Review of Entomology]] |volume=53 |issue=1 |pages=293–321 |doi=10.1146/annurev.ento.53.103106.093346 |pmid=17877452}}</ref><ref>{{Cite journal |last1=Branham |first1=Marc A. |last2=Wenzel |first2=John W. |date=December 2001 |title=The Evolution of Bioluminescence in Cantharoids (Coleoptera: Elateroidea) |journal=[[The Florida Entomologist]] |volume=84 |issue=4 |pages=565 |doi=10.2307/3496389 |jstor=3496389 |url=http://journals.fcla.edu/flaent/article/view/75005 |doi-access=free}}</ref><ref>{{cite journal |last1=Martin |first1=Gavin J. |last2=Branham |first2=Marc A. |last3=Whiting |first3=Michael F. |last4=Bybee |first4=Seth M. |date=February 2017 |title=Total evidence phylogeny and the evolution of adult bioluminescence in fireflies (Coleoptera: Lampyridae) |journal=[[Molecular Phylogenetics and Evolution]] |volume=107 |pages=564–575 |doi=10.1016/j.ympev.2016.12.017 |pmid=27998815 |doi-access=free|bibcode=2017MolPE.107..564M }}</ref>
Among the [[beetle]]s, sexual selection is common. In the [[mealworm]] beetle, ''Tenebrio molitor,'' males release pheromones to attract females to mate.<ref>{{Cite journal |last1=Pölkki |first1=Mari |last2=Krams |first2=Indrikis |last3=Kangassalo |first3=Katariina |last4=Rantala |first4=Markus J. |date=2012-06-23 |title=Inbreeding affects sexual signalling in males but not females of Tenebrio molitor |journal=Biology Letters |language=en |volume=8 |issue=3 |pages=423–425 |doi=10.1098/rsbl.2011.1135 |issn=1744-9561 |pmc=3367757 |pmid=22237501}}</ref> Females choose mates based on whether they are infected, and on their mass.<ref>{{Cite journal |last1=Worden |first1=Bradley D. |last2=Parker |first2=Patricia G. |date=2005-11-05 |title=Females prefer noninfected males as mates in the grain beetle Tenebrio molitor: evidence in pre- and postcopulatory behaviours |url=https://linkinghub.elsevier.com/retrieve/pii/S0003347205002393 |journal=Animal Behaviour |volume=70 |issue=5 |pages=1047–1053 |doi=10.1016/j.anbehav.2005.01.023}}</ref>
<!--This is NOT A LIST, we don't want an example farm here --- obviously there are thousands -->

=== In molluscs ===

Postcopulatory intersexual selection occurs in ''[[Idiosepius paradoxus]]'', the Japanese pygmy squid. Males place their spermatangia on an external location on the female's body. The female physically removes spermatangia of males she is presumed to favour less.<ref name="Sato-2016">{{Cite journal |last1=Sato |first1=Noriyosi |last2=Yoshida |first2=Masa-aki |last3=Kasugai |first3=Takashi |date=2016-11-17 |title=Impact of cryptic female choice on insemination success: Larger sized and longer copulating male squid ejaculate more, but females influence insemination success by removing spermatangia |url=https://doi.org/10.1111/evo.13108 |journal=Evolution |volume=71 |issue=1 |pages=111–120 |doi=10.1111/evo.13108 |pmid=27805265 |s2cid=8866473 |issn=0014-3820}}</ref><ref name="Sato-2013">{{Cite journal |last1=Sato |first1=Noriyosi |last2=Kasugai |first2=Takashi |last3=Munehara |first3=Hiroyuki |date=2013-03-01 |title=Sperm transfer or spermatangia removal: postcopulatory behaviour of picking up spermatangium by female Japanese pygmy squid |url=https://doi.org/10.1007/s00227-012-2112-5 |journal=Marine Biology |volume=160 |issue=3 |pages=553–561 |doi=10.1007/s00227-012-2112-5 |bibcode=2013MarBi.160..553S |issn=1432-1793|hdl=10069/31698 |s2cid=253740276 |hdl-access=free }}</ref>

