Sexual selection

File:Darwin sexual caricature.gif 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 peacock's tail, whenever I gaze at it, makes me sick!" His work divided sexual selection into male–male competition and female choice.{{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 }}{{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 }}

{{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.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 }}}}

{{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.}}

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 beetles) far too cognitively undeveloped to be capable of aesthetic feeling.{{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}}

File:Tribolium castaneum87-300.jpgs from extinction in a ten-year experiment.|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.{{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}} 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.{{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 }} A ten-year study, experimentally varying sexual selection on flour beetles with other factors held constant, showed that sexual selection protected even an inbred population against extinction.[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 }}

{{Main|Fisherian runaway}}

Ronald Fisher, the 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.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]}}. He remarked that:

{{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 exponentially, or in geometric progression. —Ronald Fisher, 1930}}

File:Euplectes progne male South Africa cropped.jpg |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.{{cite book |last=Andersson |first=M. |year=1994 |pages=115–117 |title=Sexual Selection |publisher=Princeton University Press |isbn=0-691-00057-3}} 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 expressing their genetic preference for long tails, and males showing off the coveted long tail itself.

Richard Dawkins presents a non-mathematical explanation of the runaway sexual selection process in his book The Blind Watchmaker.{{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}} 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 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, 1932Ronald 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}}

File:Peacock Flying.jpg|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.

{{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}}

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.{{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 }}{{cite book |last=O'Donald |first=Peter |year=1980 |title=Genetic Models of Sexual Selection |publisher=Cambridge University Press |isbn=9780521225335 }} Alongside this, biologists have extended Darwin's formulation; Malte Andersson's widely accepted{{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 }} 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". Despite some practical challenges for biologists, the concept of sexual selection is "straightforward".

{{Further|Bateman's principle}}

File:Irish Elk front.jpg might have helped it on its way to extinction.|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.{{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}}{{cite book |last=Starr |first=Cecie |title=Biology: The Unity & Diversity of Life |year=2013 |publisher=Cengage Learning |page=281}}{{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 }} Sexual preference creates a tendency towards assortative mating or 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 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.{{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 }} More recently, researchers have doubted whether Bateman was correct.{{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 }}

{{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 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.{{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 }}{{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 }}{{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}} 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.{{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 }}

File:Susa group, mountain gorilla.jpg, a species with very large males{{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}}|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,{{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 }} and the social situation{{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 }} 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.{{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 }} Males take many risks in such conflicts, so the value of the resource must be large enough to justify those risks.{{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}}{{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}} Winner and loser effects further influence male behaviour.{{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 }} Male–male competition may also affect a female's ability to select the best mates, and therefore decrease the likelihood of successful reproduction.{{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 }}

{{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" range of models variously attempt to relate sexual selection not only to the fundamental questions of anisogamy and parental roles, but also to mechanisms such as sex ratios – governed by Fisher's principle,{{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}} parental care, investing in sexy sons, sexual conflict, and the "most-debated effect", namely mate choice.{{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 |hdl=1885/22652 |hdl-access=free }} 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.{{cite web |last=Oufiero |first=Christopher E. |title=Sexual Selection, Costs, and Compensation |date=May 2015 |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}}

File:Protarchaeopteryx 4713.JPG was flightless, but had feathers, perhaps used in courtship, that 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 transition to flight could have been relatively smooth.{{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 }}

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{{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}} Eurasia{{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 }} (although climate-induced habitat deterioration and anthropogenic pressure are now considered more likely causes).{{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}} It may, however, also do the opposite, driving species divergence—sometimes through elaborate changes in genitalia{{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 }}—such that new species emerge.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.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. Sexual selection often interacts with natural selection to drive speciation.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.

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. 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.

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). This included studies of species that consistently subverted expected sex hierarchy norms, such as studies of the Pipefish family(Syngnathinae) or Seahorse family(Hippocampus). Females of these species are generally larger, more colorful, and more aggressive than males. 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. 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. 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. 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.

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. 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. 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. 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.

After Emlen and Oring's publications, 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). 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.

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.

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.

Sexual selection is widely distributed among the eukaryotes, 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.

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

Darwin conjectured that heritable traits such as beards, hairlessness, and steatopygia in different human populations are results of sexual selection in humans.{{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 |url=http://darwin-online.org.uk/content/frameset?itemID=F955&viewtype=text&pageseq=1}} Humans are sexually dimorphic; females select males using factors including voice pitch, facial shape, muscularity, and height.{{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 }}{{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}}

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 seals. Dominant males establish large harems 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.{{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 }}{{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 }}

Also seen in mammals is sex-role reversal, as in the highly social meerkats, 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 altruistic care to the young.{{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 }}{{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}}{{Clear}}

{{Main|Sexual selection in spiders|Sexual selection in insects}}

Sexual selection occurs in a wide range of spider species, both before and after copulation.{{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 }} 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.{{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 }}

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 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 nymphs 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.{{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 }}

Among the 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.{{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}}{{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}}{{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 }}

Among the beetles, sexual selection is common. In the mealworm beetle, Tenebrio molitor, males release pheromones to attract females to mate.{{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}} Females choose mates based on whether they are infected, and on their mass.{{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}}

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.{{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}}{{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 }}

{{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.{{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 }}{{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 }} Spikethumb frogs are suggested to engage in male-male competition with their elongated prepollex to maintain their mating site.{{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 }} 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.{{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}}

Many different tactics are used by snakes to acquire mates. Ritual combat between males for the females they want to mate with includes topping, a behaviour exhibited by most 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.{{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 }}{{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 }}

{{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 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 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.{{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}}{{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 }}{{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 }}

Many bird species make use of mating calls, the females preferring males with songs that are complex and varied in amplitude, structure, and frequency. Larger males have deeper songs and increased mating success.{{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}}{{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}}{{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}}{{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 }}{{Clear}}

{{Main|Sexual selection in flowering plants|Sexual selection in fungi}}

Flowering plants have many secondary sexual characteristics subject to sexual selection including floral symmetry if pollinators visit flowers assortatively by degree of symmetry,{{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 }} nectar production, floral structure, and inflorescences, as well as sexual dimorphisms.{{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}}{{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 }}{{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 }}

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. 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.{{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 }}{{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 }}{{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 }}{{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 }}

File:Gorillafamily.JPG|Among mammals, the male gorilla is much larger than female.

File:Phidippus putnami male.jpg|Males of many spiders, such as this Phidippus putnami, have elaborate courtship displays.

File:Toe-Biter.jpg|A male Abedus indentatus 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 seals fighting on Macquarie Island for the right to mate

File:Lily Lilium 'Citronella' Flower.jpg|Citronella flower's symmetry may have been subject to sexual selection by its pollinators.

File:RanaArvalisBlueMale3.jpg|Male moor frogs become blue to signal their fitness to females.

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{{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}}

{{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}}

• {{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:Population genetics