Tuesday, October 02, 2007

Inferences about patrilocality from genetics   posted by Razib @ 10/02/2007 11:03:00 PM

When genetics is used as a supplement to history or anthropology it should ideally offer more precision and reduce the field of possibilities. Unfortunately that is not always the case. For example, when asking big questions such as "Are humans 'naturally' patrilocal?" genetics has generally come up with conflicting results. The fact is that about 70% of "traditional" societies are patrilocal; wives move to live with husbands and their family. Some genetic evidence seems to support this (e.g., greater mtDNA [female] variation than Y [male] lineage variation), but there is also variation within populations and some skepticism as to whether such simple comparisons between different uniparental loci really can give a definitive answer.

This paper in PLOS ONE tackles the question using comparisons with chimpazees to help calibrate our expectations, The Genetic Signature of Sex-Biased Migration in Patrilocal Chimpanzees and Humans:
...Here we review some methodological reasons for these inconsistencies, and take them into account to provide an unbiased characterization of mtDNA and NRY variation in chimpanzees, one of the few mammalian taxa where males routinely remain in and females typically disperse from their natal groups. We show that patterns of mtDNA and NRY variation are more strongly contrasting in patrilocal chimpanzees compared with patrilocal human societies. The chimpanzee data we present here thus provide a valuable comparative benchmark of the patterns of mtDNA and NRY variation to be expected in a society with extremely female-biased dispersal.

Homo Sapiens, are a complex species. We're capable of a lot and our level ofsocial complexity isn't truly cognitively tractable for the mental toolkit that evolution has given us. So we make recourse to generalizations and first-order approximations. The problem with this is that sometimes the deviation away from the central tendency is as interesting and evolutionarily salient as the mode on the frequency distribution of the trait. When we say that 70% of human societies are patrilocal, that means that 30% are not. That also doesn't mean that 70% of human societies have always been patrilocal. It also doesn't quantify how much deviation there is from social expectation, e.g., perahps in the 70% of societies which are patrilocal a substantial minority of males move to the locality of the female, or, males and females from the same locality marry each other so that the whole dichotomy is rendered irrelevant (though even if you live within a village whether you reside in the relatives of the female or relatives of the male in an extended family circumstance is also an important point). Generalizing about the modal behavior of our species may leave the concept more analytically tractable, but it may also render our model inaccurate toward the point of uselessness.

On the other hand, chimpanzees aren't that complex...at least compared to humans. Yes, I know that chimps are the geniuses of the animal kingdom, but the chimp is the Peter Keating of the animal world and we are the Howard Roarks. Unlike humans with their facultative complexity chimpanzee societies are pretty uniform. While there are exceptions to the rule (as in the famous Gombe troop), female chimps tend to leave their natal group while males tend to remain in their natal group. In shot, chimps seem to be much more obligate in their patrilocality than humans. The workers above note that both chimp lineages, the bonobo and common chimp, are patrilocal, so this is likely an ancestral characteristic of the clade dating back at least 1 million years. It seems that chimpanzees exhibit a much more unambiguous genetic signature of patrilocality than humans, the ratio of Y to mtDNA variance is significantly lower than in humans.

There are some technical reasons why the human results could exhibit problems. For example, mtDNA might simply be more diverse than Y lineages for endogenous reasons (higher mutational rates?). Or there might be lower effective population size of one sex, which would skew the variation of the uniparental lineages independent of deme-to-deme gene flow. There are also issues of coarseness of analysis, the authors point out that in some human communities Fst analyses were preformed by spanning tribal groups where intermarriage was rather uncommon, while ignoring the pervasiveness of between deme gene flow on the intra-tribe level. Finally, there is the issue that human societies change. The Japanese were once matrilocal, but now the are patrilocal. Within historic times many matrilineal societies have shifted toward patrilocal practices (though sometimes there are ghosts of matrilineal practice, in ancient Egypt marriage to a woman of the royal line was often essential to solidify the claims of the male claimant). There variations over time can obscure or erase genetic patterns and replace them with new ones, and periodic oscillations would presumably result in a meta-stable level of diversity which balances out both mtDNA and Y.

Chimps can get around some of these issues. Chimpanzee societies are more homogeneous in their behavior patterns, have been studied for nearly two centuries, and have been characterized to a very fine level of demic structure in terms of their social dynamics (even to the point of familial histories). In short, chimpanzees are empirically tractable because of their small numbers and limited set of behaviors. The similarity across the two clades and their relative homogeneity should reassure one that temporal variation is minimized. Because of their charismatic nature chimpanzees have also been tracked in a way which makes assessments of male vs. female migration patterns on the level of specific demes tenable (i.e., you couldn't get funding to do such detailed research on most species, so people have to engage in a lot more guesswork in terms of how they behave when no one is looking, which can explain why ethologists were long fooled by "monogamous" birds before DNA fingerprinting cleared up some issues).

It seems only the most extremely patrilocal and polygynous human societies in the sample approached the chimp norm. To me this suggests that we'll have to be a bit more careful and qualified when talking about human patrilocality. Though we're not totally malleable and subject to great constraints, our behavioral flexibility is orders of magnitude more developed than the faculties of our chimp cousins. It shouldn't surprise if we tend to explore a far greater sample space of social systems, not only because we can, but also because we inhabit so many environmental and cultural ecosystems. Polyandry which consists of brothers marrying one woman in Tibet did not arise because it was natural; rather, it seems to be the best opportunity for reproduction that a low status male could attain in that society where resources were at a premium. It was a functional response to a specific set of circumstances. This work focuses on the dimension of male and female migration across adjacent demes, classical models of gene flow. And this likely works well for chimpanzees, and perhaps hunter-gatherers. But it seems excessively oversimplified when it comes to the mass societies which arose after the rise of agriculture. 4,000 years ago a man who was born in Switzerland was buried at Stonehenge. In their day the Mongol hordes swept from the Pacific to the plains of Pannonia, from Baikal to Baghdad. This was certainly a scattering of Y lineages of immense scale. These sorts of movements don't really work well with chimpanzee analogs...because, well, chimps have never produced transcontinental hordes! Atop the simmering activity of deme-to-deme wife and husband exchanges there will periodically flash a fire of migratory activity. Sometimes this will be a total folk movement, but I suspect that more often what you would see was a migration of males. There is much more variation in human social networks than among chimpanzees, so we should focus less on the mean outcome as opposed to the structure and pattern of the variation and its distribution.

Note: You can discuss this paper over at PLOS ONE if you so choose!

Langergraber KE, Siedel H, Mitani JC, Wrangham RW, Reynolds V, et al. (2007) The Genetic Signature of Sex-Biased Migration in Patrilocal Chimpanzees and Humans. PLoS ONE 2(10): e973. doi:10.1371/journal.pone.0000973

Related: Sperm competition.