Sexual selection in human evolution
Sexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is an element of his theory of natural selection, the process by which biological traits become either more or less common in a population as a function of differential reproduction of their bearers.
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: "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." Darwin concluded that both kinds of sexual selection had operated on humans.
Geoffrey Miller, drawing on some of Darwin's largely neglected ideas about human behavior, has hypothesized that many human behaviors not clearly tied to survival benefits, such as humor, music, visual art, verbal creativity, and some forms of altruism, are courtship adaptations that have been favored through sexual selection. In that view, many human artefacts could be considered subject to sexual selection as part of the extended phenotype, for instance clothing that enhances sexually selected traits.
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 quarter to a fifth of the energy and oxygen consumed by a human). Related to this is vocabulary, where humans, on average, know far more words than are necessary for communication. Miller (2000) has proposed that this apparent redundancy is due to individuals using vocabulary to demonstrate their intelligence, and consequently their "fitness", to potential mates. This has been tested experimentally, and it appears that males do make greater use of lower-frequency (more unusual) words when in a romantic mindset compared to a non-romantic mindset, suggesting that vocabulary is likely to be used as a sexual display (Rosenberg & Tunney, 2008).
The evolutionary biologist 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.
However, 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. In the words of Richard Dawkins:
- "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."
The German anthropologist Ferdinand Fellmann has proposed a modified form of sexual selection, termed "emotional selection," as the pivot in human emotional evolution. The survival edge is due to the talent of humans for long-term mating, which allows them to have feelings about feelings: the origin of human consciousness.
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" implies sexual selection for this characteristic. In the Descent of Man, 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, citing reports indicating that ideals of beauty did not, in fact, vary in this way around the world.
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 is loss of body hair to facilitate sweating. 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.
Indicating that a trait is under sexual selection can be difficult to prove through correlational methods, as characters may result from different selective pressures, some involving sexual selection, others natural selection, and some may be accidental and due to pleiotropy. For example, monogamous primates are known to typically exhibit little sexual dimorphism such as particularly large males armed with huge canines; however, powerful big-toothed males can provide protection against predators and may be bigger for that reason, rather than in order to win confrontations over females. Males and females differing in size can specialize in, and more fully exploit, different food resources while avoiding competing with each other; furthermore, body size can be useful in avoiding predators and may also be of assistance in securing a mate. This is further complicated by the consideration that with larger body size, the skeleton of mammals becomes much more robust and massive (relatively speaking). Bearing these caveats in mind, levels of sexual dimorphism are generally seen as a marker of sexual selection. Studies have shown the earliest homininae were highly dimorphic and that this tendency lessened over the course of human evolution, suggesting humans have became more monogamous. In contrast, gorillas living in harems exhibit a much stronger sexual dimorphism (see: homininae).
A study found that there is an evolutionary trend for men to have relatively shorter upper faces, and suggested that this trait may have been caused by sexual selection, possibly because women have preferred men who looked masculine but not aggressive.
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. The evolutionary biologist 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 an honest advertisement 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 also has a thicker penis than the other great apes, though it is 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, because sperm competition generally favors large testicles. 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 intercourse. A model study showed displacement of semen was directly proportional to the depth of thrusting, as an efficient semen displacement device.
Geoffrey Miller, drawing on some of Darwin's largely neglected ideas about human behaviour, has hypothesized that human culture arose through a process of sexual selection. He argues that cultural traits such as art, music, dance, verbal creativity and humour are of no survival value. Miller is critical of theories that imply that human culture arose as accidents or by-products of human evolution. He believes that human culture arose through sexual selection for creative traits. In that view, many human artifacts could be considered subject to sexual selection as part of the extended phenotype, for instance clothing that enhances sexually selected traits. During human evolution, on at least two occasions, hominid brain size increased rapidly over a short period of time followed by a period of stasis. The first period of brain expansion occurred 2.5 million years ago, when Homo habilis first began using stone tools. The second period occurred 500,000 years ago, with the emergence of archaic Homo sapiens. Miller argues that the rapid increases in brain size would have occurred by a positive feedback loop resulting in runaway selection for larger brains. Tor Nørretranders, in The Generous Man conjectures how intelligence, musicality, artistic and social skills, and language might have evolved as an example of the handicap principle, analogously with the peacock's tail, the standard example of that principle. Another hypothesis proposes that human intelligence is a courtship indicator of health and resistance against parasites and pathogens which are deleterious to human cognitive capabilities.
Anthropologists believe that "male genitalia represent a critical target of sexual selection for fertilization efficiency and sperm competition," and so have developed theories as to why about 25% of societies practice some form of male genital mutilation. In 2008, Charles Wilson of Cornell University suggested that in societies where competition among males is acute, male genital mutilation reduces the threat of conflict for female mates, and so a young man who accepts male genital mutilation "gains immediate access to social and sexual privileges that are suggested to outweigh the cost of the male genital mutilation itself."
The same author indicates that female genital mutilation (FGM) may play a similar paradoxical role:
- "Under this ‘sexual conflict’ hypothesis, male genital mutilation functions in a parallel context to female genital mutilation (FGM). Women who undergo vaginal infibulation or clitoridectomy experience sexual sequelae that would tend to limit EPCs (extra-pair copulations), including restriction of intromission and a reduced capacity to experience sexual pleasure. This reduces the paternity uncertainty of a husband, increasing the trust and investment he is selected to offer. These benefits to a woman and her children seem to outweigh the heavy cost of the mutilation itself in societies with high paternity uncertainty".
