The geography of genes tells us only so much about history

Gene ExpressionBy Razib KhanAug 25, 2011 8:29 AM


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L. L. Cavalli-Sforza'sThe History and Geography of Human Genes is a book I reference a great deal. Cavalli-Sforza is the godfather of the field of historical population genetics, the phylogeography of humankind. Though his work was on classical autosomal markers, the huge literature which drew inferences from Y chromosomal and mtDNA variation followed in the wake of the The History and Geography of Human Genes. Spencer Wells, the director of the Genographic Project, alluded to Cavalli-Sforza's influence in The Journey of Man. But at this point I think we have to be very careful of making inferences about the past from present patterns of genetic variation. This is made most stark by the fact that ancient DNA, which is a snapshot of the past, as opposed to an inference of it, sometimes diverges from our expectations based on present patterns of variation in surprising ways. This to me is the big lesson to draw from a new paper in The Proceedings of the Royal Society B, The peopling of Europe and the cautionary tale of Y chromosome lineage R-M269. The results focus on two issues. First, the distribution of Y chromosomal lineages in Europe, in particular R-M269. And second, the time to the last common ancestor of branches of the overall lineage. Patterns of distribution and variation of a lineage are informative, insofar as regions with higher variation are presumed to be the core zone from which the lineage expanded. This is the logic which underpins the conclusion that Africa is the locus of modern humanity; Africa has more genetic diversity than other continents. The time until the last common ancestor between two given lineages is contingent upon a "molecular clock" model. From what I have heard and read this is a very dicey proposition for Y chromosomal variation, and this paper confirms the erratic nature of these estimates. Since the paper is free, I suggest you go read it. The major takeaway seems to be that the representativeness of a sample matters a lot, and never trust estimates of coalescence between two lineages. The statistical associations between geography and R-M269 diversity found by earlier researchers disappeared when the database was expanded and the markers typed more thoroughly. Maju and Dienekes have a lot to say on this paper in the broader context. I'm not too interested in arguing in detail about the results and what they mean, I am of the opinion that ancient DNA is going to be the ultimate arbiter. But, I do believe that a lot of our models are way too simple, which is one of the reasons why inferences are so often faulty. I wouldn't be surprised for example if R-M269 is a male lineage which expanded rapidly very recently in Western Europe after the Neolithic, but that its point of origin has now come to be dominated by other lineages, obscuring the patterns of the past.

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