[dc_denizen]

sexual selection could also be at play in whitening certain races.

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[42] 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.[42][43]

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.[44] 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.[45] Over time human hair disappeared to allow better heat dissipation through sweating[46] and the skin tone grew darker to increase the epidermal permeability barrier[47] and protect from folate depletion due to the increased exposure to sunlight.[48] 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.[46] The genetic mutations leading to light skin may have experienced selective pressure due to the adoption of farming and settlement in northern latitudes.[49]

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.[50][51]