Last modified: Wednesday, September 18, 2002
Darwin was right, IU scientists show
Directional selection is responsible for differences among species
Directional natural selection, in which certain traits such as a giraffe's long neck are favored over others, causes most of the differences between species. The finding, reported by a team of Indiana University scientists in the latest Proceedings of the National Academy of Sciences, provides important evidence to a debate among scientists about the importance of natural selection versus non-selective forces in the origin of new species and the differences among them.
"For the first time, we are able to provide compelling evidence that Darwin was right -- that directional selection is the primary cause of diversification, both within species and between them," said IU biologist Loren Rieseberg, who directed the study.
Natural selection, the kind of evolution most people know, involves genetic changes to a species in response to environmental pressures. Directional selection is a specific kind of natural selection that favors less or more of a particular trait, such as smaller fingers or darker fur giving an individual advantages over other members of its species. Other kinds of natural selection might favor an intermediate-sized finger or both light and dark fur at the same time.
But species evolve by other means as well. Change also can occur through undirected, random processes -- like accidental death or immigration -- in a process called "neutral" evolution or "genetic drift." A heated discussion among evolutionary biologists currently centers on which kind of evolution -- natural selection or neutral evolution -- most strongly influences species over short and long periods of time. Charles Darwin in his famous book On the Origin of Species was the first to suggest that directional selection plays the major role. Almost 150 years later, a large group of biologists continues to champion directional selection, while others believe that neutral forces play the major role, particularly for the kind of evolution that spawns new species.
By reviewing the data presented in 84 previously published studies, Rieseberg and his team compared the effects of more than 2,500 genes affecting 572 traits from a wide range of species. Telltale changes indicating directional selection were very common among genes the researchers studied, suggesting that most trait differences were caused by selection. What's more, directional selection seemed stronger between species than among members of the same species, which points to directional selection as the primary factor in the origin of new species, Rieseberg said.
"Some of the conclusions from a recent review of selection in contemporary populations were counter-intuitive," Rieseberg said. "I decided we needed to be able to look at the action of selection over longer time periods, to connect evolution that happens over short periods of time within populations with the long-term evolution that has shaped the differences between species. And that's what we did."
Alex Widmer, A Michele Arntz and John M. Burke also contributed to the report. It was funded by grants from the National Institutes of Health and the National Science Foundation.
Article citation: "Directional selection is the primary cause of phenotypic diversification," Proceedings of the National Academy of Sciences of the United States of America, Vol. 99, No. 19, pp. 12242-12245; Sept. 17, 2002.