DNA and Darwin: Research shows that evolution repeats itself in Caribbean
lizards
Source: St. Louis Post-dispatch
Published: 4-6-01 Author: George Johnson

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.

Washington U. team

But not impossible. A team of researchers led by Washington University
biology professor Jonathan Losos has spent the past 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
traveled 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 whom.

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 specialty, than to a leaf specialist on
another island.
 

A clear answer

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 emerging would have delighted
Darwin.

On Thursday 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.
 

George Johnson is a biology professor at Washington University

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