PLoS Genet. 5, e1000500 (2009)

Neutral forces such as population migrations and the random accumulation of mutations seem to have had an important role in shaping how natural selection acts on human genes, observe Jonathan Pritchard of the University of Chicago in Illinois and his colleagues.

The team compared the relative occurrence of variations at individual nucleotides among the genomes of almost 1,000 individuals from diverse geographic locations to see in what cases variations have become fixed.

As expected, variations in and around genes showed stronger signs of fixation, indicative of selection. But only rarely did strong selective action allow a new mutation to sweep through the population and become fixed. The authors suggest that simultaneous partial sweeps may have driven adaptation, or that humans are less adaptable than many suspected.