Evolution's Day Off

"Does Evolutionary History Take Million-Year Breaks?" by Richard A. Kerr, in Science (Oct. 24, 1997), 1200 New York Ave. N.W., Washington, D.C. 20005. ------------------------------------------------------------------------------------------------------------------------------------------------------

Does evolution ever take a holiday? Strict Darwinists maintain that life is always in a state of change, with species continually coming and going. But some paleontologists, reports Kerr, a Science staff writer, are suggesting that hundreds of millions of years ago, entire communities of marine animals of various species remained virtually unchanged for millions of years, then plunged into brief frenzies of extinction and new species formation.

In putting forward this idea of "coordinated stasis," paleontologists Carlton Brett of the University of Rochester and Gordon Baird of the State University of New York, Fredonia, have built upon the concept of "punctuated equilibrium." This revolutionary concept was advanced in 1972 by Stephen Jay Gould of Harvard University and Niles Eldredge of the American Museum of Natural History, in New York, who argued that species tend to persist unchanged for millions of years before abruptly giving way to new species. Coordinated stasis, explains Douglas Erwin of the National Museum of Natural History, in Washington, "is punctuated equilibrium at a higher level," involving not just individual species but entire ecological communities.

Examining the fossils of marine animals that lived in ocean-bottom muds some 380 million to 440 million years ago, Brett and Baird identified 14 periods, each running from three million to seven million years, during which at least 60 percent of the species persisted with little change. Each period ended with a drastic turnover of species, lasting a few hundred thousand years.

"Most studies of similar fossil records have found little evidence for prolonged periods of evolutionary stasis," Kerr notes. But if even occasional episodes of coordinated stasis took place, he observes, that could have a major impact on the way in which evolution is understood.