Sexual selection in human evolution - AbsoluteAstronomy.com

Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...

described sexual selection

Sexual selection

Sexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is a significant element of his theory of natural selection...

as depending "on the advantage which certain individuals have over others of the same sex and species solely in respect of reproduction". In animals, he saw the competition for advantage as occurring between males - the most successful of which were chosen by females. However, in humans, Darwin came to think the evidence pointed toward male choice; he believed sexual selection could explain otherwise puzzling features of the human species, including some aspects of appearance which vary geographically but seem to be trivial and superficial, such as beards.

Some twentieth-century authors had suggested that Darwin was attracted to understanding human evolution through a sexual selection hypothesis because he felt it upheld the unity of humankind against pro-slavery demagoguery by explaining how black people

Black people

The term black people is used in systems of racial classification for humans of a dark skinned phenotype, relative to other racial groups.Different societies apply different criteria regarding who is classified as "black", and often social variables such as class, socio-economic status also plays a...

and white people

White people

White people is a term which usually refers to human beings characterized, at least in part, by the light pigmentation of their skin...

had come to look unalike, while emphasizing they were not separate species. Modern hypotheses which build on Darwin's ideas have mooted a geographical contrast in the intensity and direction of sexual selection between Europe and Africa. The general phenotypic differences between black people and white people are probably the result of both natural selection and sexual selection. Although skin color can be a target of sexual selection the skin color variation in humans is thought to be a result of natural selection

Natural selection

Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....

along a latitude gradient.

Sexual dimorphism

Men are 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

Photoprotection

Photoprotection is a group of mechanisms that nature has developed to minimize the damage that the human body suffers when exposed to UV radiation...

and is part of the most commonly accepted scientific explanation for the evolution of pigmentary traits.

Sexual selection can be difficult to demonstrate as features may result from an equilibrium among competing selective pressures, some involving sexual selection, others natural selections. 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 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

Homininae

Homininae is a subfamily of Hominidae, which includes humans, gorillas and chimpanzees, and some extinct relatives; it comprises all those hominids, such as Australopithecus, that arose after the split from orangutans . Our family tree, which has 3 main branches leading to chimpanzees, humans and...

were highly dimorphic and that this lessened over the course of the evolution of Homo, suggesting humans have became more monogamous, in contrast, gorillas who live in harems, have a much stronger sexual dimorphism (see homininae

Homininae

).

A study found an evolutionary trend for men to have relatively shorter upper faces may have been caused by sexual selection on the simple ratio of upper face length to broadness as a proxy for facial attractiveness, possibly because women have preferred men who looked masculine but not aggressive. In an initial test calculations derived from the research were applied to photographs of male celebrities; Will Smith

Will Smith

Willard Christopher "Will" Smith, Jr. , also known by his stage name The Fresh Prince, is an American actor, producer, and rapper. He has enjoyed success in television, film and music. In April 2007, Newsweek called him the most powerful actor in Hollywood...

conformed most closely to the ideal proportions.

Darwin and more recent hypotheses

Charles Darwin conjectured that the male beard, as well as the hairlessness of humans compared to nearly all other mammals, are 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

Natural selection

alone provided a satisfactory answer. Although not explicit, his observation that in Khoisan

Khoisan

Khoisan is a unifying name for two ethnic groups of Southern Africa, who share physical and putative linguistic characteristics distinct from the Bantu majority of the region. Culturally, the Khoisan are divided into the foraging San and the pastoral Khoi...

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 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. 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 where polygyny

Polygyny

Polygyny is a form of marriage in which a man has two or more wives at the same time. In countries where the practice is illegal, the man is referred to as a bigamist or a polygamist...

is common men face intense competition for wives and are more likely to be completely unsuccessful in reproducing, a 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

Testosterone

Testosterone is a steroid hormone from the androgen group and is found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is primarily secreted in the testes of males and the ovaries of females, although small amounts are also secreted by the adrenal glands...

mediated traits which confer an ability to successfully compete for females and points to the fact that males have higher rates of many diseases as evidence that the effect of 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. Hence the anti-infective qualities of melanin

Melanin

Melanin is a pigment that is ubiquitous in nature, being found in most organisms . In animals melanin pigments are derivatives of the amino acid tyrosine. The most common form of biological melanin is eumelanin, a brown-black polymer of dihydroxyindole carboxylic acids, and their reduced forms...

