A universal selection pressure doesn't produce dozens of distinct population groups, it would homogenize them. What would help produce distinct population groups - in addition to the genetic drift and inherent random walk behavior of all partially or wholly separated population groups with non-panmictic mating behavior which would produce large differences anyway - is different selection pressures for different things in different places, potentially varying over time due to environmental and social changes. (Such as, to name one possibility, greater disease burden making intelligence less fit compared to diverting those metabolic resources into faster maturation or more robust immune systems.) Since the Arctic is not Asia is not Africa, there is no surprise that there might be different selection pressures, and the real question is why one would expect selection pressures to be universally identical such that evolution would stop at the neck.