Why Do Europeans Have So Many Hair and EyeColors?
Université Laval (Canada) and St. AndrewsUniversity (Scotland).
[This article summarizes Frost (2006) and includes updates in response to criticism.]
Most humans have only one hair color and one eye color.Europeans are a big exception: their hair is black but alsobrown, flaxen, golden, or red; their eyes are brown but alsoblue, gray, hazel, or green. This diversity reaches a maximum inan area centered on the East Baltic and covering northern andeastern Europe. If we move outward, to the south and east, we seea rapid return to the human norm: hair becomes uniformly blackand eyes uniformly brown.
Why this color diversity? And why only inEurope? Some believe it to be a side effect of natural selectionfor fairer skin to ensure enough vitamin D at northern latitudes.Yet skin color is weakly influenced by the different alleles forhair color or eye color apart from the ones for red hair or blueeyes. Some have no effect at all on skin pigmentation.
Others put the cause down to intermixturewith Neanderthals. Yet, according to the mtDNA that has beenretrieved, no genetic continuity is discernible between lateNeanderthals and early modern Europeans. Perhaps there was somegene flow between the two groups, but certainly not enough toaccount for the large number of Europeans with neither black hairnor brown eyes.
For others still, this color diversity arosethrough random factors: genetic drift, founder effects,relaxation of natural selection, etc. But these factors could nothave produced such a wide variety of hair and eye hues in the35,000 years that modern humans have inhabited Europe. Thehair-color gene (MC1R) has at least 7 alleles that exist only inEurope and the same is probably true for the eye-color gene(OCA2). If we take the hypothesis of a relaxation of selection,nearly a million years would be needed to accumulate this amountof diversity. Moreover, it is odd that the same sort ofdiversification has evolved at two different genes whose onlypoint in common is to color a facial feature.
Thus, some kind of non-random process seemsto have targeted hair and eye color per se, that is, asvisible characteristics. But how? And why? For some, includingthe geneticist Luigi L. Cavalli-Sforza, the answer is sexualselection. This mode of selection intensifies when malesoutnumber females among individuals ready to mate, or vice versa.The sex in excess supply has to compete for a mate and resorts tothe same strategies that advertisers use to grab attention, suchas 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 also occurs in other animals. In addition, a number of bird species exhibit color polymorphisms for which the mode of selection remains unclear. Whatever the cause, color polymorphisms are relatively uncommon. They are often hindered by two evolutionary constraints: 1) high predation pressure, this being a constraint on color traits in general, and 2) presence of related species within the same geographic range, apparently because too much intraspecific variability makes it harder to recognize one's own species and leads to hybridization.
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). A potential mate will respond not simply to a bright color but also to 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.
This sort of rare-color advantage has been reported in humans. An American researcher, Thomas Thelen, prepared three series of slidesfeaturing attractive women: one with 6 brunettes; another with 1 brunette and 5 blondes; and a third with 1 brunette and 11 blondes. Male subjects then had to select the woman in each series they would most prefer to marry. For the same brunette, preference increased significantly from the first to the third series, i.e., in proportion to the rarity of the brunettes. This rare-color preference may account for the wide range ofhuman hair and eye phenotypes we see today.
But why is hair and eye color so much more diverse in Europethan elsewhere? Perhaps because sexual selection was muchstronger among ancestral Europeans than in other humanpopulations.
Sexual selection intensifies when the "Operational SexRatio" (OSR) ceases to be balanced, i.e., when too many ofone sex are competing for too few of the other. To understand whythis may have happened in ancestral humans, we can examine thedemography of present-day hunter-gatherer bands. Such groupsusually develop an OSR imbalance for two reasons: 1) huntingdistances are longer and have increased the death rates of youngmen, typically because game animals are more mobile and/or lessnumerous per unit of land area; and 2) the cost of providing fora second wife is higher and has reduced the incidence of malepolygamy (polygyny), typically because women are procuring lessfood for themselves through food gathering. As a rule, OSRs areless balanced further away from the equator. In the TemperateZone, and even more so in the Arctic, game animals roam overlarger territories and gatherable food is less available inwinter.
The most extreme OSR imbalance occurs among hunting peoples ofthe "steppe-tundra," where almost all consumablebiomass is in the form of highly mobile and spatiallyconcentrated herbivores such as caribou, reindeer, or muskox. Onthe one hand, men die younger because they have to cover longdistances in search of herds, with no alternate food sources. Onthe other, men are less polygynous because they bear almost thefull cost of feeding their families in a habitat that offerswomen little opportunity for food gathering. With fewer menaltogether and even fewer polygynous ones, women have to competefor a limited supply of potential husbands.
Ecological zonesin Europe at the last glacial maximum, ca 18,000 BP.
Steppe-tundra is now reduced to fragments along the northernfringes of Eurasia and North America. During the last ice age,however, when modern humans first arrived, the Scandinavianicecap had pushed it farther south onto the plains of Europe. Themore intense sunlight, combined with fertile loess soils, createdan expanse of steppe-tundra with unusually high bioproductivity,even at the peak of the ice age. Less productive was the Asiansteppe-tundra east of the Urals. It was drier, farther north, andlargely polar desert, especially at the glacial maximum.Prospects were better for continuous and substantial humansettlement on the European portion of this ecological zone.
The European steppe-tundra was distinctive in another way. Ittook in an area that covers almost the same area where hair andeye color is today most diverse. Could this be an imprint left onthe human genetic landscape by sexual selection?
Perhaps. But more proof is needed. One tantalizing piece ofevidence is the possibility that these new hair and eye colorsare mildly sex-linked, as would be expected if women were morestrongly selected for such characteristics. According to anunpublished British study, non-black-haired and non-brown-eyedindividuals have longer second fingers in relation to theirfourth fingers. This indicates that the new hair and eye colorsare associated with a higher ratio of estrogen to testosteronebefore birth. Interestingly, blond hair has arisen independentlyamong some Aborigines of central Australia and is more frequentthere in women than in men.
The Aborigine example points to another avenue for research:populations outside Europe among whom new hair and eye colorsseem to have appeared independently. There are a few such cases:blond hair among central Australian Aborigines, brown hair amongthe Yukhagir of eastern Siberia, and fair hair among some Inuitbands of the western Canadian Arctic. Is hair color less diversein such populations than in Europeans because sexual selectionhas been less intense or has acted over a shorter period of time?
A final avenue for research might be to extract DNA fromskeletal remains in order to chart European MC1R and OCA2variability over the last ice age. If the sexual selectionhypothesis is true, MC1R and OCA2 variability should havedeveloped almost entirely during this time window (c. 25,000 -10,000 BP).
References:
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Brooks, R. 2002. Variation in female matechoice within guppy populations: population divergence, multipleornaments and the maintenance of polymorphism. Genetica116:343-358.
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Frost, P. 2006. European hair and eye color - Acase of frequency-dependent sexual selection? Evolutionand Human Behavior 27:85-103
Hughes, K.A., L. Du, F.H. Rodd, and D.N.Reznick. 1999. Familiarity leads to female mate preference fornovel males in the guppy, Poecilia reticulata. AnimalBehaviour 58:907-916.
Makova, K, and H. Norton. 2005. Worldwidepolymorphism at the MC1R locus and normal pigmentation variationin humans. Peptides 26:1901-1908.
Sturm, R.A., and T.N. Frudakis. 2004. Eyecolour: portals into pigmentation genes and ancestry. Trendsin Genetics 20:327-332.
Thelen, T.H. 1983. Minority type human matepreference. Social Biology, 30, 162-180.