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 anti microbial function - than the latitudinal gradient in intensity of ultraviolet radiation, he points to the lack of very dark skin at equatorial latitudes of the New World

New World

The New World is one of the names used for the Western Hemisphere, specifically America and sometimes Oceania . The term originated in the late 15th century, when America had been recently discovered by European explorers, expanding the geographical horizon of the people of the European middle...

and the relatively light skin of Khoisan

Khoisan

people in Africa.

Evolutionary psychology

From the viewpoint of evolutionary psychology

Evolutionary psychology

Evolutionary psychology is an approach in the social and natural sciences that examines psychological traits such as memory, perception, and language from a modern evolutionary perspective. It seeks to identify which human psychological traits are evolved adaptations, that is, the functional...

(championed by David Buss

David Buss

David M. Buss is a professor of psychology at The University of Texas at Austin, known for his evolutionary psychology research on human sex differences in mate selection.-Biography:...

, Steven Pinker

Steven Pinker

Steven Arthur Pinker is a Canadian-American experimental psychologist, cognitive scientist, linguist and popular science author...

, Desmond Morris

Desmond Morris

Desmond John Morris, born 24 January 1928 in Purton, north Wiltshire, is a British zoologist and ethologist, as well as a popular anthropologist. He is also known as a painter, television presenter and popular author.-Life:...

, Daniel Dennett

Daniel Dennett

Daniel Clement Dennett is an American philosopher, writer and cognitive scientist whose research centers on the philosophy of mind, philosophy of science and philosophy of biology, particularly as those fields relate to evolutionary biology and cognitive science. He is currently the Co-director of...

, and others) modern humans have inherited natural traits that were adaptive in a prehistoric environment, including traits that had different advantages for males versus females (see Sexual selection

Sexual selection

Sexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is a significant element of his theory of natural selection...

). Evolutionary theory of sex considers gender differences as a result of distinct specialization of the sexes, performing relationship with preceding generations (inheritance) and with the environment (variability). Theory explains ethological

Ethology

Ethology is the scientific study of animal behavior, and a sub-topic of zoology....

and psychological sexual dimorphism

Sexual dimorphism

Sexual dimorphism is a phenotypic difference between males and females of the same species. Examples of such differences include differences in morphology, ornamentation, and behavior.-Examples:-Ornamentation / coloration:...

, more efficient "education" or "training" of females during the course of ontogenetic

Ontogeny

Ontogeny is the origin and the development of an organism – for example: from the fertilized egg to mature form. It covers in essence, the study of an organism's lifespan...

adaptation

Adaptation

An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....

as well as greater conformity

Conformity

Conformity is the process by which an individual's attitudes, beliefs, and behaviors are influenced by other people.Conformity may also refer to:*Conformity: A Tale, a novel by Charlotte Elizabeth Tonna...

of females well known to psychologists.

Direction and intensity of sexual selection

Another proposed explanation for the evolution of pigmentary traits in humans though sexual selection is by Peter Frost

Peter Frost (anthropologist)

Peter Frost is a Canadian anthropologist.He was awarded his Ph.D. from Université Laval in 1995.He is best known for hypotheses on the evolution of hair, eye and skin colors...

who suggests European hair, eye, and skin color originated during the Last glacial period when intense female-female rivalry for mates led to a series of adaptations for increased mating success. Modern research suggests that across a wide range of cultures a female face is seen as attractive if it has high eyebrows, widely spaced large eyes with dilated pupils, high cheekbones, a small nose, a narrow face with thin cheeks, a large smile, a full lower lip, a small chin, and a full hairstyle. Although these mate-choice criteria are largely universal the aesthetic preferences of men are constrained by the number of potential mates, without an excess to choose from there can be no overall selection.

