Advertisement
Research Article

Sex-Biased Evolutionary Forces Shape Genomic Patterns of Human Diversity

  • Michael F. Hammer mail,

    mfh@u.arizona.edu

    Affiliations: ARL Division of Biotechnology, University of Arizona, Tucson, Arizona, United States of America, Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona, United States of America

    X
  • Fernando L. Mendez,

    Affiliation: Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona, United States of America

    X
  • Murray P. Cox,

    Affiliation: ARL Division of Biotechnology, University of Arizona, Tucson, Arizona, United States of America

    X
  • August E. Woerner,

    Affiliation: ARL Division of Biotechnology, University of Arizona, Tucson, Arizona, United States of America

    X
  • Jeffrey D. Wall

    Affiliation: Institute for Human Genetics, University of California San Francisco, San Francisco, California, United States of America

    X
  • Published: September 26, 2008
  • DOI: 10.1371/journal.pgen.1000202

Reader Comments (2)

Post a new comment on this article

Polygyny or patrilocality?

Posted by PeterFrost on 10 Oct 2008 at 17:47 GMT

Hammer et al. correctly conclude that genetic diversity is higher on the maternally inherited X chromosome than on chromosomes inherited by both sexes (autosomes), at least for the populations they sampled. It does not follow, however, that this higher maternal diversity is due to polygyny, i.e., proportionately more women than men contributing to the gene pool. It could just as easily be explained by patrilocality. In many societies, a wife goes to live in her husband’s community after marriage. This has the effect of inflating the genetic diversity of women in any one community.

These two confounding levels of explanation, polygyny and patrilocality, were a problem with the previous methodology of comparing maternally inherited mtDNA with the paternally inherited Y chromosome. If we use Hammer et al’s methodology, patrilocality exerts an even stronger bias on the results because the Y chromosome is no longer used as a point of reference.

Hammer et al. do not discuss patrilocality in their paper, although they do discuss “sex-biased forces”. Under this heading, they propose a model where only females migrate between communities (‘demes’) and at such a rate that panmixia eventually results. The authors conclude that this factor could not be significant. In my opinion, the model is unrealistic, partly because the assumed migration rate is far too high and partly because two demes are used to represent a real world where brides are exchanged among many communities separated by varying genetic distances.

If Hammer et al. have adequately measured the impact of polygyny on the human gene pool, we must conclude that the incidence of polygyny has been relatively similar among the Mandenka (West Africa), French Basques, Han Chinese, and Melanesians. Do the authors believe this finding? If they do, they are going against almost all of the comparative literature on cross-cultural differences in mating systems. To cite only one authority, Pebley and Mbugua (1989) note:

“In non-African societies in which polygyny is, or was, socially permissible, only a relatively small fraction of the population is in polygynous marriages. Chamie's (1986) analysis of data for Arab Muslim countries between the 1950s and 1980s shows that only 5 to 12 percent of men in these countries have more than one wife. … Smith and Kunz (1976) report that less than 10 percent of nineteenth-century American Mormon husbands were polygynists. By contrast, throughout most of southern West Africa and western Central Africa, as many as 20 to 50 percent of married men have more than one wife … The frequency is somewhat lower in East and South Africa, although 15 to 30 percent of husbands are reported to be polygynists in Kenya and Tanzania.”

Reference:

Pebley, A. R., & Mbugua, W. (1989). Polygyny and Fertility in Sub-Saharan Africa. In R. J. Lesthaeghe (ed.), Reproduction and Social Organization in Sub-Saharan Africa, Berkeley: University of California Press, pp. 338-364.