Where the wild clines aren't

In the recent 'do human races' exist controversy Nick Matzke's post Continuous geographic structure is real, “discrete races” aren’t has become something of a touchstone (perhaps a post like Cosma Shalizi's on I.Q. and heritability).* In the post Matzke emphasized the idea of clines, roughly a continuous gradient of genetic change over space. Fair enough. But in the map above I traced two linear transects. I would suggest that anyone who has a general understanding of the demographics of South-Central Eurasia would immediately anticipate that these transects would reveal a relatively sharp break in allele frequencies. True, there are intermediate populations between the two end points, in Nepal, and on the fringes of India's northeastern states. But clearly about halfway through the southwest-northeast transect you'll see a rapid shift in allele frequencies. The blue transect is different, insofar as the change occurs very near its eastern pole. In Bengal, 85% ...