Of all humans, male and female, European women have the whitest skin and the most diverse range of hair and eye colors. Are European physical characteristics really female characteristics? (source)
People of European origin have an unusually diverse palette of hair and eye colors. This diversity is commonly ascribed to their unusually white skin. Ancestral Europeans became lighter-skinned, and this genetic change therefore caused other changes to hair and eye pigmentation.
Actually, the genetic changes are different in each case. European skin turned white through a replacement of alleles, primarily at TYRP1, SLC24A5, and SLC45A2. European hair and eyes diversified in color through a proliferation of new alleles, primarily at MC1R for hair color and in the HERC2-OCA2 region for eye color.
It now appears that this diversification has occurred at other gene loci as well. Zhang et al. (2013) report that a region downstream from EDNRB is associated with differences in hair color and that two other loci, VASH2 and POLS, are associated with differences in eye color. Sulem et al. (2008) report that TPCN2 is associated with differences in hair color and that ASIP is associated with red hair.
A common selection pressure, not a common gene
This is further proof that a selection pressure created the visual effect of color diversity by acting on whatever genes it could. In short, this diverse palette of hues seems to exist “just for show.”
The evolutionary problem is spelled out by Walsh et al. (2012):
People of European descent display the widest variation in pigmentation traits, such as iris (eye) and hair colouration, in the world. In particular, eye colour variation is nearly restricted to people of (at least partial) European descent. Eye colour categories here often concern blue, brown and intermediate (green, etc.). In the rest of the world, people tend to have brown eye colour, which is considered to be the ancestral human trait in agreement with the Out-of-Africa hypothesis of modern humans. The current variation in eye colour is thought to have originated via a genetic founder event involving non-brown irises in early European history. It is furthermore assumed that eye colour variation in Europe has been shaped by positive selection via sexual selection i.e., mate choice preference. Alternatively it has been proposed that eye colour variation evolved via a correlation with skin colour and its environmental adaptation e.g. maximizing vitamin D conversion in low levels of UV radiation, or as a combination of both. One suggested geographic region for the origin of blue eye colour in Europe is the southern Baltic, as indicated by concentric rings of decreasing frequency of the blue eye colour trait spreading from the southern Baltic region, resulting in a strong north–south gradient in blue eye colour frequency across Europe.
It is doubtful whether a lack of vitamin D at northern latitudes played a role in the whitening of European skin, let alone in the diversifying of European hair and eye color. As Elias and Williams (2012) note, certain northern populations whitened much more than others:
An obvious feature of the northward dispersal of humans is a quasi-geographic reduction in pigmentation (Murray, 1934; Loomis, 1967; Chaplin and Jablonski, 2009). Coloration varies greatly among northerners. Native Inuit display medium-to-dark (type III/IV), rather than light pigmentation, and both northern and central-dwelling Asians display medium (type III) pigmentation. Recent population genetic data show that the reduction in skin pigmentation occurred sporadically and incompletely in northern and Asian populations (Sturm, 2009). Moreover, while modern humans reached Central Europe ≈40 ka (thousands of years ago), they reached northern Europe only after the last ice sheets receded less than 11 ka. It is only these humans that display light pigmentation, and recent molecular genetic studies suggest that the very light pigmentation of northern Europeans did not develop until 5-6 ka (Norton et al., 2007; Norton and Hammer, 2008).
Heather Norton’s estimate for European skin whitening (which she set within a broader range of 3,000 to 12,000 years ago) has been revised upward by Sandra Beleza to a range of 11,000 to 19,000 years ago, the second estimate being now accepted as the better one by Norton (Beleza et al., 2013; Norton and Hammer, 2007; Norton, 2012). This time period still began long after the entry of modern humans into Europe, the implication being that ancestral Europeans were brown-skinned for tens of thousands of years.
Elias and Williams (2012) also note that the vitamin-D hypothesis cannot explain the changes to European hair color, since hair is not involved in vitamin-D synthesis. Their alternate hypothesis is that European skin became white as a way to cut back on unnecessary energy expenditure:
[…] a declining need to heavily pigment the epidermis favored the retention of mutations in genes that reduced pigment synthesis, thereby diverting energy toward the production of more urgently-needed proteins.
But why, then, did ancestral Europeans wait over twenty thousand years before cutting back on this unnecessary expenditure? And why would this expenditure be less unnecessary at northern latitudes in Asia and North America? Moreover, in the case of hair color, what has happened is not a loss of pigment but rather a shift from production of one kind of pigment, i.e., eumelanin (black-brown hues), to production of another, i.e., pheomelanin (yellow-red hues).
Sexual selection?
Color polymorphisms are not limited to humans. They occur in many other species for reasons that Hofreiter and Schöneberg (2010) discuss in a recent review article. One reason is crypsis—the need to blend into a background that may vary from one place to another. Deer mice, for instance, have light fur where the ground is likewise light in color and dark fur where it is dark in color. Another reason is aposematism—individuals with a rare coloration have better chances of survival, since they are a poorer match for a predator’s search image.
