A New Look at Race and Natural Selection

A New Look at Race and Natural Selection
By NICHOLAS WADE


In a worldwide survey of 50 populations, a team of geneticists has identified many fingerprints of natural selection in the human genome. These are sites on the genome where specific sequences of DNA show signs of having become more common in the population, presumably because they helped their owners adapt to new climates, diseases or other factors.

The genetic regions where natural selection has acted turn out to differ in various populations, doubtless because each has been molded by different local forces on each continent.

This chart shows the sites along the genome (listed at the left) at which natural selection has occurred in the genome of eight regional groups (shown at top). These are 1) Biaka pygmies, 2) Bantu-speaking Africans, 3) Western Europeans, 4) Middle Easterners, 5) South Asians (people of Pakistan and India), 6) East Asians, 7) Oceanians and 8) Native Americans. The colored bars show the degree of selection at each site, with yellow denoting a signal of clear but moderate statistical significance and red denoting high statistical significance.

The three West Eurasian groups show very similar patterns of selection, which probably occurred before they separated into three geographically distinct populations but after their ancestors split from those of East Asians.

Because the human genome is still so little understood, in most cases the genes at the sites of selection (shown along the right) are of unknown function. One exception is that of genes affecting skin color that have been under strong selective pressure in non-African populations. These include the gene SLC24A5 (shown in red, at center of chart), one version of which has been favored in European, Middle Eastern and South Asian populations. SLC24A5 is not under selection in East Asians, who presumably acquired their pale skin through a different set of genes, an example of what is known as convergent evolution.

Another set of genes found to be under selection in non-African populations are three NRG or neuregulin genes (the third, NRG3, is shown in red) and a receptor gene they all interact with (ERBB4, also in red). The NRG genes make signaling proteins that are active in the developing embryo in shaping tissues like the brain, heart and breast. A variant of NRG1 has been implicated in schizophrenia. The researchers do not know which of the several roles of the neuregulin genes has caused it to come under selection.

The principal human races presumably emerged as the populations of each continent responded to different evolutionary pressures. "Our work supports the notion that regional populations have adapted in a variety of ways, some shared, some not, to the selective pressures they encountered as they dispersed from the ancestral African homeland some 80,000 years ago," said Jonathan Pritchard, a population geneticist at the University of Chicago.

The authors of the new study are Dr. Pritchard and his colleagues Joseph Pickrell and Graham Coop. It was published online last month in Genome Research. It is the first to look for signals of selection in DNA samples gathered by the Human Genome Diversity Project.