Natural Selection In The Fast Lane

Prof records changes in leg length of lizards of Bahamas within a span of months

Natural selection can occur in a span of mere months—as some short-legged lizards learned the hard way.

Jonathan B. Losos ’84, a professor in the organismic and evolutionary biology department, observed rapid changes in a lizard species within a single generation due to natural selection. The findings, reported in Science this week, illustrate that “processes seen in the short-term can account for evolution over millions of years,” Losos said.

“Contrary to what many people think, you can make predictions about evolution and test them experimentally,” he added.

Losos, a former Crimson editor, studied the lizard species Anolis sagrei for a year and a half while in the Bahamas.

Losos said he and his colleagues observed the lizards on 12 small islands in the Bahamas, introducing a predatory lizard, Leiocephalus Carinatus, onto half of them.

On the islands exposed to the predator, Losos predicted that natural selection would initially favor the longer-legged lizards because they could more successfully avoid the predator. After six months, results showed that the surviving lizards did indeed have longer legs than those on the predator-free islands.

But Losos said that as the lizards became more arboreal in an attempt to avoid the predator, natural selection would favor shorter-legged lizards, who would be more adept at maneuvering in trees.

Six more months of study confirmed that the surviving lizards were now shorter-legged than those on the predator-free islands.

The “rapidity with which the reversal occurred was most interesting,” Losos said in a phone interview yesterday.

Though a hurricane wiped out his Anolis Sagrei population and cut his experiment short, Losos said he intends to reintroduce the predator within a year to repeat his experiment.

Losos also defended introducing the predator onto previously uninhabited land.

“Some people think this is a terrible thing,” he said. But lizard-eaters naturally live just 100 or 200 meters away from the predator-free islands, and “the predators have even been seen to colonize these little islands naturally. We’re merely re-creating what is out there.”

—Staff writer Christina G. Vangelakos can be reached at cvangel@fas.