Sometimes, evolutionary selection can happen within a single generation of a species, research now shows. In response to a new predator, lizards on several Caribbean islands underwent selection first for long legs and then for short legs.
When the brown anolis lizard (Anolis sagrei) lives free of predators, it stays mostly on the ground, where long legs make for fast moves. But when preyed upon, the lizard tends to move up into trees and bushes, where shorter legs are good for climbing.
Jonathan B. Losos of Washington University in St. Louis and his colleagues report in the Nov. 17 Science that they introduced the curly-tailed lizard (Leiocephalus carinatus), which eats the brown anolis, onto six baseball diamond–size islands where the brown anolis lives in the Bahamas. The predator had also been indigenous there but had been swept away by hurricanes.
Six months after introducing the predators, the researchers found that the surviving brown anolis lizards had longer legs, on average, than those on predatorfree islands.
When the researchers returned after a year, they observed that the selective forces had reversed. The surviving lizards had, on average, shorter legs than the controls did.
The team speculates that early on, many of the lizards continued to stay on the ground, where long legs and speed were good for dodging predators. But once the lizards learned to stay in the bushes, shorter legs and agility were more advantageous.
“We did a controlled, replicable experiment in nature,” Losos says. “It illustrates that evolutionary biology at its heart is no different from any other science.”