Sexual selection has continued to be suggested as a possible explanation for geographical variation in appearance within the human species; in modern hypotheses, marriage practices are proposed as the main determinant of sexual selection. John Manning suggests that where polygyny is common, men face intense competition for wives and are more likely to be completely unsuccessful in reproducing, and the result is strong selection of males for traits which are adaptive for successful reproduction. He proposes a link to skin color through selection of males for testosterone-mediated traits which confer an ability to successfully compete for females. He suggests testosterone makes the human immune system less competent to resist pathogens. In this view the antimicrobial properties of melanin help mitigate the susceptibility to disease that polygyny induces by increasing testosteronization. According to this argument, the anti-infective qualities of melanin were more important than protection from ultraviolet light in the evolution of the darkest skin types. Manning asserts that skin color is more correlated with the occurrence of polygyny – explicable by it having an antimicrobial function – than the latitudinal gradient in intensity of ultraviolet radiation, and he points to the lack of very dark skin at equatorial latitudes of the New World and the relatively light skin of Khoisan people in Africa.
Research seems to contradict Manning's explanation about skin color. In 1978, NASA launched the Total Ozone Mapping Spectrometer, which was able to measure the ultraviolet radiation reaching Earth's surface. Jablonski and Chaplin took the spectrometer's global ultraviolet measurements and compared them with published data on skin color in indigenous populations from more than 50 countries. There was an unmistakable correlation: The weaker the ultraviolet light, the fairer the skin. Rogers et al. (2004) performed an examination of the variation in MC1R nucleotide sequences for people of different ancestry and compared the sequences of chimpanzees and humans from various regions of the Earth. Rogers concluded that, at the time of the evolutionary separation of chimpanzees and humans, the common ancestors of all humans had light skin that was covered by dark hair. Additionally, our closest extant relative, the chimpanzee, has light skin covered by thick body hair. Over time human hair disappeared to allow better heat dissipation through sweating and the skin tone grew darker to increase the epidermal permeability barrier and protect from folate depletion due to the increased exposure to sunlight. When humans started to migrate away from the tropics, there was less-intense sunlight, partly due to clothing to protect against cold weather. Under these conditions there was less photodestruction of folate, and so the evolutionary pressure stopping lighter-skinned gene variants from surviving was reduced. In addition, lighter skin is able to generate more vitamin D (cholecalciferol) than darker skin, so it would have represented a health benefit in reduced sunlight if there were limited sources of vitamin D. The genetic mutations leading to light skin experienced selective pressure due to settlement in northern latitudes.
Anthropologist Peter Frost has proposed that sexual selection was responsible for the evolution of pigmentary traits of women in Northern and Eastern European populations. He contends that the diversity of hair and eye color in Northeast European populations originated as a consequence of intense female-female competition, and is an adaptation for reproductive success in women.
The role of sexual selection in human evolution has been considered controversial from the moment of publication of Darwin's book on sexual selection (1871). Among his vocal critics were some of Darwin's supporters (for example, Alfred Wallace). Darwin was accused of looking to the evolution of early human ancestors through the moral codes of the 19th century Victorian society. Joan Roughgarden, citing elements of sexual behavior in animals and humans that cannot be explained by the sexual-selection model, suggested that the function of sex in human evolution was primarily social.
Joseph Jordania suggested in 2011 that in explaining such human morphological and behavioral characteristics as singing, dancing, body painting, wearing of clothes, Darwin (and proponents of sexual selection) totally neglected another important evolutionary force, intimidation of predators and competitors with the ritualized forms of warning display. Warning display uses virtually the same arsenal of visual, audio, olfactory and behavioral features as sexual selection. According to the principle of aposematism (warning display), in order to avoid costly physical violence and to replace violence with the ritualized forms of display, many animal species (including humans) use different forms of warning display: visual signals (contrastive body colors, eyespots, body ornaments, threat display and various postures to look bigger), audio signals (hissing, growling, group vocalizations, drumming on external objects), olfactory signals (producing strong body odors, particularly when excited or scared), behavioral signals (demonstratively slow walking, aggregation in large groups, aggressive display behavior against predators and conspecific competitors). According to Jordania, most of these warning displays were incorrectly attributed to the forces of sexual selection. Jordania proposed an aposematic model of human evolution, where most of the human morphological and behavioral features that had been considered by Darwin as the result of sexual selection, via female choice, are explained by the aposematic (intimidating) display.
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- Darwin, Charles (1871). The Descent of Man and Selection in Relation to Sex 1 (1st ed.). London: John Murray. p. 256.
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- Dawkins, Richard (2006) [First published 1976]. The Selfish Gene (30th anniversary ed.). p. 158 endnote.
It is not implausible that, with natural selection refining their diagnostic skills, females could glean all sorts of clues about a male's health, and the robustness of his ability to cope with stress, from the tone and bearing of his penis.
- Richard Dawkins (2009). The Greatest Show on Earth: The Evidence for Evolution. Free Press. ISBN 1416594787, ISBN 978-1416594789.
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- Charles Darwin (1882). The Descent of Man and Selection in Relation to Sex. London: John Murray. p. 578.
- The races of man differ from each other, and from their nearest allies, in certain characters which are of no service to them in their daily habits of life, and which it is extremely probable would have been modified through sexual selection. (Darwin, 1936 , p. 908).
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- In a theoretical paper 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.
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- Evo and Proud – Peter Frost's anthropology blog.