Such a frequency dependent effect, favouring the rarer colour morphs, is also known from sexual selection, when females preferentially mate with rare colour morph males, a phenomenon also known from guppies. (Hofreiter and Schöneberg, 2010)
This kind of color polymorphism typically involves bright colors, since sexual selection is influenced by sensory biases that favor not only novel colors but also bright ones as well. In fish species, for instance, color morphs are often red because a sensory bias for this color has developed irrespective of mating contexts.
If we look at the polymorphisms for human hair and eye color, the recently evolved “European” hues tend to be brighter than the species norm of black hair and brown eyes. Eyes may be light blue, but not navy blue. Hair may be carrot red, but not beetroot red. Sexual selection is also indicated by a greater variability of hair color in women, with red hair being especially more frequent (Shekar et al., 2008).
But why?
Why would sexual selection have been more intense among ancestral Europeans? Such selection happens when too many of one sex are competing to mate with too few of the other. In most mammals, the males do the competing—because polygyny dries up the pool of available females. So the males are brilliantly colored, and the females duller in appearance.
But here we have the reverse. Hair color is brighter and more diverse in European women than in European men. We see a similar pattern with skin color. “European” physical traits seem to be female traits. It looks as though sexual selection primarily targeted women and then secondarily spilled over on to men.
This unusual color scheme seems to result from the unusual steppe-tundra that covered the plains of northern and eastern Europe during the last ice age 25,000 to 10,000 years ago. This environment offered ancestral Europeans a huge amount of edible biomass, but nearly all of it was locked up as meat in wandering herds of reindeer and other herbivores. Since male hunters provided almost all of the food for their wives and offspring, the cost of supporting a second wife and her children was prohibitive for them, being feasible for only the ablest hunters. At the same time, pursuit of migratory game greatly lengthened the mean hunting distance and boosted male death rates accordingly.
Thus, limited polygyny, combined with higher hunting-related mortality, skewed the mate market towards a shortage of available men. Women had to compete for men, unlike the situation among tropical humans and most other mammalian species. This intense mate competition in turn drove sexual selection for colorful features that could, by their brightness or their novelty, catch the attention of a prospective mate (Frost, 2006; Frost, 2008).
References
Beleza, S., A. Murias dos Santos, B. McEvoy, I. Alves, C. Martinho, E. Cameron, M.D. Shriver, E.J. Parra, and J. Rocha. (2013). The timing of pigmentation lightening in Europeans, Molecular Biology and Evolution, 30, 24-35.
Frost, P. (2006). European hair and eye color - A case of frequency-dependent sexual selection? Evolution and Human Behavior, 27, 85-103.
Frost, P. (2008). Sexual selection and human geographic variation, Journal of Social, Evolutionary,and Cultural Psychology, 2(4), 169-191. http://137.140.1.71/jsec/articles/volume2/issue4/NEEPSfrost.pdf
Hofreiter, M., and T. Schöneberg. (2010). The genetic and evolutionary basis of colour variation in vertebrates, Cellular and Molecular Life Sciences, 67, 2591–2603.
Norton, H.L., and M.F. Hammer. (2007). Sequence variation in the pigmentation candidate gene SLC24A5 and evidence for independent evolution of light skin in European and East Asian populations. Program of the 77th Annual Meeting of the American Association of Physical Anthropologists, p. 179
Norton, H.L. (2012). Personal communication
Shekar, S.N., D.L. Duffy, T. Frudakis, G.W. Montgomery, M.R. James, R.A. Sturm, and N.G. Martin. (2008). Spectrophotometric methods for quantifying pigmentation in human hair—Influence of MC1R genotype and environment, Photochemistry and Photobiology, 84, 719–726.
Sulem, P., D.F Gudbjartsson, S.N. Stacey, A. Helgason, T. Rafnar, M. Jakobsdottir, S. Steinberg, S.A. Gudjonsson, A. Palsson, G. Thorleifsson, S. Palsson, B. Sigurgeirsson, K. Thorisdottir, R. Ragnarsson, K.R. Benediktsdottir, K.K. Aben, S.H. Vermeulen, A.M. Goldstein, M.A. Tucker, L.A. Kiemeney, J.H. Olafsson, J. Gulcher, A. Kong, U. Thorsteinsdottir, and K. Stefansson. (2008). Two newly identified genetic determinants of pigmentation in Europeans, Nature Genetics, 40, 835-837.
Walsh, S., A. Wollstein, F. Liu, U. Chakravarthy, M. Rahu, J.H. Seland, G. Soubrane, L. Tomazzoli, F. Topouzis, J.R. Vingerling, J. Vioque, A.E. Fletcher, K.N. Ballantyne, and M. Kayser. (2012). DNA-based eye colour prediction across Europe with the IrisPlex system, Forensic Science International: Genetics, 6, 330–340.
Zhang, M., F. Song, L. Liang, H. Nan, J. Zhang, H. Liu, L.-E. Wang, Q. Wei, J.E. Lee, C.I. Amos, P. Kraft, A.A. Qureshi, and J. Han. (2013). Genome-wide association studies identify several new loci associated with pigmentation traits and skin cancer risk in European Americans, Human Molecular Genetics, advance access 1–12