Courtesy of Honolulu Zoo

In the estimated 5 million years since humans and chimpanzees began diverging, they've acquired major anatomical and cognitive differences. Nevertheless, some of the strongest evidence for positive selection since divergence appears to be for genes related to immunological defense and apoptosis. Building on an earlier study, Cornell University's Rasmus Nielsen and colleagues compared over 13,000 annotated human genes and their chimp equivalents, looking to the ratio of synonymous to nonsynonymous mutations as evidence of positive selection.1

Nielsen's team made several surprising discoveries, including increased positive selection on the X chromosome and a relative lack of positive selection on genes expressed in the brain. Many genes that showed signs of positive selection were involved in sensory perception or immune defenses, and some of the strongest evidence for selection was seen in genes involved in apoptosis. The latter finding offers "a possible link between selfish mutations during...

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