=== In amphibians and reptiles ===

{{Main|Sexual selection in amphibians|Sexual selection in scaled reptiles}}

Many amphibians have annual breeding seasons with male–male competition. Males arrive at the water's edge first in large numbers, and produce a wide range of vocalizations to attract mates. Among frogs, the fittest males have the deepest croaks and the best territories; females select their mates at least partly based on the depth of croaking. This has led to sexual dimorphism, with females larger than males in 90% of species, and male fighting to access females.<ref>{{cite journal |last1=Phelps |first1=S. |last2=Rand |first2=A. |last3=Ryan |first3=M. |year=2006 |title=A cognitive framework for mate choice and species recognition |journal=The American Naturalist |volume=167 |issue=1 |pages=28–42 |doi=10.1086/498538 |pmid=16475097 |bibcode=2006ANat..167...28P |s2cid=15851718 }}</ref><ref name="Wells Schwartz 2007">{{cite book |last1=Wells |first1=Kentwood D. |last2=Schwartz |first2=Joshua J. |title=Hearing and Sound Communication in Amphibians |chapter=The Behavioral Ecology of Anuran Communication |series=Springer Handbook of Auditory Research |year=2006 |volume=28 |publisher=Springer |location=New York |url=http://hydrodictyon.eeb.uconn.edu/courses/herpetology/Readings/Wells%20and%20Schwartz%202007%20Beh.%20Ecol.%20anuran%20comm..pdf |isbn=978-0-387-32521-7 |doi=10.1007/978-0-387-47796-1_3 |pages=44–86|s2cid=160384362 }}</ref> [[Spikethumb frog]]s are suggested to engage in male-male competition with their elongated prepollex to maintain their mating site.<ref name="Gonzalez-Mollinedo-2020">{{Cite journal |last1=Gonzalez-Mollinedo |first1=S. |last2=Marmol-Kattan |first2=A. |date=2020 |title=The underground sex life of the Guatemalan Spike-thumb Frog (Plectrohyla guatemalensis) |url=https://www.researchgate.net/publication/347752233 |journal=Neotropical Biology and Conservation |volume=15 |issue=4 |pages=551–559|doi=10.3897/neotropical.15.e57142 |doi-access=free }}</ref> The prepollex, which serves as a rudimentary digit, contains a projecting spine that may be used during this combat, leaving scars on the heads and forelimbs of other males.<ref name="Duellman-1992">{{Cite journal |last1=Duellman |first1=W.E. |last2=Campbell |first2=J.A. |date=1992 |title=Hylid frogs of the genus Plectrohyla: systematics and phylogenetic relationships |url=https://deepblue.lib.umich.edu/bitstream/handle/2027.42/56425/MP181.pdf?sequence=1 |journal=Museum of Zoology, University of Michigan |issue=181}}</ref>

Many different tactics are used by snakes to acquire mates. Ritual combat between males for the females they want to [[mating|mate]] with includes topping, a behaviour exhibited by most [[Viperidae|viperids]], in which one male twists around the vertically elevated fore body of its opponent and forcing it downward. Neck biting is common while the snakes are entwined.<ref name="Shine Langkilde Mason 2004">{{cite journal |last1=Shine |first1=Richard |last2=Langkilde |first2=Tracy |last3=Mason |first3=Robert T. |title=Courtship tactics in garter snakes: How do a male's morphology and behaviour influence his mating success? |year=2004 |journal=Animal Behaviour |volume=67 |issue=3 |pages=477–483 |s2cid=4830666 |doi=10.1016/j.anbehav.2003.05.007 }}</ref><ref name="Blouin-Demers Gibbs Weatherhead 2005">{{cite journal |last1=Blouin-Demers |first1=Gabriel |last2=Gibbs |first2=H. Lisle |last3=Weatherhead |first3=Patrick J. |title=Genetic evidence for sexual selection in black ratsnakes, ''Elaphe obsoleta'' |year=2005 |journal=Animal Behaviour |volume=69 |issue=1 |pages=225–34 |s2cid=3907523 |doi=10.1016/j.anbehav.2004.03.012 }}</ref>

