In Europe, especially in the north and east, skin is unusually white, almost at the physiological limit of depigmentation, eyes are not only brown but also blue, gray, hazel or green, and hair is not only black but also brown, flaxen, golden or red. Are these color traits directly or indirectly due to selection for light skin at northern latitudes? But why, then, are they absent in populations that are indigenous to similar latitudes in northern Asia and North America?
As one reader of this blog has argued, skies are more overcast in Europe than at similar latitudes in northern Asia and North America. Thus, ancestral Europeans would have experienced less selection for dark skin to protect against skin cancer and sunburn and more selection for light skin to increase synthesis of vitamin D. Since genes for hair and eye color have some effect on skin color, relaxation of selection for dark skin should have allowed defective alleles to proliferate at all pigmentation loci, including those for hair color and eye color.
Actually, at any given latitude, solar UV radiation is just as intense at ground level in Europe as it is in northern Asia and North America. (Jablonski & Chaplin, 2000; see also charts on: http://pages.globetrotter.net/peter_frost61z/European-skin-color.htm). At these latitudes, UV radiation is already weak, so a significant further reduction in solar UV requires continually overcast skies, such as exist only on the coastal fringe of northwestern Europe.
Moreover, it is doubtful that relaxed selection for dark skin could have diversified hair and eye color by allowing defective alleles to proliferate. Two papers have shown that such a scenario would have needed close to a million years to produce the hair-color and eye-color variability that Europeans now display, with the redhead alleles alone being c. 80,000 years old (Harding et al., 2000; Templeton, 2002). Yet modern humans have been in Europe for only 35,000 years or so.
Instead of relaxed selection for dark skin, perhaps there was increased selection for light skin, notably to boost synthesis of vitamin D. This hypothesis solves the time problem but does not explain the increase in the number of MC1R and OCA2 alleles. Natural selection would have simply favored one allele at the expense of all others, i.e., whichever one optimally reduced skin pigmentation.
There are other problems with either hypothesis, or with any that attribute these color traits to weaker solar UV:
1) If we examine the many homozygous and heterozygous combinations of MC1R or OCA2 alleles, most have little visible effect on skin pigmentation, except for the ones that produce red hair or blue eyes (Duffy et al., 2004; Sturm & Frudakis, 2004).
2) If we consider the estimated time of origin of these color traits, at least two of them seem to have appeared long after modern humans had entered Europe's northern latitudes about 35,000 years ago. The whitening of European skin, through allelic changes at AIM1, is dated to about 11,000 years ago (Soejima et al., 2005). No less recent are allelic changes at other skin color loci and at the eye color gene OCA2 (Voight et al., 2006). Did natural selection wait over 20,000 years before acting?
Are there other forces of natural selection that might explain the 'European exception'? Loomis (1970) and Murray (1934) have argued that Europeans are lighter-skinned than indigenous populations at similar latitudes in northern Asia and North America because the latter obtain sufficient vitamin D in their diet from marine fish. This argument may hold true for the Inuit but not for the majority of indigenous populations that live within the zone of minimal UV radiation, essentially above 47º N (Jablonski & Chaplin, 2000). Most, in fact, live far from sea coastlines.
Duffy, D.L., Box, N.F., Chen, W., Palmer, J.S., Montgomery, G.W., James, M.R., Hayward, N.K., Martin, N.G., & Sturm, R.A. (2004). Interactive effects of MC1R and OCA2 on melanoma risk phenotypes. Human Molecular Genetics, 13, 447-461.
Harding, R.M., Healy, E., Ray, A.J., Ellis, N.S., Flanagan, N., Todd, C., Dixon, C., Sajantila, A., Jackson, I.J., Birch‑Machin, M.A., & Rees, J.L. (2000). Evidence for variable selective pressures at MC1R. American Journal of Human Genetics, 66, 1351‑1361.
Jablonski, N.G., & Chaplin, G. (2000). The evolution of human skin coloration. Journal of Human Evolution, 39, 57-106.
Loomis, W.F. (1970). Rickets. Scientific American, 223, 77-91.
Murray, F.G. (1934). Pigmentation, sunlight, and nutritional disease. American Anthropologist, 36, 438-445. ,
Soejima, M., Tachida, H., Ishida, T., Sano, A., & Koda, Y. (2005). Evidence for recent positive selection at the human AIM1 locus in a European population. Molecular Biology and Evolution, 23, 179-188.
Sturm, R.A., & Frudakis, T.N. (2004). Eye colour: portals into pigmentation genes and ancestry. Trends in Genetics, 20, 327-332.
Templeton, A.R. (2002). Out of Africa again and again. Nature, 416, 45-51.
Voight, B.F., Kudaravalli, S, Wen, X, Pritchard, J.K. (2006). A map of recent positive selection in the human genome. PLoS Biology, 4(3), e72 doi:10.1371/journal.pbio.0040072