Men are often unneeded during pregnancy and infant care and so they are usually free to serve the cause of maximizing their reproductive fitness though a return to the mate market; in practice many males will be competing for too few females at any one time. Males can best maximize their reproductive fitness (leave more descendants) in the tropics, there women relied least on men as providers - being able to gather fruits, vegetables, tubers, and eggs for themselves while still caring for infants. In higher latitudes, however, opportunities for food gathering are restricted and women tend to rely on their husband to provide for the family by hunting, consequently the cost of providing for a second wife makes polygyny impossible for all but the ablest hunters. Land supports fewer game animals per unit of land area at high latitude and hunting distance increases proportionately this makes hunters vulnerable to death by exposure, exhaustion, drowning, injuries and starvation.

These risks peak in the 'continental Arctic,' of steppe-tundra

Steppe-tundra

Steppe-tundra is a very cold dry-climate vegetation type consisting of mostly treeless open herbaceous vegetation. it was widespread during Pleistocene times at mid-latitudes of North America and Eurasia. Steppe-tundra can be divided into two types...

, an environment widespread in Europe during the Late Glacial Maximum. It is likely that those hunting mobile game animals on foot in this environment would have very high mortality. The death of many male providers allied to the fact that surviving men were unable to feed more than one wife may have meant a high proportion of women could never marry and reproduce; sexual selection of women for characteristics that would enable them to get a husband was intense. Studies show that recognition of objects does not proceed by attention to all features simultaneously; attention is initially directed to cues of 'brightness', 'colour' or shape'. Priority goes to the strikingly varied and vivid colors tend to be attributes of intense sexual selection. If mate competition is relatively weak, sexual selection focuses primarily on visible markers of mate quality which signify fecundity, as mate competition intensifies the focus will shift from functional to eye-catching novelty rather than biologic 'truth in advertising'.

With a glut of potential mates animals are swayed by eye-catching stimuli as when a secondary sexual characteristic is outsized or more vividly colored for effective hyperstimulating of the algorithms for sex-identification. Eyes and facial features that function in personal interaction and identification can be similarly highlighted. Brightly colored phenotypes can diversify under selection pressure into a polymorphism of phenotypes. Hair and eye pigmentation shows a remarkable diversity in Europe and frequency-dependent sexual selection for such diversity is a more likely explanation for it than any side effect of straightforwardly decreasing distribution of melanin following the latitudinal gradient of ultraviolet radiation.

Furthermore, the date of the mutations for the multitude of eye and hair colors found in Europe are closer to the late glacial maximum than when humans first entered Europe. Skin color dimorphism means that the characterization of women as the "fair sex" is in fact accurate; men have darker complexions in comparison to women as a result of differing melanin and hemoglobin levels in outer layers of the skin. Although this constitutive sex difference - which is not due to sun exposure - is less obvious in weakly pigmented peoples a hardwired mental mechanism enables us to recognize the gender of human faces.

The main clue to gender is complexion; people can distinguish a man's face from a woman's by the complexion and contrast between facial pigmentation and eye/lip pigmentation This slight - but significant - difference in pigmentation could explain why Europeans are white skinned. In premodern cultures there is a cross-cultural tendency to associate lighter skin with femininity and a preference for lighter skinned women as mates. In this way strong sexual selection of women is hypothesized to have altered the pigmentation of the entire population. The dating of the mutations for white skin and hair and eye colors are closer to the late glacial maximum than the original modern human entering of Europe as the UV-latitude hypothesis would presumably predict; however, there are alternative, i.e. non-sexual selection, explanations for this (see below).

