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An Immunological Fitness Hypothesis to Explain the Selection of Human Color Polymorphisms in Europe

Page history last edited by Simon78 9 years, 10 months ago

 

Comments on Peter Frost's 2006 paper on The Evolution of Blond Hair and Blue Eyes among Nordics

 

In his 2006 paper in Evolution and Human Behavior (revised version), Peter Frost suggests that blond hair color, light skin and blue eyes arose in Northern Europe roughly because the food distribution forced men to hunt longer distances away from camp, leading to higher male mortality, leading to some women getting left without mating, leading to higher sexual selection for attractive females (i.e. more consequences for the least attractive females), leading to selection for diversity/polymorphisms in hair color, eye color and skin pigmentation.

 

Frost justifies the last step in the causal chain largely by pointing to a study by Thomas Thelen. Thelen prepared three series of slides featuring 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. An important control is missing in Thelen's experiment (or at least Frost's description of it): is Thelen's result due to increased attractiveness of the rare hair color, or due to increased saliency alone? This can be discovered by asking the subjects one more question: which of the women do you find least attractive? That said, my experience with people's taste in sexual matters suggests to me that Thelen's result is probably correct, and that indeed, the grass on the other side always looks greener.

 

Frost points to this preference for the rare as a byproduct of attention to salient objects, with no evolutionary advantage, and some disadvantages (increased predation due to the rare standing out to prey, and "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").

 

If Frost is on to the right reasons, there may be more to men's predilection for rare polymorphisms than he suggests: visible deviations from the average phenotype probably correlate positively with genetic distance from the average genotype, and thus negatively with susceptibility to infectious diseases in the population. Thus, a rare phenotype is likely a predictor of fitness in an age when infectious diseases were counted among the most important killers (see also my draft with Ulrik Beierholm, The Origin of Diversity). Thus, our preference for rare appearances may well be adaptive --although it is interesting to contrast this with the attractiveness of average faces reported in the literature, which may be due to the higher symmetry of average faces (symmetry is known to be a predictor of fitness).

 

What is less clear is why this happened to a great extent only in Northern and Eastern Europe --with the exception of blonde Australian aboriginals (see photo below). If rare alleles convey a fitness advantage, they should prevail more widely than just where sexual selection is heightened due to male/female ratios different from 1. That said, the short timespan in question may be insufficient to have allowed the mutations to arise in other human populations --which begs the question, why did the hair and eye color polymorphisms not develop earlier? Here, Frost proposes an answer: rare alleles expose bearers to increased predation by standing out, so until humans went from hunted to hunters, this force could have kept polymorphisms in check. Likewise, while other humanoid species coexisted with us, variance from the norm could have caused confusion with other species, and thus negative sexual selection.

An Australian aborigine with blond hair in Kakadu, Northern Territory. Photograph copyright © 2006 Alex Backer.

Frost's paper lacks evidence for his claim of male to female ratios in Northeastern Europe that are disproportionately higher than those of other ethnias that did not develop the same color polymorphisms --indeed, his claim that humans have been less hunted in that area indeed runs counter to his claims of higher male mortality. The most convincing evidence he presents ("Among 19th century Labrador Inuit, only 57 males remained for every 100 females in the 15+ age bracket because of hunting deaths from drowning or exposure (Scheffel, 1984). Among Inuit, in general, “the preponderance of adult women is generally explained by the higher death rate among men due to the natural hazards of hunting” (Weyer, 1932, pp. 135–136)") describes not Europeans, but Inuit, who have not developed blond hair or blue eyes.

Frost looks for sexual dimorphisms that might explain why why sexual selection happened when men, and not women, were scarce. The question here might be misguided, as sexual selection will happen in either case, but it is not clear that there are many societies in which there is an abudance of men and a scarcity of women --an interesting mystery, since it would seem that, given females' limitation in terms of the number of offspring they can have, a species with a M/F ratio significantly lower than one would exhibit faster growth (see Why the Sex Ratio is 1).

Frost's 'selection for diversity' hypothesis also fails to shed light on why skin became lighter rather than more diverse --Northern European skin color does not seem any more diverse than that of Africans. The traditional explanation related to sunlight levels close to the Arctic and the competing effects of skin pigmentation on vitamin production and skin cancer would seem to fare better here.

Finally, did the increase sexual selection in Northern and Eastern Europe contribute to selection in that most prominent product of human sexual selection, breast size? Alas, I could not find a study systematically comparing breast size across different races. Perhaps a graduate student can perform the sacrifice in the interest of science and add a thesis to the likes of The effect of breast size on attractiveness, a thesis presented to the faculty of the Tennessee Technological University in 1995. Hey, it's a hard job, but someone's got to do it.

Up to nail fungus natural cure Science.

 

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