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When Genes Evolve, a Staccato Rhythm
5 October 2006 (All day)
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.
The researchers counted the branch points--each point represents a new species--and measured the evolutionary distance from the root to the tips of all the branches to estimate how much evolution had occurred. Pagel's earlier work had suggested that if evolution occurs in bursts, then the number of genetic changes over time should be greater in trees with more branch points.
According the genetic analysis, bursts occurred in some trees, Pagel and his colleagues report in 6 October Science. More than a quarter of the evolution of spiral gingers took place during periods of accelerated genetic change. Evolution revved up during speciation in many of the plants and fungi examined as well. However in others, such as certain neotropical butterflies, the total number of changes in the genes indicated that evolution had been more gradual.
Pagel attributes the bursts of genetic change not just to natural selection, but also to an additional phenomenon called genetic drift, in which some genetic changes become incorporated into the genome even if they are not beneficial. Drift is likely to happen in the small populations that typically characterize an incipient species. As a new species settles into its new niche and its population grows, drift becomes less common, and the rate of genetic change slows. As a result, "We think species change very rapidly at first, and then they slow down," says Pagel.
The work is causing a stir. "This study supports the idea that punctuated equilibria exist but also suggests a limit to its overall effect on genetic change," comments Don Waller, an evolutionary biologist at the University of Wisconsin, Madison. But, despite the precautions Pagel took, Brian Charlesworth from the University of Edinburgh, U.K., worries that multiple changes at the same base in species that have been around a long time might have skewed the results.