Meanwhile, other studies suggested that the rate of selection markedly increased in recent human history. A research team led by anthropologist John Hawks at the University of Wisconsin concluded that selection was more than 100-fold faster in recent human history than before the movement of modern humans out of Africa. Two members of that team, anthropologist Henry Harpending and physicist Gregory Cochran, have used the data to speculate that many features of modern populations—such as the higher average scores of Ashkenazi Jews on IQ tests—reflect the influence of selection.

At this point, arguments about selection often acquire political overtones. But Pritchard has avoided those arguments. He wants to know whether selection actually produced the signals he and others have detected. Recently, he's come up with some surprising answers.

Last year, a group of researchers at Stanford used a new technology to measure differences in DNA sequences from the Human Genome Diversity Panel at many more genetic locations than in the past. With postdoctoral fellow Graham Coop, graduate student Joseph Pickrell, and several collaborators at Stanford, Pritchard began searching the data for signals of selection.

Previously known signals jumped out right away, including the lactase and skin pigmentation genes. Pritchard and his colleagues also found several interesting signs of selection where they hadn't been seen before, such as in a set of genes involved in the development of heart, neural, and mammary tissue. Geneticists have few clues about how these genes operate and why they might have been selected, but “we're keen to learn what these genes are and how they work,” says Coop, now an assistant professor at the University of California, Davis.

In a paper published in PLoS Genetics on June 5, 2009, Pritchard, Coop, Pickrell, and a group of colleagues, including Feldman and Cavalli-Sforza, describe an unexpected result of their analysis. Beyond the clearest signs of selection—like lactase persistence, skin color, and resistance to several infectious diseases—there appear to be few unambiguous cases of strong selection in the human genome. “Natural selection may shape the human genome much more slowly than previously thought,” says Pritchard. In fact, some of the DNA sequences identified earlier as possible signs of selection look like something else to Pritchard. They look like the patterns generated by the migration of modern humans out of Africa and by the continued movements of people since then.

Pritchard's team has concluded that selection in the human genome is often overwhelmed by the movement and expansion of populations. “Selection is a weaker force than people thought,” says Pickrell. Populations that are closely related genetically, because they split recently or exchange many migrants, have very few genetic variants that are markedly different. With more distantly related populations, demographic processes have usually had a greater influence than selective pressures. Selection may be driving groups of genes that all influence a trait in particular directions, but “simple models of strong selection pushing single variants to high frequencies appear not to be the case,” Pickrell says.

	PAGE 1 2 3 4 5 6	Go Back | Continue