Hierodule
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6. Discussion One might object that sexual selection could not have diversified European hair and eye color because there is no sexual dimorphism in these traits. Had women been selected for a diversity of hair and eye colors, they would now be more diverse in this respect than men are. It should be noted, however, that both hair and eye colors are, at best, weakly sex-linked; hence, selection acting on women should have affected men and women equally until sex-linked alleles had arisen through chance mutations. The original alleles (i.e., the non-sex-linked ones) could then have been selected out, but only if men were somehow disadvantaged by the novel hair and eye colors. For most animals, the disadvantage is an increased risk of predation, which will reduce highly visible colors in the sex that does not need them. For early Europeans, such a disadvantage would have been slight. Even wolves were more likely to be prey than predators (Hoffecker, 2002, pp. 180–183, 225, 241–242). This being said, some hair and eye colors seem to be sex linked. Blond hair darkens with age more slowly in women than in men (Olivier, 1960, p. 74). Furthermore, prenatal exposure to estrogen, as indicated by digit ratio, appears to be higher in individuals with blond hair or non-brown eyes (Mather et al., unpublished). If a sex difference does indeed exist in these novel hair and eye colors, it seems to be expressed only right after puberty. It was notably absent in the 18-to-38 year olds studied by Mather et al. (unpublished). Besides diversifying European hair and eye color, sexual selection may have accentuated existing sexual dimorphisms. Several studies have found wider hips, narrower waists, and thicker deposition of subcutaneous fat in women of European descent than in women of other origins (Hrdlička, 1898, Meredith & Spurgeon, 1980 and Nelson & Nelson, 1986). Even before birth, Euro-American fetuses show significantly more sexual dimorphism than do African-American fetuses (Choi & Trotter, 1970). The proximal cause may be lower androgen production than in women of sub-Saharan African descent (Falkner et al., 1999) and higher estrogen production and lower fecal excretion of estrogen than in women of north/east Asian descent (Adlercreutz et al., 1994, Coker et al., 1997, Key et al., 1990, Taioli et al., 1996 and Wang et al., 1991). Prenatal exposure to estrogen, as indicated by digit ratio, may also be higher in European women, albeit with much interpopulation variation (Manning et al., 2000; Manning, J. T. (2003). Personal communication). This variation may reflect a maternal-age effect: digit ratio is higher in Catholic countries like Poland and Spain, where mothers generally bear children in their 20s, than in Germany and Finland, where more mothers bear children in their 30s (Manning et al., 2000). Sexual selection may have also lightened European skin color. The extreme depigmentation of northern and eastern Europeans deviates markedly from the much weaker north–south gradient in skin color of other human populations (the latter gradient may reflect selection pressures to maintain a critical level of vitamin D synthesis). Yet the geographic extent of this extreme depigmentation does not coincide with a specific pattern of solar radiation: Skies are generally overcast over coastal northwestern Europe and become clearer and typically continental further east. It does coincide, however, with the area where hair and eye color has diversified (Fig. 4). Aside from red hair, the color of the hair and eyes is not genetically linked to skin color (Flanagan et al., 2000 and Sturm et al., 2001). It seems, therefore, that a common selective force has acted simultaneously on skin, hair, and eye color within this geographic area while being absent at similar latitudes in northern Asia and North America (Frost, 1994a and Manning et al., 2004).  Fig. 4. Skin-color depigmentation in Eurasia approximately 500 BP (after Brace, 1973, p. 344, reprinted with permission from Wiley). If this common selective force were sexual selection, it could have lightened European skin color by acting on an existing sexual dimorphism. Men and women differ in complexion because of differing amounts of melanin and cutaneous blood flow; in short, women are fairer, men browner and ruddier (Edwards & Duntley, 1939, Frost, 1988, Frost, 2005, Hulse, 1967 and Jablonski & Chaplin, 2000). The size of this sex difference is still debated, largely because most studies are poorly controlled for age (girls lighten only after puberty and immediately before are actually darker than boys). Investigators also try to exclude tanning by measuring under the arm, where there is less subcutaneous fat and probably less dimorphism in skin color, given that the lightness of a woman's skin correlates with the thickness of her subcutaneous fat (Mazess, 1967). In any event, sexual selection may have targeted this sex difference, as suggested by a cross-cultural male preference for lighter complexioned women and, conversely, by some evidence of a female preference for darker complexioned men (Aoki, 2002, Feinman & Gill, 1978, Frost, 1988, Frost, 1994b, Frost, 2005 and Van den Berghe & Frost, 1986). Among ancestral Europeans, such selection, even if acting only on women, would have lightened the complexions of both sexes because most skin-color genes are not sex linked. Nonetheless, some of these genes are; thus, there should have been some selective pressure to make European skin color more sexually dimorphic. Yet skin color actually seems to be less sexually dimorphic in light-skinned populations (Relethford et al., 1985). The reason may be a ceiling effect. As ancestral Europeans approached the phenotypic limit of maximum skin depigmentation, further lightening would have become harder to achieve for women than for men, with the result that sexual selection, although acting primarily on women, lightened men more. In conclusion, sexual selection may have acted on all three color traits in northern and eastern Europe, with hair and eye color being diversified and skin color lightened. This hypothesis is consistent with the narrow timeframe for the evolution of these traits, their geographic distribution, and the large number of alleles involved.
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