Evolution is often thought of as being such a slow process that it can’t be observed. That’s not true, but to happen quickly, there usually has to be some sort of strong pressure on a population to drive any sort of fast evolution.
For Carolina, or green, anoles, a type of lizard from the southeastern United States, that pressure comes from an invader — the Cuban, or brown, anole. Cuban anoles arrived in South Florida in the 1950s, probably as stowaways in shipments of agricultural products from Cuba (a relatively common method of accidental introduction for invasive species) and over the next decades spread across the Carolina anole’s range.
The two species have similar lifestyles and food requirements, so when they find each other living in the same neighborhood, fights for space, and probably food, occur. But the two species eventually find a balance: The Carolina anoles have not only moved to higher perches, but they also now have larger toepads that help them live at those new heights, researchers report in the Oct. 24 Science.
Yoel Stuart at the University of Texas at Austin and colleagues performed a series of experiments with Carolina anoles living on Florida islands created in the mid-20th century from material dredged to create the Atlantic Intracoastal Waterway. In May 1995, researchers deliberately introduced Cuban anoles to three of the islands.In 2010, researchers went back to survey the lizard population on 30 of the islands. By that time, Cuban anoles had invaded 25 of the 30 surveyed islands. Carolina anoles had moved to higher perches on invaded islands, whereas other members of their species on Cuban-anole-free islands lived closer to the ground. Measurements of the Carolina anoles’ toepads revealed that the lizards that live farther up in the trees have larger toepads with more lamellae (fringes that provide increased surface area), which increased the lizards’ ability to cling to narrow, unstable branches.
The researchers confirmed that the change was genetic in several ways, including by hatching eggs in the lab and raising offspring in identical conditions.
“This morphological change occurred quickly,” the researchers note. Assuming that all the Cuban anole invasions happened in 1995, the larger toepads evolved in the Carolina anoles after only 20 generations.
The speed of the lizard evolution would be equivalent to human height increasing from 5 feet 9 inches (the average for U.S. males today) to 6 feet 4 inches within just 20 generations (about 400 to 500 years), the researchers say.