Ancient Balancing Act

In evolution, “survival of the fittest” refers to genetic selection of the single best adaptation. But what happens when more than one adaptation is advantageous? Research by Molly Przeworski, an HHMI early career scientist at the University of Chicago, has found some instances in which this scenario dates back several million years.

Although evolution usually works to select a single adaptive trait, sometimes it keeps all hereditary options open by not favoring a particular version, or allele, of a gene. This phenomenon, known as balancing selection, maintains genetic variation in a population. In an effort to find examples of ancient balancing selection, Przeworski and her team analyzed the genomes of 59 humans from sub-Saharan Africa and 10 chimpanzees from western Africa, looking for variation shared between the two species since they diverged from their common ancestor. They reported their results online in Science on March 29, 2013.

The researchers found strong evidence for long-lived balancing selection in six regions of the human genome and weaker evidence in another 119 regions. Unexpectedly, not one of these six areas occurred in DNA that coded for genes. That variants in these noncoding regions are preserved in humans and chimps suggests that regulatory variation dates back millions of years before the two species split. Przeworski plans to investigate the function of these regions, which appear to play a role in fending off disease.

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