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subspecies." Wright specified that an Fst of 5 to 15% in any population of organisms constituted "moderate" genetic differentiation, and 15 to 25% should be considered "great"

genetic differentiation.240

But whether an Fst of 10 to 15% represents a large or small degree of differentiation is probably not very relevant to the question of whether or not human races can be identified. The reason is that such measures of Fst have captured something called neutral variation, which is probably not the kind that underlies most racial differences.

Neutral variation refers to mutations that don't affect the organism one way or another. Evolution doesn't care about such changes, and the frequencies of such alleles in a population will vary randomly under genetic drift. Most common variation is neutral, and most measures of FST are likely to sample common—that is, neutral—variation. But evolution pays great heed to mutations that make a significant change to a gene and its protein. If the change is adverse, the mutations are ruthlessly eliminated because the affected individual either dies or fails to reproduce. If the change enhances the individual's reproductive success, the mutation is selected for and becomes commoner in a population. These two kinds of selection, one negative and the other positive, are the two faces of natural selection. Biologists do not yet understand what genes need to be changed to make one species into two subspecies. However, it seems that the alleles involved in differentiating the human population are likely to be of the selected kind, not the neutral kind.

Versions of the two brain genes that evolved within the last 40,000 years show just this pattern. As mentioned in chapter 5, an allele of one, known as microcephalin, appeared some 37,000 years ago and is now widespread among Caucasians and East Asians but is much less common in sub-Saharan Africans. The Fst for this allele between sub-Saharans and the others is 48% or 0.48, "which indicates strong differentiation and is significantly higher than the genome average of 0.12," writes Bruce Lahn of the University of Chicago, who discovered the allele.241

A new version of another gene, ASPM, arose some 6,000 years ago in Caucasians, 44% of whom now carry this allele. The allele is less common among East Asians and rare to nonexistent in sub-Saharan Africans. The Fst for the allele between

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