A new paper posted on bioArxiv surveys Y chromosomal and mtDNA diversity using over 600 males from the HGDP data set. Their goal is to compare differences in variation and long term demography between the two sexes. This is not an unimportant topic, sex specific demographics are relevant to mating patterns, and effective population size is of broader evolutionary interest in the way they can frame different dynamics. For instance, species with high male reproductive skew and genetic polygyny tend to be subject to more powerful sexual selection, as males have a much larger potential upper reproductive bound (a phenomenon which drives greater sexual dimorphism). This sort of evaluation with the Y and mtDNA has been attempted before, but the power of this data set is greater because they have a much larger fraction of the Y chromosome (500 kilobases) and whole mtDNA genomes (often in the past they used STRs for the Y and only the HVR in the mtDNA). These data from 600 males allowed them to construct the figure you see above, which shows the branching pattern of human migration out of Africa at given time periods, male and female effective population sizes, and later expansions (female red, male blue).
Even accounting for the possible smaller effective population sizes of uniparental lineages due to greater drift (haploid inheritance cuts down the pool of parents) I am struck again by the shockingly small number of inferred individuals who left Africa. It does not seem unreasonable to give a number closer to 100 as opposed to 1,000 for the ur-population which settled the world outside of Africa, at least when it comes to the neo-African ancestry (i.e., excludes the residual archaic admixture). But is the answer so simple? What I’m primarily concerned with is the treelike representation of human migration above. It is highly likely that less than half of the ancestry of modern Europeans dates to the original settlement date given above. So those inferred male and female effective population sizes need to be taken with a grain of salt. Much the same can be said for elsewhere. Additionally, there may have been two Out-of-Africa events in close succession (here I’m alluding to ‘basal Eurasian’ vs. the ancestors of western and eastern Eurasians proper).
Overall I can accept the broad picture painted of greater female effective population sizes, and the rise in reproductive skew in the more recent past is intriguing, and dovetails with particular conjectures of some anthropologists and economists (i.e., a rise of “winner-take-all” stratification). And the small effective population sizes at particular bottleneck events is broadly creditable looking at genome-wide patterns (e.g., high sequence level diversity in Africa vs. non-Africa). But these surveys need more textured detail in this data and age. I hope later revisions will flesh that out, as well as acknowledge the findings of Sayres et al.
Citation: Human paternal and maternal demographic histories: insights from high-resolution Y chromosome and mtDNA sequences, Sebastian Lippold, Hongyang Xu, Albert Ko, Mingkun Li, Gabriel Renaud, Anne Butthof, Roland Schroeder, Mark Stoneking, bioRxivdoi: 10.1101/001792
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