Predators Provoke Behavioral Shifts and Rapid Evolution of Island Lizards

Biologists have shown that natural selection can function even in few months, not only over the geological ages.

They revealed that island lizards pressured by the introduction of a predator suffered heavy natural selection in just one year, first for longer and then shorter hind legs.

"Because of its epochal scope, evolutionary biology is often caricatured as incompatible with controlled experimentation," says Jonathan Losos, professor of organismic and evolutionary biology in Harvard's Faculty of Arts and Sciences.

"Recent work has shown, however, that evolutionary biology can be studied on short time scales and that predictions about it can be tested experimentally. We predicted, and then demonstrated, a reversal in the direction of natural selection acting on limb length in a population of lizards."

The team looked on populations of the lizard called brown anole (Anolis sagrei) (photo above) on islets ( cays ) in the Bahamas.

In six cays introduced larger, predatory Northern curly-tailed lizard (Leiocephalus carinatus) (photo bellow) common on nearby islands and cays.

Other six cays were kept predator-free and the researchers exhaustively counted, marked, and measured lizards on all 12 islets.

In predator free habitats, the anole is predominantly ground dweller, but in the presence of a predator, it turns arboreal.

The team guessed that immediately following a predator introduction, longer-legged faster-running anoles would elude capture more frequently.

But, as the lizards turn more arboreal in habitat, natural selection would stress for shorter limbs, better suited for arboreal movement.

Six months after the beginning of the experiment, anole population had decreased by half or more on the islands with curly-tailed compared to control islands and the survivors were longer-legged than previous populations.

After another six months, natural selection turned back its pressure to shorter legs compared to control cays.

"The behavioral shift from the ground to higher perches apparently caused this remarkable reversal," Losos said.

"Behavioral flexibility may often drive extremely rapid shifts in evolution." he added.

"Evolutionary biology is by its nature an historical science, but the combination of microevolutionary experimentation and macroevolutionary historical analysis can provide a rich understanding about the genesis of biological diversity".