Université Laval (Canada)
Most humans have only one hair color and one eye color. Europeans are a big exception: their hair is black but also brown, flaxen, golden, or red; their eyes are brown but also blue, gray, hazel, or green. This diversity reaches a maximum in an area centered on the East Baltic and covering northern and eastern Europe. If we move outward, to the south and east, we see a rapid return to the human norm: hair becomes uniformly black and eyes uniformly brown.
Why this color diversity? And why only in Europe? Some believe it to be a side effect of natural selection for fairer skin to ensure enough vitamin D at northern latitudes. Yet skin color is weakly influenced by the different alleles for hair color or eye color, apart from the ones for red hair or blue eyes. Some have no effect at all on skin pigmentation (Duffy et al. 2004; Sturm and Frudakis 2004).
Others put the cause down to intermixture with Neanderthals. Yet, according to the mtDNA that has been retrieved, no genetic continuity is discernible between late Neanderthals and early modern Europeans. Perhaps there was some gene flow between the two groups, but certainly not enough to account for the large number of Europeans with neither black hair nor brown eyes.
For others still, this color diversity arose through random factors: genetic drift, founder effects, relaxation of natural selection, etc. But these factors could not have produced such a wide variety of hair and eye hues in the 35,000 years that modern humans have inhabited Europe. The hair-color gene (MC1R) has at least 7 alleles that exist only in Europe and the same is probably true for the eye-color gene (OCA2) (Rana et al. 1999). If we take the hypothesis of a relaxation of selection, nearly a million years would be needed to accumulate this amount of diversity (Harding et al. 2000; Templeton 2002). Moreover, it is odd that the same sort of diversification has occurred at two different genes whose only point in common is to color a facial feature (Frost 2006; Makova & Norton 2005).
Thus, some kind of non-random process seems to have targeted both the hair and the eyes as visible characteristics. But why? And how? For some, including the geneticist Luigi L. Cavalli-Sforza, the answer is sexual selection. This mode of selection intensifies when males outnumber females among individuals ready to mate, or vice versa. The sex in excess supply has to compete for a mate and resorts to the same strategies that advertisers use to grab attention, such as the use of bright or striking colors.
In other animals, bright colors are usually due to sexual selection. Sometimes the result may be a «color polymorphism» (see box). This is because a potential mate is attracted not just by a bright color but also by a rare one that stands out from the crowd. By enhancing reproductive success, however, such a color will also become more common and less eye-catching. Sexual attraction will then shift to less common variants, the eventual result being an equilibrium that maximizes color diversity (Brooks 2002; Frost 2006; Hughes et al. 1999).
This frequency dependence has been shown in humans. Thelen (1983) presented male participants with slides showing attractive brunettes and blondes and asked them to choose, for each series, the woman they would most like to marry. One series had equal numbers of brunettes and blondes, a second 1 brunette for every 5 blondes, and a third 1 brunette for every 11 blondes. Result: the rarer the brunettes were in a series, the likelier any one brunette would be chosen. This rare-color preference is also shown in a Gene Expression (2008) study that found an overrepresentation of blondes and dark brunettes on the front covers of Maxim magazine in relation to the white American population. Women with the more common light brown hair were underrepresented. This frequency-dependent preference may have produced the wide range of human hair and eye phenotypes we see today.