Polygyny and X-chromosome diversity

Last year, a team of University of Arizona researchers found evidence of widespread polygyny in five different human populations: Biaka (Central African Republic), Mandenka (Senegal), San (Namibia), Basques (France), Han (China), and Melanesians (Papua New Guinea). In short, the maternally inherited X chromosome was genetically more diverse than the chromosomes inherited by both sexes (autosomes) (Hammer et al., 2008). So more women than men seem to have contributed to the gene pool. Surprisingly, there was little difference in this respect between the Mandenka (known to be highly polygynous) and the Basques and the Han (among whom the incidence of polygyny is much lower).

I was frankly skeptical. For one thing, maternally inherited genetic diversity reflects not only the number of women who contribute to the gene pool but also their own genetic diversity. If these women are drawn from a larger geographic area than the men are, the female gene pool will be more genetically diverse than the male gene pool. This is often the case. In a patriarchal society, land ownership is vested in the man’s lineage, so women are usually the ones who move to their mate’s community when they get married. We see this ‘patrilocality’ even in societies where land ownership is matrilineal. Among the Iroquois, wives were often abducted from other tribes through warfare.

In any case, the above findings have now been challenged. Another study has found much less maternally inherited genetic diversity in East Asians and Europeans than in West Africans (Keinan et al., 2008).

So what gives? The methodology is similar in both studies. John Hawks points out that the second study scales X-chromosome diversity to the human-macaque divergence whereas the first study uses the human-orangutan divergence. While this might explain differences in calculation of mutation rate and hence X-chromosome diversity, I don’t see how it could explain why one study found geographic differences (i.e., African versus non-African) and the other did not.

I suspect that the key difference is that the first study just did not have enough resolution to pick out these geographic differences, i.e., its dataset was too small. The second study used 130,000 loci (SNPs) whereas the first one used 40.


Please check out my latest article: “Sexual selection and human geographic variation” in The Journal of Social, Evolutionary & Cultural Psychology.

References

Frost, P. (2008). Sexual selection and human geographic variation, Special Issue: Proceedings of the 2nd Annual Meeting of the NorthEastern Evolutionary Psychology Society. Journal of Social, Evolutionary, and Cultural Psychology, 2(4), pp. 169-191.
http://www.jsecjournal.com/articles/volume2/issue4/NEEPSfrost.pdf

Hammer, M.F., Mendez, F.L., Cox, M.P., Woerner, A.E., & Wall, J.D. (2008). Sex-biased evolutionary forces shape genomic patterns of human diversity. PLoS Genet, 4(9), e1000202. doi:10.1371/journal.pgen.1000202

Keinan, A., Mullikin, J.C., Patterson, N., & Reich, D. (2008). Accelerated genetic drift on chromosome X during the human dispersal out of Africa. Nature Genetics, early view December 2008; doi:10.1038/ng.303