BioEd Online

Researchers have confirmed a theory about the driving force behind natural selection in lizards — by taking charge of selection pressures themselves on tiny Caribbean islands.

Anole lizards, energetic little creatures with colourful throat fans, have been studied for decades. Some 400 species of the lizard exist in the Caribbean and the Americas. In the Caribbean they are particularly numerous, with many islands hosting several species, each living in its own microhabitat.

Evolutionary biologists have hypothesized that the dominant pressures on mainland and island anoles differ. On Caribbean islands, where predators are limited but food is scarce and lizards are many, competition between lizards is thought to drive adaptation. On the mainland, where lizards are fewer and food is plentiful, but predators are also abundant, predation is thought to drive adaptation.

Ryan Calsbeek and Robert Cox of Dartmouth College in Hanover, New Hampshire, decided to go beyond observing correlations between different environments and physical traits and to get into the selection business themselves. As their stage, they used a scattering of small islands in the Bahamas — each petite enough to play a game of catch from one end to the other. They removed the resident brown anole lizards (Anolis sagrei) and replaced them with individuals from the nearby island of Great Exuma. The size of these animals had been measured, and they had been tested for running stamina on treadmills, and individually marked.

Island life

Without straying out of the range of variability normally seen in the Caribbean, the duo set up islands that were dense with lizards and islands with few lizards. And for each density type, they established islands inhabited by lizard-eating birds, islands containing both lizard-eating birds and snakes, and islands free of predators. On predator-free islands, they used many metres of netting to keep out birds.

To do this, they stitched together squares of netting and used plastic zip ties to secure them to the rocky shores below the high-tide line. It was hard work. "The first year we did this, we motored up to the island in our boat which was full to the gills with nets, and we looked at the island and we looked at each other and we said, 'Oh man, this is never going to work'", says Calsbeek.

But they managed it. Their reward was a mountain of data on the fate of all the lizards that survived on the islets for the four months of the breeding season. An analysis of those data confirmed the current hypothesis that competition is a more powerful selective force on Caribbean islands than predation. There were no significant differences between the characteristics of lizards that survived or died according to which predators they had to face. But lizards surviving on crowded islands were significantly bigger and tougher (as measured by treadmill stamina) than their counterparts on low-density islands. Competition between lizards was pushing the population as a whole in a clear direction. The research is published online in Nature today.¹

Hypothesis tests

It is not clear that this result says anything about natural selection in species other than anoles, Calsbeek explains. But for him, that isn't the point — his work pushes forward the project of experimentally testing the hypotheses of evolutionary biology, a field that isn't well known for working with real organisms. "This represents one of the first large-scale experimental manipulations of a process that is central to evolution," he says. "This really is a hard experimental science. You can manipulate agents of selection and test hypotheses about how the process works."

One of the pioneers of such work is David Reznick of the University of California, Riverside. He has used guppies in separate pools in Tanzania to run similar experiments. Calsbeek has followed these manipulations since his undergraduate days, reading about them in textbooks. Now Reznick is returning the favour. "His work is great," says Reznick. "When I saw the abstract, I wanted to see how this guy pulled this one off. And he did it. After I read this paper, I read the whole sequence of his papers. I was very impressed."

The hits may well keep coming. Calsbeek has been working on another experimental manipulation of anoles, this time testing hypotheses relating to sexual selection. "We are putting little tutus on 'em," he jokes.