Mechanisms of specification: new examples of sympatric speciation revive some nagging questions.(Feature)

"A new species develops if a population which has become geographically isolated from its parental species acquires during this period of isolation characters which promote or guarantee reproductive isolation when the external barriers break down."

--Ernst Mayr, Systematics and the Origin of Species, 1942 (1)

The duration of a cell cycle lasts anywhere from one hour to one day; Drosophila melanogaster lives for a couple of weeks. But the origin of a species, otherwise known as speciation, takes thousands, maybe millions of years, a fact that makes it extraordinarily difficult to study.

Consequently, the process of speciation has baffled biologists for nearly two centuries. Even Charles Darwin got it wrong. At least that's what evolutionary biologist Ernst Mayr argued in Systematics and the Origin of Species in 1942, and that's how Mayr still sees it today.

Darwin believed that speciation was the ultimate adaptational answer to competition among individuals for scarce resources. Not so, says Mayr, who has argued for decades that speciation is a nonadaptational byproduct of geographic isolation. Although Mayr credits 19th-century naturalists, including German explorer Leopold van Buch, for developing what has become known as the allopatric (or geographic) model of speciation, evolutionists did not fully embrace it until Mayr did so in his 1942 book. Since then, it has dominated speciation theory. "We've been laboring under Mayr's shadow," says evolutionary geneticist Jerry Coyne, University of Chicago.

But today, some evolutionists, including University College London's lames Mallet, are calling for a back-to-Darwin approach to study speciation, arguing that growing evidence suggests that Darwinian natural selection may be more important than isolation after all, even in allopatric situations. Loren Rieseberg, an Indiana University professor and recipient of a 2003 MacArthur Genius Award, explains: "We are realizing that it is the strength of selection rather than the amount of gene flow that controls rates of speciation."

"That's just nonsense," …