Editing Sexual selection (section)

== 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>