I hear the voice of physicist turned evolutionary biologist Greg Cochran in Wade's writing :-)
Wade: Historians [and social scientists!] often assume that they need pay no attention to human evolution because the process ground to a halt in the distant past. That assumption is looking less and less secure in light of new findings based on decoding human DNA.
People have continued to evolve since leaving the ancestral homeland in northeastern Africa some 50,000 years ago, both through the random process known as genetic drift and through natural selection. The genome bears many fingerprints in places where natural selection has recently remolded the human clay, researchers have found, as people in the various continents adapted to new diseases, climates, diets and, perhaps, behavioral demands. ...
Cochran: There is something new under the sun — us.
Thucydides said that human nature was unchanging and thus predictable — but he was probably wrong. If you consider natural selection operating in fast-changing human environments, such stasis is most unlikely. We know of a number of cases in which there has been rapid adaptive change in humans; for example, most of the malaria-defense mutations such as sickle cell are recent, just a few thousand years old. The lactase mutation that lets most adult Europeans digest ice cream is not much older.
There is no magic principle that restricts human evolutionary change to disease defenses and dietary adaptations: everything is up for grabs. Genes affecting personality, reproductive strategies, cognition, are all able to change significantly over few-millennia time scales if the environment favors such change — and this includes the new environments we have made for ourselves, things like new ways of making a living and new social structures. I would be astonished if the mix of personality types favored among hunter-gatherers is "exactly" the same as that favored among peasant farmers ruled by a Pharaoh. In fact they might be fairly different.
(Larger version here.)
A new article in BMC Genomics discusses the issue of predicting continental origin using randomly selected markers. The pdf is freely available.
One of the arguments of those who deny the existence of biological races is that their reality is subjective. Some extremists have argued that race is totally socially constructed; this is, however, disproven by the fact that socially constructed race is correlated with physical characteristics. Thus, rather than being separated from biology, the social phenomenon of race is rooted in biology.
A different argument holds that race is correlated with biology, but the differences are "skin-deep", i.e., involve only superficial, visible, (and by some strange logic unimportant) characteristics. According to the proponents of this view, the idea of biological race places an undue emphasis on a set of traits: it is a result of the subjective choice of a set of traits as race-defining. Thus, the commonly recognized races of traditional physical anthropology are discounted as subjective organizations of the biological data: we could just as simply speak of a "lactose-intolerant race" according to this view.
In forensic science and admixture analysis scientists often discover and use polymorphisms which exhibit large inter-population differences. Decoding DNA isn't free, thus, it makes sense to use the most informative, most "biased" markers when one is trying to discover the origin of a biological sample. For example, if Africans have 55% of gene version A and 45% of gene version B, and Europeans have 53% of A and 47% of B, it makes little sense to type this particular gene, since it cannot really tell us whether a sample is European or African. A gene where Africans have 90% of A while Europeans have 5%of A would be much more useful. Race skeptics claim, as with the physical anthropological data, that to privilege such carefully chosen genes is to stress the differences between groups; the implication is that in randomly chosen genes these differences are minor.
The new paper is one of many (you can click on the Clusters label to find more) recent papers that have discovered that no matter what genetic markers you choose: SNPs, STRs, no matter how you choose them: randomly or based on their "informativeness", it is relatively easy to classify DNA into the correct continental origin. Depending on the marker types (e.g., indel vs. microsatellite), and their informativeness (roughly the distribution differences between populations), one may require more or less markers to achieve a high degree of accuracy. But, the conclusion is the same: after a certain number of markers, you always succeed in classifying individuals according to continental origin.
Thus, the emergent pattern of variation is not at all subjectively constructed: it does not deal specifically with visible traits (randomly chosen markers could influence any trait, or none at all), nor does it privilege markers exhibiting large population differences. The structuring of humanity into more or less disjoint groups is not a subjective choice: it emerges naturally from the genomic composition of humans, irrespective of how you study this composition. Rather than proving that race is skin-deep, non-existent, or unimportant, modern genetic science is both proving that it is in fact existent, but also sets the foundation for the study of its true importance, which is probably somewhere in between the indifference of the sociologists and the hyperbole of the racists.