Wednesday, October 1, 2008
The latest PLoS Genetics hit is Sex-Biased Evolutionary Forces Shape Genomic Patterns of Human Diversity, by M.F. Hammer et al.
We already knew, or at least believed, that the effective population of males has generally been lower than that of females. It seems obvious considering that in principle any fertile man has the potential to engender thousands of children, provided he finds the sufficient number of mothers, while women can only have a limited number of children each (I think the highest single figure I have ever heard of is something like thirty - and one shivers at the idea of giving birth so many times, plus breastfeeding and rearing all them). Much lower figures are more normal of course but the difference of reproductive potential between sexes remains and can be very high.
But we were not certain on how that worked in reality through the generations. We typically find more mtDNA than Y-DNA diversity but these items may not be fully comparable anyhow. This paper adresses this issue by comparing the diversity at the X chromosome and at autosomal DNA. If the effective population (the people effectively reproducing) would be the same for both genders, we should find that the X-DNA diversity should be 75% of the autosomal one. What Hammer and colleagues have found is that it is not the case that X-DNA diversity is generally higher than under neutral expectations and in some cases even higher than autosomal diversity itself.
That basically means that some men have been more succesful than others and that the average woman as well. Expected indeed, as the difference in biological potential is certainly there.
But the confusion comes with the use of the term polygyny (one man married or otherwise reproducing, like in concubinage, with several women). While the conclusions are more imprecise, the term arises in the abstract: our results point to a systematic difference between the sexes in the variance in reproductive success; namely, the widespread effects of polygyny in human populations. The term is adressed at times confusingly, as normally this means formal poygyny and not mere infidelity. Both Dienekes and Kambiz emphasize the term polygyny suggesting more or less explictly that it has been a formal feature of human societies. True that part of the fault may lie on Hammer's shoulders, quoted at New Scientist saying: I don't know how long monogamy has been with us. It seems it hasn't been around long, evolutionarily.
This is far from clear, I think. Even without formal polygamy, there has always been opportunities for some men to be more succesful than average in this aspect.
The most important and revealing graph in that paper is figure 2, where the six sampled populations are shown with their respective ratios of X-DNA diversity in contrast with a hypothetical neutral 0.75 line. With the possible exception of the San (Bushmen), all clearly exceed the neutral model. When we take the point extimate (ignoring the possible error) they appear like this:
- San (Bushmen): 0.85 (13% over neutral expectation)
- Biaka Pygmies: 0.90 (20% excess)
- Mandenka (West Africans): 0.93 (24% excess)
- Han Chinese: 0.94 (25% excess)
- Melanesians: 1.04 (39% excess)
- Northern Basques: 1.05 (40% excess)
The first interesting thing is that the hunter-gatherer peoples are the ones showing lower deviation from the neutral expectation, i.e. the ones where polygyny has been less important overall. The difference between the San and the Biaka is significative but I wonder if it can reflect Bantu influences among the latter.
Excluding hunter-gatherers, there are two (geografically meaningless) groups: Han and Mandenka appear moderately polygynistic, while Melanesians and Basques appear rather extreme. It would be interesting to contrast these peoples with their culturally akin neighbours, for instance compare Chinese with Japanese, Mongols or Thais... or Basques with other Europeans. But we will have to wait for that, I guess.
Anyhow, it is the "highly polygynistic" group the most intriguing set. Melanesians (at least Papuans, I could not find which ethnicity they are specifically) are known for actually practicing polygyny with normality: they are a highly patriarchal Neolithic society. But Basques are a totally different case: there is no record of such institution ever existing at all and the monogamic (patriarchal maybe but softly so in the European context) household is a national institution. So I have been chewing on the issue for several days now because it really stroke me as very odd.
I cannot know how was Basque society in Prehistory (and that, with the exception of some brief accounts, lasts until the late Middle Ages) but what we know does not point in that institutional polygynistic direction at all. Wilhelm von Humboldt, who visited the country in the early 19th century, reported instead that Basques had then frequent pre-marital relations and that marriage was normally formalized when the girl got pregnant (possibly but not necesarily from the groom, he implied). From Inquisitorial accounts we also know that Basques from both sides of the mountains used to engage on pagan orgiastic celebrations (community sex and drug parties probably with religious meaning) as late as the 16th century. We also know that the status of women was rather high and that they enjoyed nearly as much freedom as men (and were specially important as leaders of the old religion: "witches"). Earlier reports, even if way too limited, do not contradict this picture in any case.
Informal polygamy (both sexes) was frequent among Basques in the past and was socially accepted, in other words. But patrilineal monogamic family was still the basic cell and that is very clear in the central importance of the undivisible household in Basque law.
So I guess that such a high X-DNA excess diversity among Northern Basques, rather than formal polygyny (unattested, quite unlikely to have ever existed in my opinion) what reflects is informal polygamy or promiscuity and the implicit ability of women to choose the father of their children to some extent in this context, moreso if some anticonceptives were known, what is probable considering that witches (sorginak for their Basque name) were primarily herborists.
That way the more desirable men would get some advantage... but not because they wielded particular power over women, as in typical polygyny, but because women would tend to choose them with preference over the rest as the biological fathers, that, as it is well known, is not necesarily the same as the legal father.
The child born inside marriage is son of the husband, Napoleon dixit. Biology shrugs, cheats and laughs last.
Three months later... Natura publishes something behind a paywall (authored by Keinan et al.) that finds exactly the opposite results: that in Africa the X-DNA to A-DNA ratio is c. 0.75, while in Eurasia is c. 0.65, much lower. They scratch their heads wondering what may have caused a loss of female efefctive population, what makes little sense (hey, why no massive polyandry?, it's as logical - or not - as massive polygyny in the previous one).
I don't think I can handle such a variety of contradictory results and make any sense. But, as Manjunat said, the post needed an update.