=== In birds ===
{{Main |Sexual selection in birds}}

Birds have evolved a wide variety of mating behaviours and many types of sexual selection. These include intersexual selection (female choice) and intrasexual competition, where individuals of the more abundant sex compete with each other for the privilege to mate. Many species, notably the [[Bird-of-paradise|birds-of-paradise]], are sexually dimorphic; the differences such as in size and coloration are energetically costly attributes that signal competitive breeding. Conflicts between an individual's fitness and signalling adaptations ensure that sexually selected ornaments such as coloration of plumage and courtship behaviour are [[honest signal|honest]] traits. Signals must be costly to ensure that only good-quality individuals can present these exaggerated sexual ornaments and behaviours. Males with the brightest plumage are favoured by females of multiple species of bird.<ref name=saino2013>{{cite journal|last=Saino|first=Nicola|author2=Romano, Maria |author3=Rubolini, Diego |author4=Teplitsky, Celine |author5=Ambrosini, Roberto |author6=Caprioli, Manuela |author7=Canova, Luca |author8=Wakamatsu, Kazumasa |author9=Roulin, Alexandre |display-authors=3 |title=Sexual Dimorphism in Melanin Pigmentation, Feather Coloration and Its Heritability in the Barn Swallow (Hirundo rustica) |journal=PLOS ONE |year=2013 |volume=8 |issue=2 |pages=e58024 |doi=10.1371/journal.pone.0058024 |pmid=23469134 |pmc=3585210 |bibcode=2013PLoSO...858024S|doi-access=free}}</ref><ref name=edwards2012>{{cite journal |last=Edwards |first=D.B. |title=Immune investment is explained by sexual selection and pace-of-life, but not longevity in parrots (Psittaciformes) |journal=PLOS ONE |year=2012 |volume=7 |issue=12 |pages=e53066 |pmid=23300862 |doi=10.1371/journal.pone.0053066 |pmc=3531452 |bibcode=2012PLoSO...753066E |doi-access=free }}</ref><ref name="Doutrelant 2012">{{cite journal |last1=Doutrelant |first1=C. |last2=Grégoire |first2=A. |last3=Midamegbe |first3=A. |last4=Lambrechts |first4=M. |last5=Perret |first5=P. |title=Female plumage coloration is sensitive to the cost of reproduction. An experiment in blue tits |journal=[[Journal of Animal Ecology]] |date=January 2012 |volume=81 |issue=1 |pages=87–96 |pmid=21819397 |doi=10.1111/j.1365-2656.2011.01889.x |doi-access=free|bibcode=2012JAnEc..81...87D }}</ref>

Many bird species make use of [[mating call]]s, the females preferring [[Bird vocalization|males with songs]] that are complex and varied in amplitude, structure, and frequency. Larger males have deeper songs and increased mating success.<ref>{{cite journal |last=Hall |first=L. |author2=Kingma, S. A. |author3=Peters, A. |title=Male songbird indicates body size with low-pitched advertising songs |journal=PLOS ONE |year=2013 |volume=8 |issue=2 |pages=e56717 |pmid=23437221 |doi=10.1371/journal.pone.0056717 |pmc=3577745 |bibcode=2013PLoSO...856717H |doi-access=free}}</ref><ref name="Pfaff 2007">{{cite journal |last=Pfaff |first=J. A. |author2=Zanette, L. |author3=MacDougall-Shackleton, S. A. |author4=MacDougall-Shackleton, E. A. |title=Song repertoire size varies with HVC volume and is indicative of male quality in song sparrows (Melospiza melodia) |journal=[[Proceedings of the Royal Society B]] |date=22 August 2007 |volume=274 |issue=1621 |pages=2035–40 |pmid=17567560 |doi=10.1098/rspb.2007.0170 |pmc=2275172}}</ref><ref name="Nemeth 2012">{{cite journal |last=Nemeth |first=E. |author2=Kempenaers, B. |author3=Matessi, G. |author4=Brumm, H. |title=Rock sparrow song reflects male age and reproductive success |journal=PLOS ONE |year=2012 |volume=7 |issue=8 |pages=e43259 |pmid=22927955 |doi=10.1371/journal.pone.0043259 |pmc=3426517 |bibcode=2012PLoSO...743259N |doi-access=free}}</ref><ref>{{cite journal |author1= Mikula, P. |author2= Valcu, M. | author3= Brumm, H. | author4= Bulla, M. | author5= Forstmeier, W. | author6= Petrusková, T. | author7= Kempenaers, B. | author8= Albrecht, T | year=2021| title= A global analysis of song frequency in passerines provides no support for the acoustic adaptation hypothesis but suggests a role for sexual selection. | journal= Ecology Letters | volume= 24 | issue = 3 | pages= 477–486|doi=10.1111/ele.13662|pmid= 33314573|s2cid= 229176172 | doi-access= free |bibcode= 2021EcolL..24..477M }}</ref>{{Clear}}

=== In plants and fungi ===
{{Main|Sexual selection in flowering plants|Sexual selection in fungi}}

