Scientists have long recognized that, faced with new environmental conditions, populations of organisms will eventually speciate. What they didn't know was just how quickly those populations that share common ancestry can become reproductively isolated from one another. The fastest known examples of such change, taking several hundred generations, had been documented in certain insects. Now new research, reported today in the journal Science, describes a run of salmon that colonized a river and a lake beach, and evolved partial reproductive isolation in fewer than 13 generations. Natural selection, it appears, can spur the emergence of new species far faster than expected.

Lead author Andrew P. Hendry, who was a graduate student at the University of Washington when the research was conducted, and his colleagues examined sockeye salmon (right) in Seattle's Lake Washington and the Cedar River, which flows into the lake. Originally from Baker Lake in northwest Washington state, the sockeye were introduced into Lake Washington between 1937 and 1945. In the 60 years since then, the salmon have split into two morphologically distinct populations. Males that breed along the lake beach, for example, are deeper-bodied than males that spawn in the river, where a slim body is better suited to swimming against strong currents. Similarly, river females are larger, which enables them to dig deeper nests that offer better protection to their eggs during flooding. Although the fish swam between the sites, genetic analyses reveal that fish hatched in the river had little success in breeding at the beach.

The results of this study, along with those of a related study of fruit flies that also appears today in Science, provide strong evidence that reproductive isolation evolves rapidly. "This raises the question: Why do we not see more species?" Nick Barton of the University of Edinburgh writes in an accompanying commentary. Perhaps, he offers, new species may in fact form often, but only rarely to the extent that they are recognized as separate species by biologists or that they find a distinct ecological niche. "For ecologists," he remarks, "the question is then whether the number of established species that we see is determined by a balance between the rate of speciation and the rate of extinction, or instead is set by the range of distinct niches that are available."