A recent lizard invasion shows how quickly evolutionary change can occur when the competition for food and space gets fierce. On a group of small islands in Florida, native green lizards were pushed to higher tree perches after brown lizard invaders forced their way in. As a result, their little green feet became bigger and stickier -- all the better to grip those higher branches with. This study, published in Science this week, provides a rare glimpse of real-time evolution.
When two closely related species rely on the same resources, intense pressure from competition could drive rapid evolution in the form of “phenotypic divergence,” or different traits to take advantage of different niches. But documented cases of character displacement on observable time scales in the wild are extremely rare.
Well, southern Florida’s green anole (Anolis carolinensis, pictured right), which liked to hang out on the trunks and lower branches of trees, were forced to move to higher perches following the 1970s invasion by the brown anole (Anolis sagrei, pictured above), introduced from Cuba. These pets and agricultural stowaways have since spread across the southeastern U.S. and even to Hawaii.
For their experiment, Harvard’s Yoel Stuart and colleagues took advantage of the Spoil islands in Mosquito Lagoon, a byproduct of dredging for the Intracoastal Waterway in the 1950s, The Scientist reports. In 1995, the team introduced small populations of the brown anole to three islands where mainland natives have already colonized. Another three islands served as invasion-free controls.
Before and at various points after the invasion for the next 15 years, the team recorded the height at which the lizards perched and took toepad measurements on the expanded scales (or lamellae) at the end of their longest toe. As a response to the invasion, the native lizards relocated and adaptively evolved feet that are better at gripping the thinner, smoother branches higher up. (The lower branches are rougher and wider.)
Their toepads became larger and stickier after only 20 lizard generations, or between 10 and 15 years. That’s a biological blink of an eye. To the right is a flatbed digital scan of the left hind foot of Anolis carolinensis.
On islands where invaders haven't landed, Anolis carolinensis toepads remained the same. Adults of both species are known to eat the hatchlings of the other species, "so it may be that if you're a hatchling, you need to move up into the trees quickly or you'll get eaten," Stuart explains in a news release. "Maybe if you have bigger toe pads, you'll do that better than if you don't." Even in the lab, baby lizards of females from invaded islands had larger, stickier toepads than babies conceived on invader-free islands.
To help put this into perspective, “if human height were evolving as fast as these lizards' toes, the height of an average American man would increase from about 5 foot 9 inches [1.75 meters] today to about 6 foot 4 inches [1.9 meters] within 20 generations," Stuart explains. Humans live longer than lizards, he adds, but such an increase would make the average U.S. male the height of an NBA shooting guard.
Images: Todd Campbell (top, middle), Yoel Stuart (bottom)