Friday, June 16, 2006

Hearing, blacks & whites   posted by Razib @ 6/16/2006 11:54:00 PM

Shelley Batts has some interesting commnets about the new finding which suggests that blacks and women have better hearing than whites and men, respectively. Shelley goes on to support the idea that melanin might have a protective effect on hair cells because of physiological factors by drawing from her own research. It seems that there is a way to test this: find the correlation between index of reflectance (skin color) and hearing loss. Do comparisons on sibships, especially in populations with lots of skin color variation. If this phenomenon is due to the effect of the phenotype, that is, the end product of melanin, as opposed to pleiotropy on one of the skin color genes then the covariation should work across populations. On the other hand, if this is pleiotropy of some sort coupled with one of the genes which controls skin color, then you would see variation across populations for the correlation because different populations of the same skin reflectance have different genetic profiles (i.e., East Asians and Europeans are pale for different genetic reasons, or, more precisely, a different combinations of alleles on the same loci).

Additionally, if memory serves it seems that humans have had very dark skin for something on the order of 200,000 years if the "fur loss" research pans out. That is, unexposed skin color in most primates is depigmented, and the implication is that dark skin emerged after we lost our hair and were exposed to the full force of solar radiation. Simulatenously, if the FOXP2 research program is confirmed we might have gained speech around the same time frame, or, at least fully articulate and recursive speech. This facility would require greater auditory capacities, so the two concomitant transitions might have fed back into each other.

Addendum: Background for those of you who are a bit perplexed by my allusions re: skin color. The short of it is that very dark skin is generated by a constrained "consensus sequence." Black skinned the world over, from Africa to southern India to Melanesia tend to exhibit this consensus sequence (for example, a particular form the MC1R gene). In contrast, light skin has no consensus sequence, but is characterized by variation. In Europeans MC1R is very polymorphic (i.e., Peter Frost's sexual selection/frequency dependent hypothesis), while in East Asians it seems to be subject to positive selection which is pushing it toward fixation on an alternative consensus sequence. The locus which controls around 1/3 of the variation between Europeans and Africans in regards to skin color is shared between Africans and East Asians.