The above map is from a new preprint on the patterns of genetic variation as a function of geography for humans, Genetic landscapes reveal how human genetic diversity aligns with geography. The authors assemble an incredibly large dataset to generate these figures. The orange zones are “troughs” of gene flow. Basically barriers to gene flow. It is no great surprise that so many of the barriers correlate with rivers, mountains, and deserts. But the aim of this sort of work seems to be to make precise and quantitative intuitions which are normally expressed verbally.
To me, it is curious how the borders of the Peoples’ Republic of China is evident on this map (an artifact of sampling?). Additionally, one can see Weber’s line in Indonesia. There are the usual important caveats of sampling, and caution about interpreting present variation and dynamics back to the past. But I believe that these sorts of models and visualizations are important nulls against which we can judge perturbations.
As I said, these methods can confirm rigorously what is already clear intuitively. For example:
Several large-scale corridors are inferred that represent long-range genetic similarity, for example: India is connected by two corridors to Europe (a southern one through Anatolia and Persia ‘SC’, and
a northern one through the Eurasian Steppe ‘NC’)
We still don’t have enough ancient DNA to be totally sure, but it’s hard to ignore the likelihood that “Ancestral North Indians” (AN) actually represent two different migrations.
India also illustrates contingency of these barriers. Before the ANI migration, driven by the rise in agricultural lifestyles, there would likely have been a major trough of gene flow on India’s western border. In fact a deeper one than the one on the eastern border. And if the high genetic structure statistics from ancient DNA are further confirmed then the rate of gene flow was possibly much lower between demes in the past. Perhaps that would simply re-standardize equally so that the map itself would not be changed, but I suspect that we’d see many more “troughs” during the Pleistocene and early Holocene.
Because there are so many geographically distributed samples for humans, and frankly some of the best methods developers work with human data (thank you NIH), it is no surprise that our species would be mapped first. But I think some of the biggest insights may be with understanding the dynamics of gene flow of non-human species, and perhaps the nature and origin of speciation as it relates to isolation (or lack thereof).