Six islands, 1,300 lizards and the evolutionary biologists
By covering large swaths of land with netting, making lizards jog on treadmills and turning six Caribbean islands into Darwinian laboratories, evolutionary biologists have made important discoveries about what drives the evolution of island lizards.
The report, published online Sunday in the journal Nature, has broader implications for the practical study of evolution, researchers said. It serves as a rare demonstration that theories about natural selection can be directly tested in the field.
Ryan Calsbeek and Robert Cox of Dartmouth College traveled to the Bahamas to figure out what had been more important in shaping the evolution of brown anole lizards that live there: competition among one another for resources such as food or shelter, or evasion of predators.
To do so, the scientists set up several scenarios. They covered two small islands more or less the size of a baseball diamond entirely in bird-netting to protect the lizards from seabirds, thus removing the animals’ only predators.
On another pair of islands, they introduced a few snakes (male, so that they wouldn’t reproduce and permanently invade the islands). This meant the lizards had more predators — more “predation pressure” — to worry about.
They left another two islands alone, leaving the lizards to contend with just the bird predators.
They packed some islands with lizards and left others scantly populated, so they could gauge what effect competition might have on the populations. A higher lizard density meant there would be more lizards vying for the same resources.
Setting up the experiment was exhausting, said Calsbeek, lead author of the paper. That first sweaty day, they picked their way across the cactus-studded island encircled by the jagged edges of an old coral reef, checking for birds and spreading netting over the vegetation.
A tourist in a boat puttered by, and asked them — were they doing science, or making art?
“When you pulled away from the island, it really did look like a sculpture in bird-netting,” Calsbeek said.
A total of 1,300 lizards were released on the islands in May 2008 and 2009, at the beginning of each breeding season. Before the release, the researchers had measured the length of each male lizard’s hind limb and put each animal on a 3-foot-long treadmill surrounded by plexiglass to record its running speed.
Each lizard had been individually tattooed so the scientists would know exactly which ones survived.
Four months later, the scientists went back to the islands, recaptured as many lizards as they could and examined the traits of the survivors.
They found no significant physical differences between the lizards on islands where predators had been removed (courtesy of bird-netting), where predators had been added (in the form of snakes), or islands that had been left as they were.
However, on the islands that were crowded with lizards, the scientists found that the surviving males had slightly longer hind limbs, larger body size and greater endurance on a treadmill. They concluded that the biggest pressures faced by the island lizards — the ones that would drive their evolution — came not from predators but from competition.
The finding plays into a larger debate about the extent to which these two forces drive natural selection, said Thomas B. Smith, director of the Center for Tropical Research at UCLA, who was not involved in the study.
“What this experiment does is tease apart those two things, in a very elegant way,” Smith said.
The authors said the finding applies strictly to island lizards. They expect that for brown anole lizards living on the mainland the opposite may be true — that predation will be more important than competition.
But the most important thing about the study, they added, was it showed that evolution is not just a theoretical science but one that can be tested in the field.
The ability to test for evolutionary forces is “particularly important in the United States, where more than 50% of Americans are skeptical of” Darwin’s theory of evolution, Calsbeek said. “I think what we’ve done here should speak directly to that. Even a skeptic could go out and perform an experiment [and] hopefully, if they approach it with an open mind, it might persuade them.”