رنگ مو و چشمان اروپاییان:یک مورد انتخاب جنسی وابسته به فرکانس؟
|کد مقاله||سال انتشار||مقاله انگلیسی||ترجمه فارسی||تعداد کلمات|
|35750||2006||19 صفحه PDF||سفارش دهید||8960 کلمه|
Publisher : Elsevier - Science Direct (الزویر - ساینس دایرکت)
Journal : Evolution and Human Behavior, Volume 27, Issue 2, March 2006, Pages 85–103
Human hair and eye color is unusually diverse in northern and eastern Europe. The many alleles involved (at least seven for hair color) and their independent origin over a short span of evolutionary time indicate some kind of selection. Sexual selection is particularly indicated because it is known to favor color traits and color polymorphisms. In addition, hair and eye color is most diverse in what used to be, when first peopled by hunter-gatherers, a unique ecozone of low-latitude continental tundra. This type of environment skews the operational sex ratio (OSR) of hunter-gatherers toward a male shortage in two ways: (1) men have to hunt highly mobile and spatially concentrated herbivores over longer distances, with no alternate food sources in case of failure, the result being more deaths among young men; (2) women have fewer opportunities for food gathering and thus require more male provisioning, the result being less polygyny. These two factors combine to leave more women than men unmated at any one time. Such an OSR imbalance would have increased the pressures of sexual selection on early European women, one possible outcome being an unusual complex of color traits: hair- and eye-color diversity and, possibly, extreme skin depigmentation.
Human hair and eye color is unusually diverse in a geographic area centered on the East Baltic and covering northern and eastern Europe (Fig. 1 and Fig. 2). Within this area, eyes are not only brown but also blue, gray, hazel, or green, while hair is not only black but also brown, flaxen, golden, or red (Beals & Hoijer, 1965, pp. 212–214). As one moves outward from this area, color diversity declines markedly with eyes becoming uniformly brown and hair uniformly black. Full-size image (89 K) Fig. 1. Hair-color diversity in and near Europe (after Beals & Hoijer, 1965, p. 214). (Reprinted with permission from Beals et al., “An Introduction to Anthropology,” 3rd ed. Published by Allyn and Bacon, Boston, MA. Copyright © 1965 by Pearson Education.) Figure options Full-size image (89 K) Fig. 2. Eye-color diversity in and near Europe (after Beals & Hoijer, 1965, p. 213). (Reprinted with permission from Beals et al., “An Introduction to Anthropology,” 3rd ed. Published by Allyn and Bacon, Boston, MA. Copyright © 1965 by Pearson Education.) Figure options Is this diversity due to chance? In particular, could it reflect founder effects during the repeopling of glaciated Europe 15,000 to 10,000 years ago? When a founder group breaks off from its parent population, such “sampling” may indeed increase the frequency of a variant hair- or eye-color allele. It is less probable that two alleles of the same gene would become more frequent, and this probability would decline exponentially with each additional allele. Yet the hair-color gene, MC1R, has at least seven phenotypically distinct alleles that exist only in Europe (Box et al., 1997, Harding et al., 2000 and Rana et al., 1999). Furthermore, eye-color diversity results from another set of alleles at a locus that is at best weakly linked to hair color (Eiberg & Mohr, 1987). Is this diversity due to relaxation of selection and a resulting accumulation of variant alleles? Harding et al. (2000) have investigated this evolutionary scenario and found that the time to the most recent common ancestral hair color would be about a million years, with the redhead alleles alone being approximately 80,000 years old. Templeton (2002) has come to a similar conclusion: If the cause were relaxation of selection, the current level of hair-color diversity would have taken 850,000 years to develop. Yet modern humans have been in Europe for approximately 35,000 years. Is this diversity due to admixture with older European populations, notably the Neanderthals? Recently, human mtDNA has been retrieved from skeletal material on both sides of the transition from Neanderthals to modern humans: No genetic continuity is discernible between the late Neanderthals and the early modern Europeans (Caramelli et al., 2003). In addition, the mtDNA and dental traits of Neanderthals are no more similar to those of present-day Europeans than they are to those of any other modern human population (Krings et al., 1999, Ovchinnikov et al., 2000 and Tyrrell & Chamberlain, 1998). Neanderthal admixture seems to have been minor, if not negligible, and could hardly account for the high proportion of Europeans who deviate from the species norm of black hair and brown eyes. Is this diversity due, then, to some selective force, either natural or sexual selection? The first kind of selection is unlikely. As a rule, highly visible color traits are not adaptations to the natural environment, which typically favors an unobtrusive, cryptic coloration as a means to evade predators. It has been suggested that a lighter colored iris may offer more visual acuity in dim light, such as in the misty maritime environments of northwestern Europe (Short, 1975). Eye color, however, is polymorphic over a much larger area of Europe, most of which is typically continental in climate. It is also unclear why selection for visual acuity would have favored more variability in eye color as opposed to a simple reduction in eye pigment. The alternative, sexual selection, has already been advanced to explain Europe's hair- and eye-color diversity (Cavalli-Sforza et al., 1994, p. 266). This kind of selection is known to favor colorful traits, but there is little consensus on the reasons why. It may be that bright colors stimulate sexual attraction in the brain through (1) mate-assessment algorithms