Sexual selection
is a mode of natural selection in which members of one biological sex choose mates of the other sex to 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 better reproductive success than others within a population, either because they are more attractive or prefer more attractive partners to produce offspring.[1][2] For instance, in the breeding season, sexual selection in frogs occurs with the males first gathering at the water's edge and making their mating calls: croaking. The females then arrive and choose the males with the deepest croaks and best territories. In general, males benefit from frequent mating and monopolizing access to a group of fertile females. Females can have a limited number of offspring and maximize the return on the energy they invest in reproduction.
The concept was first articulated by Charles Darwin and Alfred Russel Wallace who described it as driving species adaptations and that many organisms had evolved features whose function was deleterious to their individual survival,[3] and then developed by Ronald Fisher in the early 20th century. Sexual selection can lead males to extreme efforts to demonstrate their fitness to be chosen by females, producing sexual dimorphism in secondary sexual characteristics, such as the ornate plumage of birds such as birds of paradise and peafowl, or the antlers of deer, or the manes of lions, 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 1:1 almost without exception. Sexual selection is also found in plants and fungi.[4][5]
The maintenance of sexual reproduction in a highly competitive world is one of the major puzzles in biology given that asexual reproduction can reproduce much more quickly as 50% of offspring are not males, unable to produce offspring themselves. Many non-exclusive hypotheses have been proposed,[6] including the positive impact of an additional form of selection, sexual selection, on the probability of persistence of a species.[***
The concept was first articulated by Charles Darwin and Alfred Russel Wallace who described it as driving species adaptations and that many organisms had evolved features whose function was deleterious to their individual survival,[3] and then developed by Ronald Fisher in the early 20th century. Sexual selection can lead males to extreme efforts to demonstrate their fitness to be chosen by females, producing sexual dimorphism in secondary sexual characteristics, such as the ornate plumage of birds such as birds of paradise and peafowl, or the antlers of deer, or the manes of lions, 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 1:1 almost without exception. Sexual selection is also found in plants and fungi.[4][5]
The maintenance of sexual reproduction in a highly competitive world is one of the major puzzles in biology given that asexual reproduction can reproduce much more quickly as 50% of offspring are not males, unable to produce offspring themselves. Many non-exclusive hypotheses have been proposed,[6] including the positive impact of an additional form of selection, sexual selection, on the probability of persistence of a species.[***
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 height.[33] 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 preferences 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.[33] 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.[33] 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,[33] and may be one of several survival strategies.[34] 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.[35]
Another factor affecting the selection process is the environment which the person inhabits. In biological terms, certain environmental conditions may bring about demands for or the disregarding of certain traits. One such example is a preference for males whose facial structure indicates certain hormonal ratios, such as testosterone-cortisol levels (sex and stress hormones). Research shows that, for example, in countries with varying Human Development Index (HDI) levels, females have different preferences for sex-stress hormone ratios, as expressed in the male's face. A Royal Society research showed a significant correlation between a measure of societal development and preferences for indication of lower testosterone levels, as manifested in facial features, and the interaction between preferences for testosterone and cortisol.[36] It was concluded that societal-level ecological factors impact the valuation of traits by combinations of sex- and stress-hormones.[36]
Selection preferences in males
Like their female counterparts, males also use visual information about a potential mate, as well as voice, body shape, and an assortment of other factors in selecting a partner. Research shows that males tend to prefer feminine women's faces and voices as opposed to women with masculine features in these categories.[37] Furthermore, males also evaluate skin coloration, symmetry, and apparent health, as a means by which the select a partner for reproductive purposes.[37] Males are particularly attracted to femininity in women's faces when their testosterone levels are at their highest, and the level of attraction to femininity may fluctuate as hormone levels fluctuate.[38] Studies on men have also been done to show the effects of exogenous testosterone and its effects on attraction to femininity, and the results concluded that throughout several studies, men have shown decreased preference for feminine female faces in the long-term context, when given exogenous testosterone, but this difference did not occur with placebo.[39]
Common preferences in either sex
