The concept of sexual selection was introduced by Charles Darwin as an element of his theory of natural selection. Sexual selection is a biological way one sex chooses a mate for the best reproductive success. Most compete with others of the same sex for the best mate to contribute their genome for future generations. This has shaped human evolution for many years, but reasons why humans choose their mates are not fully understood. Sexual selection is quite different in non-human animals than humans as they feel more of the evolutionary pressures to reproduce and can easily reject a mate. The role of sexual selection in human evolution has not been firmly established although neoteny has been cited as being caused by human sexual selection. It has been suggested that sexual selection played a part in the evolution of the anatomically modern human brain, i.e. the structures responsible for social intelligence underwent positive selection as a sexual ornamentation to be used in courtship rather than for survival itself, and that it has developed in ways outlined by Ronald Fisher in the Fisherian runaway model.Fisher, R.A. (1930) The Genetical Theory of Natural Selection. Edwards, A.W.F. (2000) Perspectives: Anecdotal, Historical and Critical Commentaries on Genetics. The Genetics Society of America (154) 1419:1426Andersson, M. (1994) Sexual selection. Andersson, M. and Simmons, L.W. (2006) Sexual selection and mate choice. Trends, Ecology and Evolution (21) 296:302Gayon, J. (2010) Sexual selection: Another Darwinian process. Comptes Rendus Biologies (333) 134:144 Fisher also stated that the development of sexual selection was "more favourable" in humans.
General hypotheses Some hypotheses about the evolution of the human brain argue that it is a sexually selected trait, as it would not confer enough fitness in itself relative to its high maintenance costs (a fifth to a quarter of the energy and oxygen consumed by a human). Current consensus about the evolutionary development of the human brain accepts sexual selection as a potential contributing factor but maintains that human intelligence and the ability to store and share cultural knowledge would have likely carried high survival value as well.
Sexual selection's role in human evolution cannot be definitively established, as features may result from an equilibrium among competing selective pressures, some involving sexual selection, others natural selection, and others pleiotropy. Richard Dawkins argued that "When you notice a characteristic of an animal and ask what its Darwinian survival value is, you may be asking the wrong question. It could be that the characteristic you have picked out is not the one that matters. It may have "come along for the ride", dragged along in evolution by some other characteristic to which it is pleiotropically linked."
Darwin's sexual selection hypothesis Charles Darwin described sexual selection as depending on "the advantage which certain individuals have over others of the same sex and species, solely in respect of reproduction". Darwin noted that sexual selection is of two kinds and concluded that both kinds had operated on humans: "The sexual struggle is of two kinds; in the one it is between the individuals of the same sex, generally the male sex, in order to drive away or kill their rivals, the females remaining passive; whilst in the other, the struggle is likewise between the individuals of the same sex, in order to excite or charm those of the opposite sex, generally the females, which no longer remain passive, but select the more agreeable partners."
Charles Darwin conjectured that the male beard, as well as the hairlessness of humans compared to nearly all other mammals, were results of sexual selection. He reasoned that since the bodies of females are more nearly hairless, the loss of fur was due to sexual selection of females at a remote prehistoric time when males had overwhelming selective power, and that it nonetheless affected males due to genetic correlation between the sexes. He also hypothesized that contrasts in sexual selection acting along with natural selection were significant factors in the geographical differentiation in human appearance of some isolated groups, as he did not believe that natural selection alone provided a satisfactory answer. Although not explicit, his observation that in Khoisan women "the posterior part of the body projects in a most wonderful manner" (known as steatopygia) implies sexual selection for this characteristic. In The Descent of Man, and Selection in Relation to Sex, Darwin viewed many physical traits which vary around the world as being so trivial to survival that he concluded some input from sexual selection was required to account for their presence. He noted that variation in these features among the various peoples of the world meant human mate-choice criteria would also have to be quite different if the focus was similar, and he himself doubted that, citingDarwin, C. (1936) . The Descent of Man and Selection in Relation to Sex. reprint of 2nd ed., The Modern Library, New York: Random House. reports indicating that ideals of beauty did not, in fact, vary in this way around the world.
