Caribbean Lizards Evolve Independently

A team led by Jonathan B. Losos, Ph.D., associate professor of biology in Arts and Sciences at Washington University, has discovered that remarkably similar lizard communities have evolved independently on different islands in the Caribbean. Losos and his colleagues examined DNA of 56 species found throughout the large Caribbean Islands of Puerto Rico, Cuba, Jamaica and the Greater Antilles. Using several common genes of different species they developed a 'family tree' of these species, the most commonly observed in the Caribbean, to test theories on evolutionary history of the Anoles.

The study reveals a perfect example of an evolutionary concept known as "convergence", where species evolve in similar adaptations to the environment despite living geographically apart. Although evolutionary convergence has been taken as evidence for the working of natural selection, the study is unique in showing that entire communities have converged. This goes against the grain of most evolutionary thought which stresses that random events -- a meteorite striking Earth or a hurricane wiping out island species , for example -- play unpredictable roles that send evolutionary diversification down different pathways.

The results were published in the March 27, 1998 issue of Science magazine.

For the past decade Losos and various collaborators have surveyed the Caribbean Island Anolis populations and documented how species differ in their habitat use and body proportions. Detailed studies on more than 50 of the 150 Caribbean anole species indicate that species have adapted to use different parts of the environment by evolving differences in limb length, toepad size and other characteristics. These studies indicate that, within an island, species use different parts of the environment and have evolved different features to adapt to their particular habitat.

For instance, one type that lives near tree trunks in the rain forest has long legs that favor running and jumping quickly; another, which lives on twigs in the rain forest canopy, has short legs, letting it creep on the smaller diameter living surface. These are extreme examples of the different types of lizards, and the types are called habitat specialists.

In Puerto Rico's Luquillo Forest, different anole species have adapted to use different parts of the environment. One has extremely short legs and crawls slowly on narrow twigs; another has long legs and runs rapidly on the ground; a third lives in the grass. Moreover, species that live high in the trees tend to have big toepads, important for clinging, whereas those that are more terrestrial have small toepads. The interesting thing is that Cuba, for example, has the same set of habitat specialists, yet none of them are the same species as the set of specialized species in Puerto Rico. The same is true on all four islands, for the most part.

Losos and his colleagues developed the family tree to test two theories on how the lizards evolved. One possibility is that each of these habitat specialists evolved only one time. For example, the twig specialist may have evolved on a single large island that then fragmented into the four islands that exist today. Alternatively, a twig specialist may have evolved on one island and then managed to cross the Caribbean to colonize the other islands. If this were the case, then each of the specialists would have evolved only a single time. the other possibility is that each of the specialists have evolved repeatedly on each island.

The lizard family tree strongly suggests the second mechanism.

In Science, Losos and his colleagues report that the Anolis evolutionary tree shows the habitat specialists from the different islands genetically are not closely related, despite exact similarities in their physical traits.

"Our results are very clear-cut that similar communities on the different islands have evolved independently," Losos said. "The same habitat specialists on different islands are not closely related, and that's very interesting because it suggests that there is something about the environment on these islands that elicits similar evolutionary responses on each island. This is rare proof of a community convergence, in which each component of the community on the different islands is identical."

The study is believed to be the first well-documented case that shows both communities and components of the communities to be similar, a very difficult thing to find in nature. For instance, there are various parts of the world where the Mediterranean Climate occurs -- South Africa, Chile, southern California and parts of Australia, as well as Mediterranean regions Botanists studying plants have long searched to find exact plant communities. with the exact components and distribution patterns, but in most cases the plants and their communities evolved differently in the various regions, despite having the same warm, generally wet climate. The differences heavily outweigh the similarities.

"The lizard populations on the islands not only have very similar communities, but they are composed of identical components, and that's really unique," said Losos. "The biggest surprise of our result is that it is opposite to a general trend in evolutionary biology in which evolution proceeding in different areas or times lead to very different results."

The study is part of a larger effort by Losos and colleagues to study evolutionary diversification of this very diverse group. In a paper published last year in Nature, Losos and others studied populations of one species of Anolis that had been transplanted to small islands with scrubby vegetation. Based on their knowledge of how species have adapted over millions of years to different habitats, they predicted that these populations should develop shorter limbs to move with greater efficiency on the narrow vegetation in their new habitat.

Examination of the populations 15 years after their establishment confirmed this prediction.

"This finding is very exciting because it allows us to understand how species adapt to their environment at two very different time scales," said Losos. "Our studies throughout the Caribbean reveal that species adapt to the environment in the same way, time and again, over millions of years. By contrast, our work in the Bahamas illustrates what happens at the very first stages when a population encounters a new habitat. The congruence in results allows us to integrate short- and long-term perspectives to understand evolutionary adaptation and diversification."