Doomed to die on unfamiliar islands, small populations of lizards have defied the experts and adapted to their new homes by undergoing the kind of body changes that could in time transform each island's population into a separate species. If these adaptations, described in tomorrow's issue of Nature, are rooted in the genes, they offer strong evidence for natural selection over genetic drift as the driving force behind divergence in isolated populations.
Twenty years ago, a team led by evolutionary biologist Jonathan Losos of Washington University in St. Louis transplanted small populations of Anolis sagrei lizards from Staniel Cay in the Bahamas to several nearby islands with much different terrain in an attempt to study extinction in action. Instead, the lizards thrived. That surprising result gave the researchers a chance to study another aspect of evolution--how a change in habitat affects leg length. For instance, anole species living on tree trunks like those found on Staniel Cay have longer legs than do those living on twigs, apparently because they sacrifice the agility--crucial on bushy vegetation--that comes with shorter legs for the increase in speed--necessary to cover greater distances--that's provided by longer limbs.
But the 14 lizard-free islands that the researchers seeded with the Staniel Cay pioneers have only a few trees; most of their vegetation is bushy and narrow-leafed. "From the kind of vegetation on the new islands, we predicted that the lizards would develop shorter hindlimbs," says Losos. His analysis bears that out: After 10 to 14 years on the new islands, the anoles have shorter rear legs than their ancestors.
But limb length must be a genetic trait for this change to be the first step toward the formation of new species. And some researchers doubt that such rapid morphological change, coming in a few generations, could have a genetic basis. "They have seen a lot of evolution in a very short time," notes evolutionary biologist David Wake of the University of California, Berkeley. He speculates that the leg-length change could be environmental--the equivalent of a body-builder's muscles.
Other experts, however, are convinced that Losos has observed the same process, called adaptive radiation, that led to the great diversity of finches that Darwin spotted on the Galápagos Islands. Says evolutionary biologist Trevor Price of the University of California, San Diego, "It's the first attempt to make a prediction about how the theory of evolution will work--and then show that it does happen as predicted."