Why Do Europeans Have So Many Hair and Eye Colors?
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.
Guppy males (Poecilia reticulata) caught on a single morning from a single creek (Brooks 2002).
Rare-color advantage has been studied mainly in guppies and fruit flies but it also occurs in other animals. In addition, a number of bird species exhibit color polymorphisms whose mode of selection remains unclear. Whatever the cause, this diversification of bright colors is relatively uncommon because it cannot develop in two common situations: 1) high predation pressure, which penalizes color traits in general; and 2) geographic cohabitation with one or more related species. In the latter situation, too much intraspecific variability makes it harder to recognize one's own species and leads to hybridization.(Hughes et al. 1999) more, more
Representative eye colors (Sturm and Frudakis 2004)
|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.
But why do we see more of this color diversity in Europe than elsewhere? Perhaps because sexual selection was stronger in ancestral Europeans, particularly during the long period when they lived from hunting and gathering.
Among contemporary hunter-gatherers, the ratio of single men to single women is most unequal in "steppe-tundra" environments where almost all consumable biomass is in the form of highly mobile and spatially concentrated herbivores such as caribou, reindeer, or muskox. On the one hand, men die younger because of the distances they must cover in search of herds, with no alternate food sources. On the other, men are less polygynous because they bear almost the full cost of feeding their families in a habitat that offers women little opportunity for food gathering. With fewer men altogether and even fewer polygynous ones, women have to compete for a limited supply of potential husbands. They are thus under stronger sexual selection.
Steppe-tundra is now reduced to fragments along the northern fringes of Eurasia and North America. As recently as 10,000 years ago, however, its European portion lay well below 60° N, having been pushed far to the south by the Scandinavian icecap. This low-latitude treeless plain was quite unlike the arctic barrens we see today. The stronger sunlight, combined with the Gulf Stream's moderating and humidifying influence, ensured high bioproductivity even at the peak of the ice age (Hoffecker 2002: 21-26, 32-34). Living conditions were much harsher on the Asian steppe-tundra, which lay further north and further within the Eurasian interior. Polar desert dominated its landscape during glacial advances (Ray and Adams 2001). Thus, it was in the European portion of this ecological zone that modern humans had the best prospects for establishing a substantial and lasting population.
|Ecological zones of Europe at last glacial maximum, c. 18,000 BP.|
This steppe-tundra was distinctive in another way. It covered the same geographic area where, today, human hair and eye color is most diverse. This is also an area where human skin is whiter than in aboriginal populations at similar latitudes elsewhere and where human body build is most sexually dimorphic. more. Could this be an imprint left on the human phenotypic landscape by sexual selection?
Perhaps. But more proof is needed. One tantalizing piece of evidence is the possibility that hair and eye color are mildly sex-linked, as would be expected if women were more strongly selected for such characteristics. According to one study (unpublished), the ratio of second finger length to fourth finger length is higher in non-black-haired and non-brown-eyed individuals. This indicates that these hair and eye colors are associated with a higher ratio of estrogen to testosterone before birth. Interestingly, blond hair has arisen independently among some Aborigines of central Australia and is more frequent there in women than in men.
The Aborigine example points to another avenue for research: populations outside Europe that seem to have independently evolved non-black hair, i.e., blond hair among central Australian Aborigines, brown hair among the Yukaghir of eastern Siberia, and fair hair among some Inuit bands of the western Canadian Arctic. Are these cases due to sexual selection that has been less intense or shorter in duration than it was for Europeans?
A final avenue for research might be to extract DNA from skeletal remains in order to chart the evolution of European MC1R and OCA2 variability over the last 35,000 years. If the sexual selection hypothesis is true, the European-specific alleles would have arisen almost entirely during the last ice age (25,000 - 10,000 BP).
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