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A higher N for Tibetans may be explained by a more stable population where diverse subsets and across individuals the reproductive value may be more equitable.
In other words, an effective population size is a statistic which is bundling together a lot of evolutionary history, and is not a simple measure of perceived census sizes (the Tibetans may also be something of a melange of a diverse set of ancient groups which took refuge in the highlands, while the Han are the descendants of early adopters of agriculture which expanded demographically; so they’re opposite ends of the demographic tunnel).
The researchers didn’t go looking for EPAS1 is also known as hypoxia-inducible factor 2 (HIF-2).
The HIF family of transcription factors consist of two subunits, with three alternate subunits (HIF-1, HIF-2/EPAS1, HIF-3) that dimerize with a β subunit encoded by ARNT or ARNT2.
The strongest signal of natural selection came from endothelial Per-Arnt-Sim (PAS) domain protein 1 (EPAS1), a transcription factor involved in response to hypoxia.
One single-nucleotide polymorphism (SNP) at EPAS1 shows a 78% frequency difference between Tibetan and Han samples, representing the fastest allele frequency change observed at any human gene to date.
It’s an enormous outlier, with SNPs where Tibetans and Han differ a great deal.
This SNP’s association with erythrocyte abundance supports the role of EPAS1 in adaptation to hypoxia.
Thus, a population genomic survey has revealed a functionally important locus in genetic adaptation to high altitude.
The supplements lays out the details a bit more than the press reports, so below is figure 2: It looks like to get a better sense of the model you’ll have to read the cited paper, and I’m not sure that that will satisfy the archaeologists.
They did use a large number of neutral markers though, so I’m not too worried about biases in their data set.