DNA and Darwin: Evolution repeats itself in Caribbean lizards

Darwin can rest a little easier tonight. I’m sure he would have been puzzled at the average American’s reluctance to accept his theory of evolution. The evidence supporting Darwin’s theory is clear, and every year more supporting evidence accumulates.

There is a sticky point in Darwin’s argument, however. If evolution is indeed guided by natural selection, as Darwin claims, then two environments that are similar should select in the same way — similar habitats should select for the same sorts of critters, all else being equal.

Is Darwin right? Do two communities of animals living in similar habitats evolve to be the same? Does evolution repeat itself at the community level?
This is not an easy question to answer, simply because it’s difficult to find an array of similar but independent habitats to compare.

But not impossible. A team of researchers led by Washington University biology professor Jonathan Losos has spent the last several years studying lizards of the genus anolis (commonly called “anoles”) that live on large Caribbean islands. He has focused on Puerto Rico, Cuba, Haiti, and Jamaica. All four islands are inhabited by a diverse array of anole lizards (there are 57 species on Cuba alone), and all four islands have quite similar habitats and vegetation.

Unlike rats and cockroaches, which are generalists and much the same wherever you find them, anole lizards are specialists. In Puerto Rico, for example, one slender anole species with a long tail lives only in the grass. On narrow twigs at the base of trees you find a different species, also slender, but with stubby legs. On the higher branches of the tree a third species is found, of stocky build and long legs. High up in the leafy canopy of the tree lives a fourth giant green species.

Do the four Caribbean islands have similar lizard communities? Yes. If you go to Cuba, to Haiti, or to Jamaica, you can find on each island a species that looks nearly identical to each of the specialists on Puerto Rico, living in the same type of habitat and behaving in much the same manner.

Does this striking similarity of anole communities on the four islands indicate that the “Darwin experiment” has given the same result four times running?

The striking similarity of anole communities living on the four islands might be explained two different ways:

hypothesis A — lizards migrated between the islands. A specialist anole like the one that lives in grass may have evolved only once, but then travelled to the other islands, perhaps on floating driftwood. If this is true, the similarity of communities is not the result of evolution repeating itself, but just a matter of specialists finding their way to the habitats they prefer.

hypothesis B — lizards evolved in parallel on the four islands. The anole communities on the four islands may have evolved their similarity independently, evolution taking the same course again and again.

Working with Allan Larson of Washington University and Todd Jackman (now at Villanova University), Losos was able to choose between these two hypotheses by looking at the DNA of the lizards. The team compared several genes from more than 50 anole species. Points of similarity allowed them to construct a “phylogenetic tree,” a family tree that showed who was related to who.

If hypothesis A is correct, then all the leaf specialists should be closely related to one another, whatever island they live on. The same would be expected for the four twig species, and also for the branch and canopy species.

On the other hand, if hypothesis B is correct, then a leaf specialist on one island should be more closely related to the other lizards on the same island, regardless of their speciality, than to a leaf specialist on another island.

Has evolution repeated itself? Yes. The DNA data are clear-cut: specialist species on one island are not closely related to the same specialists elsewhere, and are closely related to other anoles inhabiting the same island. Hypothesis B is correct. The four lizard communities evolved independently to be similar to one another.

The Losos research team has gone on to examine the functional consequences of Caribbean anole specializations, to see if natural selection can reasonably explain how each species has evolved. Why do some anole species have long legs, for example, while others have short stubby ones? These studies, involving both field and laboratory experiments, are science at its very best, insightful and fun. The rich picture of lizard evolution that is immerging would have delighted Darwin.

Last night Losos received the St. Louis Academy of Science’s “2001 Innovation Award,” given to a young scientist under age 40 for highly innovative work. It is an honor I am sure Darwin would applaud.

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