Monday, June 22, 2009

African populations have greater genetic diversity, and Eurasians somewhat more than East Asians

In human beings, natural selection appears to work most of the time on dozens of genes in small and hard-to-detect ways. In contrast to fruit flies in the lab, useful traits involving body size, immunity, metabolism and behavior do not come about because one or two genes become ascendant.

The short stature of rain-forest dwellers such as the pygmies of central Africa, for example, appears not to be the product of a single derived allele for shortness carried by virtually everyone in the population. Instead, dozens of gene variants that slightly decrease height have each become slightly more common, and it is their total effect that results in the group's dramatically shorter stature.

"Adaptations to the environment absolutely do occur," said Joseph K. Pickrell, a graduate student at the University of Chicago.

SHOOT: I can vouch for the lack of diversity in East Asia having lived for 4 years in the Far East. Everyone looks like everyone else, and even the Japanese closely resemble the Koreans, and the Koreans the Chinese, the Chinese the Mongols, the Tibetans and Philippinos the Indians (well, sort've), the Indians the Arabs etc.

Vitamin D is made in the skin through a chemical reaction requiring ultraviolet light. Mutations in genes that lighten skin pigment -- at least a half-dozen have been found -- swept through populations as they moved away from the Equator and had less-constant sunlight.

Among West Africans, a chance mutation in the blood protein hemoglobin turned out to partially protect against malaria. It rapidly became common in places where malaria was a huge threat to survival. Similarly, a mutation allowing adults to digest milk became valuable when Middle Easterners and Europeans domesticated cattle. About 90 percent of Scandinavians now carry it.

Bacteria, fruit flies and other rapidly reproducing organisms were (and still are) the workhorses of genetic research.

When experimenters subject populations of them to extreme conditions, mutant genes can become pervasive in just a few generations.

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