Faced with competition from an invading species, a type of lizard in Florida took just 20 generations — about 15 years — to evolve feet better suited to climbing trees, a new study says.

Biologists have long believed that evolution could occur rapidly among some species facing sudden, intense competition. In 1995, as part of an unrelated study, researchers introduced a foreign species of brown anole lizards to three islands off the coast of Florida that were already home to green anole lizards. The green lizards are known to move to higher perches after an invasion of brown ones.

To test the rapid-evolution theory, researchers returned to the islands 15 years later and examined the feet of the green lizards (which had, as predicted, relocated to higher perches). They found that the lizards had developed larger and stickier toe pads, a characteristic not shared by green lizards on nearby islands that had not been populated with brown lizards.

“First, we saw the behavioral response,” said Yoel Stuart, a biologist at the University of Texas and the lead author of the study. “And when we compared the anoles to islands with no invaders, we saw there was also this evolutionary response.”