Criticism

The main critics of hypotheses which suggest that the extremes of skin pigmentation have resulted from sexual selection are those who see skin color as an obvious adaptation to excessive or inadequate ultraviolet

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

radiation, they object that skin pigmentation accompanied loss of body hair while dark pigmentation evolved soon after the genus Homo in Africa, moreover a conspicuous latitude gradient of skin tones follows the out of Africa dispersion, these are conclusive. Therefore - it is argued - natural selection for sufficient ultraviolet penetration to enable vitamin D

Vitamin D

Vitamin D is a group of fat-soluble secosteroids. In humans, vitamin D is unique both because it functions as a prohormone and because the body can synthesize it when sun exposure is adequate ....

production while protecting folic acid

Folic acid

Folic acid and folate , as well as pteroyl-L-glutamic acid, pteroyl-L-glutamate, and pteroylmonoglutamic acid are forms of the water-soluble vitamin B9...

gave rise to the evolution of skin pigmentation; light skin preference by males in some cultures may have - at most - slightly affected sexual dimorphism.

Indeed the case for human skin color as an adaptation to UV radiation is so clear an example of the mechanism of evolution by natural selection that it is a perfect model to demonstrate it. Deleterious health effects of insufficient vitamin D

Vitamin D

Vitamin D is a group of fat-soluble secosteroids. In humans, vitamin D is unique both because it functions as a prohormone and because the body can synthesize it when sun exposure is adequate ....

are also pointed to as confirmation that skin lightening was in response to strong selection pressure for maximizing vitamin D.

A variation of the vitamin D argument has recently gained credence as awareness of the late dates for the origin of European specific skin, eye and hair lightening has grown and made an explanation for humans living in Europe for several thousand years without becoming white required.

The replacement of hunting by agriculture is claimed to have led to a dearth of vitamin D , therefore it was the elimination of game meat, fish, and some plants from the diet which resulted in skin turning white long after modern human settlement in Europe.

An anomaly for this conjecture is that if the consequence of switching from meat to an agricultural diet is skin becoming less pigmented then historically hunter gatherer population in Africa such as the Khoisan

Khoisan

and Oromo

Oromo

The Oromo are an ethnic group found in Ethiopia, northern Kenya, .and parts of Somalia. With 30 million members, they constitute the single largest ethnic group in Ethiopia and approximately 34.49% of the population according to the 2007 census...

peoples, who are paler than other sub Saharan Africans and basal

Basal (phylogenetics)

In phylogenetics, a basal clade is the earliest clade to branch in a larger clade; it appears at the base of a cladogram.A basal group forms an outgroup to the rest of the clade, such as in the following example:...

to them, ought to have darker skin than historically agriculturist groups. However, a leading proponent of the agriculturally caused pigmentation hypothesis maintains that sexual selection can not account for European pigmentation as it would have almost certainly have resulted in the traits of lighter skin, hair and eyes exhibiting strong sexual dimorphism

Sexual dimorphism

Culture and sexual selection

Another cultural practice which may actually be related to culturally enforced of male-male competition is circumcision

Circumcision

Male circumcision is the surgical removal of some or all of the foreskin from the penis. The word "circumcision" comes from Latin and ....

, which may reduce younger men's potential to illicitly father a child with an older man's wife. Some forms handicap sperm delivery in an obvious way; sub-incision is where cuts are made in the base of the penis causing sperm to be ejaculated from the base rather than the end (it is performed in several Aboriginal Australian societies). In some African and Micronesian cultures youths have one of their testicles crushed. One suggestion is that lack of a foreskin could make insertion or ejaculation slower, meaning brief, illicit sex is less likely to result in pregnancy. Illicit sex is proposed to be a worry of polygynous males. An anthropological database revealed that predictions of male circumcision tending to be a practice of non-monogamous societies was borne out - 48% of highly polygynous societies practice some form of male genital mutilation, and in societies in which wives live in separate households that increases to 63% while only 14% of the monogamous societies in the database practised genital mutilation of young men.

