Humans Have Spread Globally, and Evolved Locally (The New York Times):
No one yet knows to what extent natural selection for local conditions may have forced the populations on each continent down different evolutionary tracks. But those tracks could turn out to be somewhat parallel. At least some of the evolutionary changes now emerging have clearly been convergent, meaning that natural selection has made use of the different mutations available in each population to accomplish the same adaptation. This is the case with lactose tolerance in European and African peoples and with pale skin in East Asians and Europeans.
Nothing new to readers of this weblog, but Wade does a good job surveying the various angles. Anyone with a model of evolution in their head shouldn't be surprised, the range of human variation is to be expected; we are a species which spans Arctic and tropical biomes, evolutionary pressures generally reshape populations into localized ecotypes. Dogs are similar except their selection pressure was our species, and our preferences (as opposed to environmental conditions) served as evolution's sculpting tool. Note that Wade mentions that selection seems notable on both disease and metabolically salient genomic regions. This illustrates the dynamic and multi-layered texture of evolutionary processes, pathogen resistance is always something which all complex species are always tinkering with as we attempt to stay ahead of the race. In regards to the changes in metabolism Wade alludes to the shift between hunter-gather and farming lifestyles. In most of the world this transition occurred between 5 to 10 thousand years before the present, suggesting rapid and recent evolutionary processes which allow for localized adaptation. Lactose tolerance is another example of this, it seems to have emerged in northern Europe only in the last 5 to 6 thousand years, but is now the dominant phenotype across broad swaths of northwest Eurasia. While particular evolutionary dynamics are always bubbling in the background, others are responses to local conditions in time and space. Shifting from a high protein low starch diet to a low protein high starch diet as populations transitioned between modal hunting and gathering toward agriculture was a definite shock to our metabolic systems. Though there is evidence that peasant populations were always physiologically sub-optimal compared to hunter-gatherers (they were smaller for example, and showed more stress during growth in their bone development) their metabolic systems co-evolved with their lifestyles to produce a "good enough" solution and result in more natural increase than non-agricultural peoples. It is a classic illustration that all evolution cares about is replication, not quality of life or some idealized perfection. Finally, note that Wade emphasizes the convergent evolutionary patterns throughout the world. Selection operates upon traits, the phenotype, the underlying genetic architecture that produces this is irrelevant. In the case of light skin for example alternative alleles (genetic variants) arose which produced the same phenotype via mutation. This might not be too difficult if you imagine that light skin is simply a loss of function and there are many ways to do that. One can imagine that these mutants were always bubbling in the background but in the higher latitudes the dampening pressure of selection was removed (and perhaps reversed), and out of the random sample spaces of mutants western and eastern Eurasia exhibited different clusters. This is likely to have happened quite a bit in the past 10,000 years as cultural innovations, such as agriculture and mass societies, have swept across the world to a far greater extent than populations. The implication being that cognate selective pressures arose at about the same time across disparate regions. Related articles on recent human evolution.