[[Flowering plant]]s have many secondary sexual characteristics subject to sexual selection including [[floral symmetry]] if [[pollinator]]s visit flowers assortatively by degree of symmetry,<ref name="Møller Eriksson 1995">{{cite journal |last1=Møller |first1=Anders Pape |last2=Eriksson |first2=Mats |date=1995 |title=Pollinator Preference for Symmetrical Flowers and Sexual Selection in Plants |journal=Oikos |volume=73 |issue=1 |pages=15–22 |doi=10.2307/3545720 |jstor=3545720|bibcode=1995Oikos..73...15M }}</ref> nectar production, floral structure, and inflorescences, as well as sexual dimorphisms.<ref name="Ashman Delph 2006">{{Cite journal |last1=Ashman |first1=Tia-Lynn |last2=Delph |first2=Lynda F. |date=1 August 2006 |title=Trait selection in flowering plants: how does sexual selection contribute? |journal=Integrative and Comparative Biology |volume=46 |issue=4 |pages=465–472 |doi=10.1093/icb/icj038 |pmid=21672758 |doi-access=free}}</ref><ref name="Moore Pannell 2011">{{cite journal |last1=Moore |first1=Jamie C. |last2=Pannell |first2=John R. |title=Sexual selection in plants |journal=Current Biology |year=2011 |volume=21 |issue=5 |pages=R176–R182 |doi=10.1016/j.cub.2010.12.035 |pmid=21377091 |s2cid=18044399 |doi-access=free |bibcode=2011CBio...21.R176M }}</ref><ref>{{Cite journal |last=Wilson |first=Mary F. |date=June 1979 |title=Sexual Selection In Plants |journal=The American Naturalist |volume=113 | issue=6 |pages=777–790 |doi=10.1086/283437 |bibcode=1979ANat..113..777W |s2cid=84970789 }}</ref>

[[Fungi]] appear to make use of sexual selection, although they also often reproduce asexually. In the [[Basidiomycetes]], the sex ratio is biased towards males, implying sexual selection there. [[Male–male competition]] to fertilise occurs in fungi including yeasts. [[Sex pheromone#Signalling|Pheromone signaling]] is used by female gametes and by conidia, implying male choice in these cases. Female–female competition may also occur, indicated by the much faster evolution of female-biased genes in fungi.<ref name="Nieuwenhuis 2012">{{cite journal |doi=10.1111/jeb.12017 |pmid=23163326 |volume=25 |issue=12 |title=Sexual selection in fungi |year=2012 |journal=Journal of Evolutionary Biology |pages=2397–2411 |last1=Nieuwenhuis |first1=B. P. S. |last2=Aanen |first2=D. K. |s2cid=5657743 |doi-access=free }}</ref><ref>{{cite journal |last=Leonard |first=Janet L. |date=1 August 2006 |title=Sexual selection: lessons from hermaphrodite mating systems|journal=Integrative and Comparative Biology |volume=46 |issue=4 |pages=349–367 |doi=10.1093/icb/icj041 |pmid=21672747 |doi-access=free }}</ref><ref name="Beekman Nieuwenhuis Ortiz-Barrientos Evans 2016">{{cite journal |last1=Beekman |first1=Madeleine |last2=Nieuwenhuis |first2=Bart |last3=Ortiz-Barrientos |first3=Daniel |last4=Evans |first4=Jonathan P. |title=Sexual selection in hermaphrodites, sperm and broadcast spawners, plants and fungi |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |publisher=The Royal Society |volume=371 |issue=1706 |date=19 October 2016 |issn=0962-8436 |doi=10.1098/rstb.2015.0541 |page=20150541|pmid=27619704 |pmc=5031625 }}</ref><ref name="Whittle Johannesson 2013">{{cite journal |last1=Whittle |first1=Carrie A. |last2=Johannesson |first2=Hanna |title=Evolutionary Dynamics of Sex-Biased Genes in a Hermaphrodite Fungus |journal=Molecular Biology and Evolution |publisher=Oxford University Press |volume=30 |issue=11 |date=20 August 2013 |doi=10.1093/molbev/mst143 |pages=2435–2446|pmid=23966547 |doi-access=free }}</ref>

