Sex linkage of human skin, hair, and eye color

Much of my writing has focused on sexual selection of women and how it may have structured certain pigmentary traits in our species—specifically by lightening skin color and by diversifying hair and eye color. Since most of the relevant alleles are not sex-linked, this selection would have spilled over on to men as well.

Yet, if this selection acted primarily on women, shouldn’t it have tended to favor sex-linked alleles that confine these pigmentary traits to females? All things being equal, wouldn’t such alleles have come to replace those that are not sex-linked? Indeed, Mother Nature loves organisms that don’t waste their energy on things they don’t need. We see this principle in the loss of pigmentation by organisms that live solely in dark caves. It’s not because albino skin is now more useful. It’s because pigmented skin is now useless and may be dispensed with.

Human skin color does show sex linkage. From puberty on, women are lighter-skinned than men in all human populations. This sexual dimorphism seems to be greater in populations that are medium in skin color, perhaps because floor and ceiling effects constrain its expression in populations that are either very dark or very light-skinned (Frost, 2006, pp. 54-60; Frost, 2007; Jablonski & Chaplin, 2000; Madrigal & Kelly, 2006). In women, lightness of skin correlates with thickness of subcutaneous fat, apparently because of a common hormonal causation and not because of a mechanical effect of fat on skin color (Mazess, 1967). It also correlates with digit ratio, which in turn correlates with prenatal estrogenization (Manning et al., 2004). It is this exposure to estrogen before birth that seems to “program” the lightening of female skin after puberty.

Hair color too shows some sex linkage. Hair is darker in girls than in boys before puberty and then lighter afterwards (Keiter, 1952; Leguebe & Twiesselmann, 1976; Olivier, 1960, p. 74; Steggerda, 1941). In a still unpublished British study, digit ratios were found to be higher in blond participants than in darker-haired ones. This finding, if true, suggests increased prenatal estrogenization among people with blond hair.

For eye color, we have no studies that track variation by sex and age. A study of Icelander and Dutch adults found green eyes to be much more prevalent in women than in men (by at least a factor of two). Blue eyes were less prevalent and brown eyes somewhat more prevalent. The participants, however, seem to have been very heterogeneous for age. Many had been recruited for a prostate cancer study among the men or for a breast cancer study among the women (Patrick et al., 2007). Razib discusses this topic on ‘Brown eyed girl’ at GNXP. In the above unpublished British study, digit ratios were found to be higher in light-eyed participants than in brown-eyed ones. This finding, if true, suggests increased prenatal estrogenization among people with non-brown eyes.

References

Frost, P. (2007). Comment on Human skin-color sexual dimorphism: A test of the sexual selection hypothesis, American Journal of Physical Anthropology, 133, 779-781.

Frost, P. (2006). European hair and eye color - A case of frequency-dependent sexual selection? Evolution and Human Behavior, 27, 85-103.

Frost, P. (2005). Fair Women, Dark Men. The Forgotten Roots of Color Prejudice. Cybereditions: Christchurch (New Zealand).

Jablonski, N.G., and Chaplin, G. (2000). The evolution of human skin coloration. Journal of Human Evolution, 39, 57-106.

Keiter, F. (1952). Über ͈Nachdunkeln” und Vererbung der Haarfarben. Z. Morph. Anthrop. 44, 115-126.

Leguebe, A., & Twiesselmann, F. (1976). Variations de la couleur des cheveux avec l’âge. Z. Morph. Anthrop. 67, 168-180.

Madrigal, L, and Kelly, W. (2006). Human skin-color sexual dimorphism: A test of the sexual selection hypothesis. American Journal of Physical Anthropology, 132, 470-482.

Manning, J.T., Bundred, P.E., and Mather, F.M. (2004). Second to fourth digit ratio, sexual selection, and skin colour. Evolution and Human Behavior, 25, 38-50.

Mazess, R.B. (1967). Skin color in Bahamian Negroes. Human Biology, 39, 145‑154.

Olivier, G. (1960). Pratique anthropologique. Paris: Vigot Frères.

Patrick, S., D.F. Gudbjartsson, S.N. Stacey, A. Helgason,T. Rafnar, K.P Magnusson, A. Manolescu, A. Karason, A. Palsson, G. Thorleifsson, M. Jakobsdottir, S. Steinberg, S. Pálsson, F. Jonasson, B. Sigurgeirsson, K. Thorisdottir, R. Ragnarsson, K.R. Benediktsdottir, K.K. Aben, L.A. Kiemeney, J.H. Olafsson, J. Gulcher, A. Kong, U. Thorsteinsdottir, and K. Stefansson. (2007). Genetic determinants of hair, eye and skin pigmentation in Europeans. Nature Genetics Published online: 21 October 2007 doi:10.1038/ng.2007.13

Steggerda, M. (1941). Change in hair color with age. Journal of Heredity, 32, 402-403.