A scan of the green anole's well-adapted toes. (Yoel Stuart)

Organisms are evolving all the time, but most animals change too slowly for us to see their adaptations develop. That's not the case for Anolis carolinensis, otherwise known as the green anole. An invasive species has driven this little lizard to adapt super fast as a way of avoiding extinction.

A new study published in Science reports that these critters changed their behavior within months of their competition's arrival, and that their populations had made notable physical changes within just 20 generations -- or 15 years.

The researchers were studying the anoles to see how they'd react to the invasion of a competing species. They knew that green anoles on tiny islands in Florida were being invaded by brown anoles from Cuba.

Brown and green anoles are similar in size and habitat, and eat the same things. Because they'd be in such direct competition, the researchers knew that something would have to give.


The green and brown anole might just be able to coexist, thanks to the former's quick evolution. (Todd Campbell and Adam Algar)

But the speed of the green anoles' adaptation took them by surprise. At the beginning, both species of lizard liked to perch on the trunks and low branches of trees. "Within a month or so, the natives were moving up into the trees in response to the invaders," said Yoel Stuart, a postdoctoral researcher at The University of Texas and lead author of the study.

When they went looking for an evolutionary response to match the behavior change, they found one only 20 generations later: The lizards had developed larger toe pads and sticky toes to help keep them on the smoother, thinner high branches they now favored.

This happened on six different islands with invasive brown lizards, while the lizards on non-invaded islands kept their typical feet. By comparing the rate of foot growth to these controls, Stuart and his colleagues saw just how dramatic the evolution was.

"If human male height evolved at the same rate as these toe pads, after 20 generations the average male in the U.S. would be the height of an NBA shooting guard," Stuart said. That's an increase from 5 foot 9 inches to 6 foot 4 inches.

"The common wisdom until the last decade or two was that evolution happens really slowly, and it should be hard to see it happening in front of us," Stuart said. "But we’re starting to realize that you can see an evolutionary response quite quickly if the selection is strong enough."

We'd probably see species adapting all the time if we looked closely enough, Stuart said. But whether or not those changes actually work to compete with an invading species is another question entirely. The green anoles are pretty lucky.

"In this case, it does seem like the natives should be able to adapt to coexist with their invaders," Stuart said.