Change We Can Believe In: "Race" and Continuing Selection in the Human Genome
Krause, Kenneth W., The Humanist
"People, including me, would rather believe that significant human biological evolution stopped between 50,000 and 100,000 years ago, before the races diverged, which would ensure that racial and ethnic groups are biologically equivalent."--Steven Pinker
STEVEN PINKER might be right about what most people would prefer to believe. But equivalency, I would argue, is a concept better left to mathematicians-should they choose to keep using it. In any other, less antiseptic context, however, the notion is utterly bankrupt. That we have had to work so hard in recent centuries to construct and maintain political equality among individuals and classifications of individuals should tell us how persistent and pervasive inequality really is. We should never confuse the social construct with the scientific reality. Denial is the least mature and, certainly, the least progressive response to fear.
Like all other species, human beings continue to change. But until very recently, both the popular and scientific assumptions had been that if humans were still evolving at all, it was through the very slow and completely random process of genetic drift. The alternatives, natural and sexual selection, of course turn on differential reproductive success based on fitness, attractiveness, or both. So the prevailing argument has long been that, because we civilized humans have for the most part managed to insulate ourselves from the natural environment, to nurse our sick back to health, and to provide mates for nearly all persons among us, the march of Darwinian selection had finally reached an impasse. Similarly, many have claimed that because selection had long ago relaxed its discriminating grip on the human genome, our collective abilities to think and to resist disease, for example, have steadily degenerated.
But there have been exceptions--like University of California at San Diego biologist Christopher Wills, who in his 1998 book, Children of Prometheus, defiantly pronounced that "It]he powerful effects of our culture have, if anything, accelerated our biological evolution" Wills' powerful and prescient hypothesis was that genes and an increasingly rich--or at least complex--culture have combined to create a positive feedback loop in which human minds in particular benefit from frequent adaptive boosts. Sure enough, recent genome projects and surveys along with new and controversial genetic studies seem to bear Wills out.
In 2005, for example, University of Chicago geneticist Bruce Lahn published a pair of studies concluding that two genes thought to regulate brain growth have continued to evolve under selective pressures until very recently, if not to the present day (Science 309, 1717-1720 and Science 309, 1720-1722). An intellectual controversy erupted because Lahn, a Chinese-born lifetime member of the NAACP, also discovered that the mutated alleles were less common among sub-Saharan Africans than in other populations.
Different mutations of the Microcephalin and ASPM genes were known to cause primary microcephaly, a condition marked by severely reduced brain size (typically 400 cubic centimeters in affected adults compared to 1200 to 1600 cubic centimeters in normal adults). It was also commonly understood that phylogenic analyses of both genes had revealed strong positive selection in the primate lineage leading to Homo sapiens. The question for Lahn, then, was whether certain variants of Microcephalin and ASPM had continued to evolve by natural selection during the last 200,000 years, since humans became anatomically modern.
After sifting through a vast cache of DNA broadly representative of global diversity, Lahn's team located an allele for each gene that occurred so frequently that it simply had to have been adaptive rather than merely the stray product of genetic drift or group migration. Then, using past mutation rates as a reliable molecular clock, they estimated the dates when these alleles originated. …