Life can be really funny. When I was in college I was incorrigibly curious and I asked a Korean American friend if his ear wax was dry (I’d read that East Asians had dry ear wax once) and his response was, “Isn’t everybody’s?” When it comes to interpersonal differences there are many things we take for granted and extrapolate to others that aren’t necessarily true.1 Nick Wade in The New York Times has an interesting write up about the genetics of the ear wax phenotype. While the populations of Europe and Africa have wet ear wax, those of East Asia have dry ear wax. Other populations are somewhere in between. Nature Genetics has the original paper (don’t be surprised if the link takes a while to load).
Here are the worldwide distribution of the alleles:
The trend is pretty clear, modal in Northeast Asia and decreasing everywhere else. These figures show why the authors assume that selection is at play here, linkage disequilibrium implies that recombination has not had time to break apart the correlations as neighboring portions of the genome were dragged along with the increase of frequency on the single nucleotide of interest. Wade finds a quote from a scientist who says this could be the result of random genetic drift. I don’t buy it, because drift is a default explanation offered by many when they don’t have any other plausible model on hand (runaway sexual selection is another explanation of this sort). The LDE noted above suggests that positive selection was occurring at that locus. One can posit models that generate these distributions via drift, perhaps fixation in a small population which has an enormous demographic expansion into the surrounding populations. This would have had to have happened before 8-10,000 years BP because the New World populations in North America possess the allele. It just seems more parsimonious, along with the LDE data, that selection is at work (rather than reinvent the wheel, here is RPM’s “Detecting Natural Selection” series, I, II, III, IV, V, VI, VII).
The authors posit many hypotheses for why selection for dry ear wax, or, more properly for the allele, A, which in the homozygotes generates dry ear wax, should have increased its frequency in Northeast Asia. To them it seems likely that the ear wax phenotype is a byproduct of pleiotropy, that is, another trait was generating the fitness differentials correlated with this allele, and the fact that it had other phenotypic effects was incidental. I found this “fact” amusing:
They write that earwax type and armpit odor are correlated, since populations with dry earwax, such as those of East Asia, tend to sweat less and have little or no body odor, whereas the wet earwax populations of Africa and Europe sweat more and so may have greater body odor.
The adults I have known who do not need to make recourse to deodorants to ward off body odor have all been East Asian females. No surprise.
Addendum: One point made Wade’s article is that South and Central Asians who exhibit intermediate frequencies of the alleles might have gotten them via admixture. In the case of Central Asians this is plausible, genetically and historically they tend to bridge East and West. I am skeptical in the case of South Asians, it seems more plausible to me that the same selection pressures which benefited the allele in East Asians might have some advantage in Southern Asia, as Indians are not a recently admixed population (at least that the balance of the current research).
Via John Hawks.
Reference: Yoshiura et al. 2006, An SNP in the ABCC11 gene is the determinant of human earwax type, Nature Genetics.
1 – Another example seemed to be that many of the females in my dorm assumed that men had an urge to urinate when they laughed hard, while most of the males were pretty confused about this.