Several of the 25 skin genes bear strong signatures of natural selection, but natural selection has taken different paths to lighten people’s skin in Europe and in Asia. A special version of the golden gene, so called because it turns zebrafish a rich yellow color, is found in more than 98 percent of Europeans but is very rare in East Asians. In them, a variant version of a gene called DCT may contribute to light skin. Presumably, different mutations were available in each population for natural selection to work on. The fact that the two populations took independent paths toward developing lighter skin suggests that there was not much gene flow between them.

East Asians have several genetic variants that are rare or absent in Europeans and Africans. Their hair has a thicker shaft. A version of a gene called EDAR is a major determinant of thicker hair, which may have evolved as protection against cold, say a team of geneticists led by Ryosuke Kimura of Tokai University School of Medicine in Japan.

Most East Asians also have a special form of a gene known as ABCC11, which makes the cells of the ear produce dry earwax. Most Africans and Europeans, on the other hand, possess the ancestral form of the gene, which makes wet earwax. It is hard to see why dry earwax would confer a big survival advantage, so the Asian version of the gene may have been selected for some other property, like making people sweat less, says a team led by Koh-ichiro Yoshiura of Nagasaki University.

Most variation in the human genome is neutral, meaning that it arose not by natural selection but by processes like harmless mutations and the random shuffling of the genome between generations. The amount of this genetic diversity is highest in African populations. Diversity decreases steadily the further a population has migrated from the African homeland, since each group that moved onward carried away only some of the diversity of its parent population. This steady decline in diversity shows no discontinuity between one population and the next, and has offered no clear explanation as to why one population should differ much from another. But selected genes show a different pattern: Evidence from the new genome-wide tests for selection show that most selective pressures are focused on specific populations.

One aspect of this pattern is that there seem to be more genes under recent selection in East Asians and Europeans than in Africans, possibly because the people who left Africa were then forced to adapt to different environments. “It’s a reasonable inference that non-Africans were becoming exposed to a wide variety of novel climates,” says Dr. Stoneking of the Max Planck Institute.

The cases of natural selection that have been tracked so far take the form of substantial sweeps, with a new version of a gene being present in a large percentage of the population. These hard sweeps are often assumed to start from a novel mutation. But it can take a long time for the right mutation to occur, especially if there is a very small target, like the region of DNA that controls a gene. In the worst case, the waiting time would be 300,000 generations, according to a calculation by Jonathan Pritchard, a population geneticist at the University of Chicago. And indeed, there are not many hard sweeps in the human genome.

But the new evidence that humans have adapted rapidly and extensively suggests that natural selection must have other options for changing a trait besides waiting for the right mutation to show up. In an article in Current Biology in February, Dr. Pritchard suggested that a lot of natural selection may take place through what he called soft sweeps.