When Genes Evolve, a Staccato Rhythm

For some biologists, punctuated equilibrium is a radical idea. The term was coined in the 1970s to describe an uneven pace of evolution in the fossil record. But because it posits that evolution happens in bursts, punctuated equilibrium goes against the notion that evolution inches forward in tiny steps guided by natural selection. Now evolutionary biologists have shown that evolution in the genome also has fast and slow speeds, and that natural selection isn't always governing genetic change.

Mark Pagel from the University of Reading, U.K., and his colleagues searched for telltale signs of punctuated evolution in a hodgepodge of family trees. They culled DNA data from 122 papers about various plants and animals. For each set of organisms, they used differences in the number of mutations in certain genes to determine where each organism sat in its particular group's tree. In many cases, they examined clusters of closely related organisms, such as tiger beetles or a group of tropical plants called spiral gingers that belong to one genus. But they also looked more broadly, at a family of snails and an order of frogs, for example.