that interpret pigment production as a sign of health and, hence, mate quality; (2) sex-recognition algorithms that pick out sex-specific color stimuli and respond open-endedly with stronger responses to more intense colors; and (3) general monitoring algorithms that respond to highly visible stimuli and indirectly alert other systems, including those related to sexual attraction (Farr, 1980, Hamilton & Zuk, 1982, Kirkpatrick, 1987 and Manning, 1979, pp. 66–75). The opposite sex may exploit all three algorithm types by intensifying its color stimuli until functional constraints intervene or until the cost of easier detection by predators exceeds the benefit of stronger sexual attraction (Endler, 1980 and Endler, 1991). Under certain conditions, sexual selection may also diversify color traits within a single population. When an individual is faced with potential mates of equal value, it will tend to select the one that “stands out from the crowd,” that is, that has the rarest color morph. The selection is frequency-dependent, declining in strength as the rare morph becomes more common and tending toward an equilibrium that maximizes color diversity. This rare-color advantage has been studied mainly in fruit flies and guppies but has also been reported in a parasitic wasp, in red flour beetles, in ladybugs, and in leafroller moths (Anderson, 1969, Brooks, 2002, Farr, 1980, Grant et al., 1974, Hughes et al., 1999, Muggleton, 1979, Simchuk, 2001 and Sinnock, 1970). There are also a number of bird species that exhibit color polymorphisms for which the mode of selection remains unclear (Lank, 2002). 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) the presence of related species within the same geographic range, apparently because too much intraspecific variability interferes with species recognition and leads to hybridization (Endler, 1980). Many evolutionary biologists dislike the concept of rare-color advantage. There is no gain in fitness from sexual attraction to unusual colors; therefore natural selection should eliminate such nonadaptive behavior. Yet it is difficult to see how, just as it is difficult to see how we can counter the many subterfuges that advertisers use to attract our notice. There are good adaptive reasons for paying attention when an eye-catching object enters our field of view, and it is impossible to disable this response in advance for sexual attraction, given that the nature of the object (animate/inanimate, conspecific/nonconspecific, male/female) is determined at a later stage of mental processing. At that stage, the increased attention could be reversed or given a negative meaning. But there would be a cost: not only in additional processing time but also in overcorrection and undercorrection–like a spam-filter that fails to screen out all unwanted e-mails while blocking some legitimate ones. The cost may be justified if attraction to rare-color morphs leads to hybridization or if the color itself is somehow maladaptive. Otherwise, the benefit will not justify the cost. Rare-color advantage has been reported in humans. Thelen (1983) presented three series of slides showing blonde and brunette females and asked male participants to select the one from each series that they would most prefer to marry. The first series showed 6 brunettes, the second 1 brunette and 5 blondes, and the third 1 brunette and 11 blondes. For the same brunette, preference increased significantly from the first to the third series, that is, in proportion to the rarity of her hair color. The same effect was observed, albeit to a lesser extent, when the study was repeated with male photos and female participants. These findings have some support from other studies. Schweder (1994) found that women tended to change their hair color and hair form to a type that was less common in the general population. Riedl (1990) found that men tended to prefer female faces that diverge from the norm. Finally, Ellis (1928, pp. 182–183) noted less preference for blonde women in England than in France, which he ascribed to the higher prevalence of blondness among the English. Rare-color advantage may have caused hair and eye color to diversify in ancestral humans, there being neither of the evolutionary constraints mentioned above, that is, high predation pressure or likelihood of hybridization. Outside Africa, there were only two potential predators: wolves and bears, the latter being uncommon and the former only an occasional threat to recent hunter-gatherers (Hoffecker, 2002, pp. 238, 240). Hybridization was just as nonproblematic. All other Homo populations had been reduced to extinction or relic status by 30,000 BP. It is less clear, though, why hair and eye color diversified in Europe and not elsewhere. Rare-color advantage is a special case of sexual selection, and the intensity of sexual selection normally varies with the operational sex ratio (OSR; the ratio of unmated males to unmated females). The usual pattern is too many males competing for too few females (pregnancy and early infant care exclude some females from mating at any one time). But why would there have been more competition for women in northern and eastern Europe? If anything, there should have been more in sub-Saharan Africa or Papua New Guinea, where a high incidence of polygyny leaves fewer women unmated. I will argue here that the usual pattern of too many males and too few females was reversed among ancestral Europeans, specifically among the highly mobile groups that once inhabited the continental tundra of ice-age Europe. This environment exposed men to a higher risk of hunting mortality while limiting their ability to provide for more than one wife. With fewer men altogether and even fewer polygynous ones, women had to compete for a limited supply of potential husbands. There was thus sexual selection, but it acted primarily on women—not on men.