Evolution's "Driving Force" Shifts Based on Behavior, Study Says

Scott Norris
for National Geographic News
November 16, 2006

Brown anole lizards on tiny islands in the Bahamas were enjoying the good life, untroubled by a lizard predator found on larger islands nearby.

But all that changed when biologist Jonathan Losos of Harvard University in Cambridge, Massachusetts, appeared on the scene.

Losos's team experimentally introduced predatory curly-tail lizards onto six islands where the ground-dwelling anoles had been living free of predators, sparking a see-saw year of natural selection.

For the smaller anole lizards, a trait that was advantageous in November—six months after the introduction—had become a liability by May.

Initially the longer-legged anoles were more likely to avoid being eaten, due to their faster running speed.

But as the anoles increasingly sought safety in trees, where the bulky curly-tails could not pursue them, shorter-legged lizards were favored for their superior climbing ability.

"Once they moved up into the trees, the lizards had to move along narrow surfaces," Losos said. "The long-legged individuals were quite inept."

The evolutionary experiment, reported in tomorrow's issue of the journal Science, reveals that, even though evolution can seem like a slow process, its driving force—natural selection—can shift like the wind.

The study also supports a somewhat controversial idea in biology: Animals' behavior in response to environmental change can spur evolutionary adaptations.

Experimental Islands

Curly-tail lizards already exist elsewhere in the 700 islands that make up the Bahamas, and the species has been known to periodically colonize small islands naturally.

So Losos, a National Geographic Society Committee for Research and Exploration grantee, simply sped up this process on six islands where the brown anoles lived.

Continued on Next Page >>


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