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Growing apart. After just 13 generations, two salmon populations could be on their way to becoming two species.

Evolution on the Fast Track

Evolution may happen a lot faster than you think. New studies of fish and flies, described in the 20 October issue of Science, show that the crucial first steps toward the birth of a species may occur in only a dozen generations.

New species arise when a population splits into groups that become reproductively isolated--that is, individuals breed mainly within their own group. Once this happens, the populations start to diverge, and eventually they may develop into two very different species. Scientists have been unsure of how fast reproductive isolation can evolve in the wild.

A case study comes from Andrew Hendry of the University of Massachusetts, Amherst, who examined two populations of sockeye salmon in Washington state that were known to interbreed 56 years ago. Because the two populations now breed in different areas, Hendry and his colleagues wondered whether the populations showed any signs of reproductive isolation. They did. Fish that hung out near the lake beach were genetically distinct from their river-dwelling neighbors. They were also physically different: At the beach, males had deeper bodies than those in the river, while females were significantly shorter. Such differences, the researchers say, mean that interbreeding has decreased in only 13 generations.

On the other side of the world, Megan Higgie of the University of Queensland in Australia tried to mimic evolution in the lab. She and her colleagues studied two fruit fly species, Drosophila serrata and D. birchii, that give off different pheromones, or chemical signals, to attract mates. She took serrata flies that had never before seen birchii and found that after nine generations of cohabitation with birchii in the lab, they had evolved new chemical signals--signals similar to those given off by serrata flies that already lived with birchii in the wild. It's not exactly clear why the changes occurred, but apparently, natural selection can directly influence sexual signals, and thus mate choice, which would strongly favor reproductive isolation, the authors say.

Nick Barton of the University of Edinburgh says the studies are "nice," as "they emphasize that reproductive isolation may evolve rather readily and rapidly." The question, says Barton, is: If reproductive isolation happens so fast, "why don't we see more species?"