As discussed in my last post, one puzzle of human evolution is the diverse palette of European hair and eye colors. Although these two polymorphisms have largely developed at separate genes, they share a similar geographic range and similar conspicuous hues. They also appear on or near the face—the focus of human visual attention. Could a common selection pressure be responsible? And could it be sexual selection?
This topic came up a month ago on Steve Sailer’s blog, specifically the evolution of blue eyes. Greg Cochran pointed out that sexual selection could not be responsible because blue eyes are recessive:
First, an advantageous allele whose action is purely recessive is far more likely to be lost when new than a dominant allele with an equivalent advantage. Second, assuming that it is not lost and that the population mates randomly, it takes much longer to reach 50% frequency than a dominant allele. Third, if the population is spread out over space, the Fisher wave spreads far more slowly, something like 20 times more slowly.
Mutations are fairly common, but a potentially adaptive one—like an allele for blue eyes—is usually rare. In this case, the same rare allele must occur twice and come together in the same person before sexual selection can do its work. And this work would be lost in the next generation.
All of this assumes, of course, that blue eyes are recessive. Although eye color is polygenic, alleles at two STPs (rs12913832 and rs1129038) seem to account for most cases of blue eyes (Eiberg et al., 2008). In a Polish sample, 89% of the blue-eyed individuals had both copies of the ‘C’ allele at rs12913832 and no copies of the alternate ‘T’ allele (Branicki et al., 2009).
But the C allele is far from silent if only one copy is present, as seen in the same Polish sample. Among CT heterozygotes, 16% had blue or grey eyes, 10% green eyes, 47% hazel eyes, and only 27% brown eyes.
Although the C allele is relatively recessive for expression of blue eyes, it shows strong heterozygote effects for expression of green or hazel eyes:
Blue or grey-eyed individuals: 89% had both copies, 10% one copy, 9% no copies
Green-eyed individuals: 67% had both copies, 30% one copy, 2% no copies
Hazel-eyed individuals: 9% had both copies, 80% one copy, 11% no copies
Brown-eyed individuals: 0% had both copies, 84% one copy, 16% no copies
In short, the C allele is less dominant, but not truly recessive. Even in the heterozygous state, it usually produces hues that visibly diverge from the human norm of brown eyes.
Greg also forgets that evolution can reach an initially inaccessible state by passing through intermediate states. When the C allele first appeared, it produced only green or hazel eyes for sexual selection to act upon. As copies of this allele increased in the population, there was a corresponding increase in the probability of homozygotes that could produce blue eyes—which became a new target for sexual selection.
The selection here is not for a single color, be it blue, green, hazel, or brown, but rather for any colors that can catch attention by their novelty or brightness. The end result is more and more eye colors—a balanced polymorphism where sexual selection is always on the lookout for new and interesting hues. Needless to say, this outcome is possible only when the operational sex ratio is very lopsided, thus favoring the evolution of ‘eye candy’ among members of the sex in excess supply.
Branicki, W., U. Brudnik, and A. Wojas-Pelc. (2009). Interactions between HERC2, OCA2 and MC1R may influence human pigmentation phenotype, Annals of Human Genetics, 73,160–170.
Eiberg, H., J. Troelsen, M. Nielsen, A. Mikkelsen, J. Mengel-From, K.W. Kjaer, & L. Hansen. (2008). Blue eye color in humans may be caused by a perfectly associated founder mutation in a regulatory element located within the HERC2 gene inhibiting OCA2 expression, Human Genetics, 123, 177–187
Sailer, S. (2011). Old Blue Eyes, May 10