Lizards may not get the limelight in beer commercials, but thanks to Washington University biologists the slighted creatures now have marquee value in evolution and genetics.
A team led by Jonathan B. Losos, Ph.D., associate professor of biology in Arts and Sciences, has discovered that remarkably similar lizard communities have evolved independently on different islands in the Caribbean. Losos and his colleagues examined the DNA of 56 species of anole lizards found in 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 the lizards' evolutionary history.
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The study reveals a perfect example of an evolutionary concept known as convergence, whereby different species evolve with similar adaptations to the environment despite living geographically apart. Although evolutionary convergence has been taken as evidence for the working of natural selection, this study is unique in showing that entire communities in different locations have converged. This finding goes against the grain of most evolutionary thought that 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 anole populations and documented how species differ in their habitat use and body proportions. Their studies indicate that, within an island, species have adapted to use different parts of the environment by evolving differences in limb length, toepad size and other characteristics.
In Puerto Rico's Luquillo Forest, for instance, one anole species 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 ground-dwelling lizards have small toepads. These are extreme examples of the different types of lizards, known as habitat specialists.
The interesting thing is that Cuba, for example, has the same set of habitat specialists as Puerto Rico, yet none of them is the same species as its counterpart 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 might have evolved on a single large island that then fragmented into the four islands that exist today, or it might 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 has evolved repeatedly on each island.
The lizard family tree strongly suggests the second mechanism.
In Science, Losos and his colleagues report that the anole evolutionary tree shows the habitat specialists from the different islands are not closely related genetically, 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 habitat specialist on the different islands is identical."
The study is believed to be the first well-documented case that shows both communities and habitat specialists in 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 plant communities with matching 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 habitat specialists, 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 leads to very different results."
Todd Jackman, Ph.D., a post-doctoral researcher, and Allan Larson, Ph.D., professor of biology, contributed to the study, along with Kevin de Queiroz, Ph.D., a curator at the National Museum of Natural History, and Lourdes Rodriguez-Shettino of the Institute of Ecology and Systematics, Havana.
--Tony Fitzpatrick