Geoffrey Miller

Geoffrey Miller (evolutionary psychologist)

Geoffrey F. Miller , Associate Professor of Psychology at the University of New Mexico, is an American evolutionary psychologist.Miller is a 1987 graduate of Columbia University, where he earned a B.A. in biology and psychology. He received his PhD in cognitive psychology from Stanford University...

, 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 artefacts could be considered subject to sexual selection as part of the extended phenotype, for instance clothing that enhance 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

Archaic Homo sapiens

Archaic Homo sapiens is a loosely defined term used to describe a number of varieties of Homo, as opposed to anatomically modern humans , in the period beginning 500,000 years ago....

. 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

Tor Nørretranders

Tor Nørretranders is a Danish author of popular science. He was born in Copenhagen, Denmark. His books and lectures have primarily been focused on science and its role in society, often with Nørretranders' own advice about how society should integrate new findings in science...

, in The Generous Man conjectures how intelligence, musicality, artistic and social skills, language might have evolved as an example of the handicap principle

Handicap principle

The handicap principle is a hypothesis originally proposed in 1975 by biologist Amotz Zahavi to explain how evolution may lead to "honest" or reliable signaling between animals who have an obvious motivation to bluff or deceive each other...

, 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

Parasitism

Parasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. Traditionally parasite referred to organisms with lifestages that needed more than one host . These are now called macroparasites...

and pathogen

Pathogen

A pathogen gignomai "I give birth to") or infectious agent — colloquially, a germ — is a microbe or microorganism such as a virus, bacterium, prion, or fungus that causes disease in its animal or plant host...

s which are deleterious to human cognitive capabilities.

Human sexual anatomy

The theory of sexual selection has been used to explain a number of human anatomical features. These include rounded breasts, facial hair

Facial hair

Facial hair is a secondary sex characteristic of human males. Men often start developing facial hair in the later years of puberty or adolescence, approximately between 17–20 years of age, and most do not finish developing a fully adult beard until their early 20s or even later...

, pubic hair

Pubic hair

Pubic hair is hair in the frontal genital area, the crotch, and sometimes at the top of the inside of the legs; these areas form the pubic region....

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

Richard Dawkins

Clinton Richard Dawkins, FRS, FRSL , known as Richard Dawkins, is a British ethologist, evolutionary biologist and author...

has speculated that the loss of the penis bone

Baculum

The baculum is a bone found in the penis of most mammals. It is absent in humans, but present in other primates, such as the gorilla and chimpanzee.The bone aids in sexual intercourse.-Purpose:...

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

Homo (genus)

Homo is the genus that includes modern humans and species closely related to them. The genus is estimated to be about 2.3 to 2.4 million years old, evolving from australopithecine ancestors with the appearance of Homo habilis....

also has by far the largest penis of the great apes and this may be sexually selected in much the same way as the larger testicles of Pan, another possibility is that the size of the human penis has resulted from selection for efficiency in displacing the sperm of rival males during intercourse.

Implications of differences in penis length and morphology

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 and a small penis as in the chimpanzee. However, the human penis has a larger glans ridge than is found in many other primates and this may function to displace other males' seminal fluid from a vagina by forcing it behind the glans during the thrusting of intercourse and thereby expelling rival semen away from the area of the cervix. A model study showed displacement of semen was directly proportional to the depth of thrusting as an efficient semen displacement device.

External links

Evo and Proud - Peter Frost
Peter Frost (anthropologist)
Peter Frost is a Canadian anthropologist.He was awarded his Ph.D. from Université Laval in 1995.He is best known for hypotheses on the evolution of hair, eye and skin colors...

's anthropology blog.

The source of this article is wikipedia, the free encyclopedia. The text of this article is licensed under the GFDL.

Sexual dimorphism

Darwin and more recent hypotheses

Evolutionary psychology

Direction and intensity of sexual selection

Criticism

Culture and sexual selection

Human sexual anatomy

Implications of differences in penis length and morphology

Further reading

External links