<gallery mode="packed" caption="Example sexual selection mechanisms in the named taxa">
File:Gorillafamily.JPG|[[Sexual selection in mammals|Among mammals]], the male gorilla is much larger than female.
File:Phidippus putnami male.jpg|Males of many [[spider]]s, such as this ''[[Phidippus putnami]]'', have elaborate courtship displays.
File:Toe-Biter.jpg|A male ''[[Abedus indentatus]]'' [[Belostomatidae|belostomatid bug]] carries eggs on its back.
File:Fireflies, Georgia, US (detail).jpg|Each [[firefly]] species attracts mates with its own flash pattern.
File:Dendropsophus microcephalus - calling male (Cope, 1886).jpg|Male ''[[Dendropsophus microcephalus]]'' calling
File:Indian rat snake,Ptyas mucosa, Territorial Fight.jpg|Territorial fight in the Indian rat snake, ''[[Ptyas mucosa]]''
File:Victoria's Riflebird courtship - Lake Eacham - Queensland S4E8070 (22198704599) (cropped).jpg|Male [[Victoria's riflebird]] displaying to a female
File:Satin Bowerbird nest.jpg|A male [[satin bowerbird]] guards its bower from rival males in the hope of attracting females with its decorations.
File:MacquarieIslandElephantSeal.JPG|Male [[southern elephant seal]]s fighting on [[Macquarie Island]] for the right to mate
File:Lily Lilium 'Citronella' Flower.jpg|''[[Citronella (genus)|Citronella]]'' flower's symmetry may have been subject to sexual selection by its [[pollinator]]s.
File:RanaArvalisBlueMale3.jpg|Male [[moor frog]]s become blue to signal their fitness to females.
</gallery>

== References ==
{{reflist|28em}}

<ref name="Eens et al.">{{cite journal |last1=Eens |first1=Marcel |last2=Pinxten |first2=Rianne |title=Sex-Role Reversal in Vertebrates: Behavioural and Endocrinological Accounts |journal=Behavioural Processes |date=Oct 2000 |volume=51 |issue=1-3 |pages=135–147 |doi=10.1016/s0376-6357(00)00124-8}}</ref>

<ref name="Emlen et al">{{cite journal |last1=Emlen |first1=Stephen T |last2=Oring |first2=Lewis W |title=Ecology, Sexual Selection, and the Evolution of Mating Systems |journal=Science |date=July 1977 |volume=197 |issue=4300 |pages=215–223 |url=https://doi.org/10.1126/science.327542}}</ref>

<ref name="Fritzsche et al">{{cite journal |last1=Fritzsche |display-authors=etal |first1=Karoline |title=The 150th Anniversary of the Descent of Man: Darwin and the Impact of Sex-Role Reversal on Sexual Selection Research |journal=Biological Journal of the Linnean Society |date=Aug 2021 |volume=134 |issue=3 |pages=525–540 |doi=10.1093/biolinnean/blab091|doi-access=free }}</ref>

<ref name="Gwynne">{{cite journal |last1=Gwynne |first1=Darryl T |title=Role-Reversal in Katydids: Habitat Influences Reproductive Behaviour (Orthoptera: Tettigoniidae, Metaballus Sp.) - Behavioral Ecology and Sociobiology.” |journal=Springer-Verlag |date=May 2016 |doi=10.1007/BF00295549}}</ref>

<ref name="Merrell">{{cite journal |last1=Merrell |first1=David J. |title=Mating Preferences in Drosophila |journal=Evolution |date=Dec 1960 |volume=14 |issue=4 |pages=525 |doi=10.2307/2406000}}</ref>

<ref name="Vincent et al">{{cite journal |last1=Vincent |first1=Amanda |last2=Ahnesjö |first2=Ingrid |last3=Berglund |first3=Anders |last4=Rosenqvist |first4=Gunilla |title=Pipefishes and Seahorses: Are They All Sex Role Reversed? |journal=Trend in Ecology and Evolution |date=July 1992 |volume=7 |issue=7 |pages=237–241 |doi=10.1016/0169-5347(92)90052-d}}</ref>

<ref name="Williams et al">{{cite book |last1=Williams |first1=George C |last2=Dawkins |first2=Richard |title=Adaptation and natural selection: A critique of some current evolutionary thought. |date=2019 |publisher=Princeton University Press |location=Princeton, N.J}}</ref>

== Further reading ==

* {{cite book |last=Richards |first=Evelleen |title=Darwin and the Making of Sexual Selection |date=2017 |publisher=[[University of Chicago Press]] |location=Chicago |isbn=9780226436906 |url=https://press.uchicago.edu/ucp/books/book/chicago/D/bo25338514.html |ref=none}}

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[[Category:Sexual selection| ]]
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