Sexual selection preferences are general terms by which the mating and reproductive process are understood. As one article states, sexual selection is in essence a process which favors sexual displays for attraction, aggressiveness, dominance, size, and strength, and the ability to exclude competitors by force if necessary, or by using resources to win.[40] Both male and female use voice, face, and other physical characteristics[32] to assess a potential mate's ability to reproduce, as well as their health.[31] Together with visual and chemical signals, these crucial characteristics which are likely to enhance the ability to produce offspring, as well as long term survival prospects, can be assessed and selections made
Some of the factors that affect how females select their potential mates for reproduction include voice pitch, facial shape, muscular appearance, and height.[33] 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 preferences 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.[33] 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.[33] 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,[33] and may be one of several survival strategies.[34] 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.[35]
Another factor affecting the selection process is the environment which the person inhabits. In biological terms, certain environmental conditions may bring about demands for or the disregarding of certain traits. One such example is a preference for males whose facial structure indicates certain hormonal ratios, such as testosterone-cortisol levels (sex and stress hormones). Research shows that, for example, in countries with varying Human Development Index (HDI) levels, females have different preferences for sex-stress hormone ratios, as expressed in the male's face. A Royal Society research showed a significant correlation between a measure of societal development and preferences for indication of lower testosterone levels, as manifested in facial features, and the interaction between preferences for testosterone and cortisol.[36] It was concluded that societal-level ecological factors impact the valuation of traits by combinations of sex- and stress-hormones.[36]
Selection preferences in males
Like their female counterparts, males also use visual information about a potential mate, as well as voice, body shape, and an assortment of other factors in selecting a partner. Research shows that males tend to prefer feminine women's faces and voices as opposed to women with masculine features in these categories.[37] Furthermore, males also evaluate skin coloration, symmetry, and apparent health, as a means by which the select a partner for reproductive purposes.[37] Males are particularly attracted to femininity in women's faces when their testosterone levels are at their highest, and the level of attraction to femininity may fluctuate as hormone levels fluctuate.[38] Studies on men have also been done to show the effects of exogenous testosterone and its effects on attraction to femininity, and the results concluded that throughout several studies, men have shown decreased preference for feminine female faces in the long-term context, when given exogenous testosterone, but this difference did not occur with placebo.[39]
Common preferences in either sex
Sexual selection preferences are general terms by which the mating and reproductive process are understood. As one article states, sexual selection is in essence a process which favors sexual displays for attraction, aggressiveness, dominance, size, and strength, and the ability to exclude competitors by force if necessary, or by using resources to win.[40] Both male and female use voice, face, and other physical characteristics[32] to assess a potential mate's ability to reproduce, as well as their health.[31] Together with visual and chemical signals, these crucial characteristics which are likely to enhance the ability to produce offspring, as well as long term survival prospects, can be assessed and selections made
Sexual selection in humans
Sexual selection in humans concerns the concept of sexual selection, introduced by Charles Darwin as an element of his theory of natural selection,[1] as it affects humans. 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 our evolution for many years, but reasons why humans choose their mates are hardly understood. Sexual selection is quite different in animals than humans as they feel more of the evolutionary pressures to reproduce and can easily reject a mate.[2] 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.[3] 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,[4] and that it has developed in ways outlined by Ronald Fisher in the Fisherian runaway model. Fisher also stated that the development of sexual selection was "more favourable" in humans
Sexual dimorphism
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.[25] 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.[26]
Homo has a thicker penis than the other great apes, though it is on average no longer than the chimpanzee's.[27] 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.[28] 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
Homo has a thicker penis than the other great apes, though it is on average no longer than the chimpanzee's.[27] 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.[28] 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
Selection preferences and biological drivers
There are a variety of factors that drive sexual selection in humans. Current available research indicates that selection preferences are biologically driven,[30] 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.[31] 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 persons social status, or what is perceived to be an ideal partner in various cultures
There are a variety of factors that drive sexual selection in humans. Current available research indicates that selection preferences are biologically driven,[30] 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.[31] 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 persons social status, or what is perceived to be an ideal partner in various cultures
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