Sexual dimorphism
The effects on the human brain formation during puberty is directly linked to hormones changes. The mismatch timing between biological puberty and age of social maturity in western society has a psychological expectation on children. With puberty, men are generally hairier than women, and Darwin was of the opinion that hairlessness was related to sexual selection; however, several other explanations have been advanced to explain human hairlessness; a leading one being that loss of body hair facilitated sweating.Jablonski, N. G. (2006). Skin: a natural history. Berkeley, CA: University of California Press. p. PP13. This idea closely relates to that of the suggested need for increased photoprotection and is part of the most-commonly-accepted scientific explanation for the evolution of pigmentary traits.
Sexual dimorphism suggests the presence of sexual selection. The earliest homininae were highly dimorphic and that this tendency lessened over the course of human evolution, suggesting humans have become more monogamous. In contrast, gorillas living in harems exhibit a much stronger sexual dimorphism (see: homininae).Principles of Human Evolution, by Roger Lewin, Robert Foley.
Sexual anatomy
The theory of sexual selection has been used to explain a number of human anatomical features. These include rounded breasts, facial hair, pubic hair and penis size. The breasts of primates are flat, yet are able to produce sufficient milk for feeding their young. The breasts of non-lactating human females are filled with fatty tissue and not milk. Thus it has been suggested the rounded female breasts are signals of fertility. Richard Dawkins has speculated that the loss of the penis bone in humans, when it is present in other primates, may be due to sexual selection by females looking for a clear sign of good health in prospective mates. Since a human erection relies on a hydraulic pumping system, erection failure is a sensitive early warning of certain kinds of physical and mental ill health.
Homo has a thicker penis than the other great apes, though it is on average no longer than the chimpanzee's. It has been suggested the evolution of the human penis towards larger size was the result of female choice rather than sperm competition, which generally favors large testicles.Miller, G.F. (1998), "How mate choice shaped human nature: A review of sexual selection and human evolution" in Handbook of Evolutionary Psychology. However, penis size may have been subject to natural selection, rather than sexual selection, due to a larger penis' efficiency in displacing the sperm of rival males during sexual intercourse. A model study showed displacement of semen was directly proportional to the depth of pelvic thrusting, as an efficient semen displacement device.In a published in the journal Evolutionary Psychology in 2004, Gallup and coauthor, Rebecca Burch, conjecture that, "A longer penis would not only have been an advantage for leaving semen in a less-accessible part of the vagina, but by filling and expanding the vagina, it also would aid and abet the displacement of semen left by other males as a means of maximizing the likelihood of paternity." – "Secrets of the Phallus: Why Is the Penis Shaped Like That?", ScientificAmerican.com.
Selection preferences and biological drivers
Several factors drive sexual selection in humans. Selection preferences are biologically driven, that is, by the display of phenotypic traits that can be both consciously and unconsciously evaluated by the opposite sex to determine the health and fertility of a potential mate. This process can be affected, however, by social factors, including in cultures where arranged marriage is practiced, or psychosocial factors, such as valuing certain cultural traits of a mate, including a person's social status, or what is perceived to be an ideal partner in various cultures.
Selection preferences in females
Some of the factors that affect how females select their potential mates for reproduction include voice pitch, facial shape, muscular appearance, and particularly height; with the possibility of a Fisherian runaway in the making.
Several studies suggest that there is a link between hormone levels and partner selection among humans. In a study measuring female attraction to males with varying levels of masculinity, it was established that women had a general masculinity preference for men's voices, and that the preference for masculinity was greater in the fertile phase of the menstrual cycle than in the non-fertile phase. There is further evidence from the same study that in fertile stages of the menstrual cycle, women also had a preference for other masculine traits such as body size, facial shape, and dominant behavior, which are indicators of both fertility and health. This study did not exclude males with feminine traits from being selected, however, as feminine traits in men indicate a higher probability of long-term relationship commitment, and may be one of several evolutionary strategies. Further research also backs up the idea of using phenotypic traits as a means of assessing a potential mate's fitness for reproduction as well as assessing whether a partner has high genetic quality.
One study proposed a link between Human Development Index levels and female preference for male facial appearance. While women from the United Kingdom preferred the faces of men with low cortisol levels, women from Latvia did not discriminate between men with either high or low levels of cortisol. It was concluded that societal-level ecological factors impact the valuation of traits by combinations of sex- and stress-hormones.
A 2020 study reported that women tend to find a man more attractive if the man's previous relationships ended mutually, and less attractive if the man was dumped.
Selection preferences in males