Losos’ lizards give evolutionary clues in island experiments

Tiny islets in the Bahamas have proven useful laboratories to illustrate natural selection’s effects on island lizards, which saw their legs lengthen, then shorten as ground-dwelling predators drove them into the trees.

The experiments capped years of research into a type of lizard called an anole on the Caribbean islands. The research, conducted by Jonathan Losos, the Monique and Philip Lehner Professor of the Study of Latin America, examined the relationships between lizards that shared similar habitats and characteristics but lived on different islands.

Losos described his research Tuesday (Sept. 18) during the kickoff talk in this year’s lecture series sponsored by the Harvard Museum of Natural History (HMNH).

HMNH Executive Director Elisabeth Werby introduced Losos, saying that his research has “profoundly shaped” our understanding of species biology.

Losos’ examination of the lizards focused on physical characteristics that might affect their survival, such as how fast they were, how far they jumped, and how well they were able to grip with their toe pads.

Lizards that were commonly found lower in trees and on the ground had longer legs and were able to run faster and jump farther regardless of which island they lived on. Lizards that lived on the thin twigs in the canopy, by contrast, were smaller, with shorter legs, and were slower-moving on all islands.

Losos examined whether similar lizards on different islands were closely related or whether they developed their traits separately, in response to the needs placed on them by their habitat.

By examining the lizards’ DNA and constructing a family tree, Losos found that the similar lizards were not closely related, meaning they had evolved separately in response to their environment in a process called convergent evolution.

“That’s true of each type of habitat specialist, they’re not closely related,” Losos said.

He then began to explore the evolution of these traits, using tiny islands that pepper the Bahamian archipelago as living laboratories. These islands, some only a few dozen yards across, stand just offshore of larger islands and are commonly colonized from nearby islands, Losos said.

Losos and his team identified a handful of lizard-free islands and established populations of lizards there, setting up experimental populations on one island and controls on others.

In one experiment, researchers established populations of brown anoles on several islands, measuring physical characteristics such as leg length and marking them so they could be re-identified later. They then released the larger curly tail lizard, a predator that is mostly confined to the ground.

After six months, they found that the average leg length of the anole population had grown longer, most likely due to the ground-based predator catching and eating slower, shorter-legged anoles.

“This is natural selection showing that long-legged lizards survive better,” Losos said.

Six months later, however, the trend had begun to reverse itself, Losos noted. Because the predator was on the ground, the anoles that survived best over the longer term were those that could live in the twiggy bushes and small trees on the islands. Earlier experiments had shown that short-legged lizards were considerably more sure-footed than longer-legged lizards when walking on smaller branches and less prone to slipping and falling off.

The island experiments bore those findings out, painting a picture of natural selection in action. The predatory curly tailed lizard ensured first that only faster, long-legged anoles survived and then, by driving the survivors higher into the bushes and trees, prompted a shift that favored survival of those with shorter legs.

Losos said researchers expected to see both changes, but were surprised to see the effects occur so rapidly, within a year.

Unfortunately, the experiment ended prematurely in 2004 when two hurricanes, Frances and Jeanne, swept across the islands, wiping them clean of lizards. Though researchers were disappointed initially, some lizard eggs did survive and the populations have slowly re-established themselves. By next year, researchers expect the islets’ lizard populations to recover to where they had been when the effort began.

“We hope to follow the experiment for quite a long time,” Losos said.