Last year, scientists described how partial reproductive isolation between two sockeye salmon populations had evolved at the astonishingly rapid rate of about 13 generations. This was stunning to many biologists, who think of reproductive isolation as a process that evolves over tens of thousands, or even millions of years, but certainly not decades.1 Researchers led by Andrew Hendry, a postdoctoral scientist at the University of Massachusetts, Amherst, used microsatellite markers and morphological data to piece together the recent evolutionary history of two neighboring populations of sockeye salmon in the Pacific Northwest. One fish group spawns on a lakeside beach, the other in one of the lake's major tributaries. Both populations originated about 60 years ago from the same ancestral stock, when then-barren Lake Washington had salmon reintroduced to it from a nearby hatchery.

Hendry's team observed that male river sockeyes are slimmer than their lake counterparts (to better...

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