Study Illustrates Diversification, Speciation In Biological "Islands"

The bigger the island, the faster species proliferate and diversify.

Jonathan B. Losos, Ph.D., associate professor of biology in Arts and Sciences at Washington University in St. Louis, proved this species-area relationship in a study of 143 species of Caribbean Anolis lizards on 147 islands. Focusing on the four largest islands, Cuba, Hispaniola, Jamaica and Puerto Rico, collectively known as the Greater Antilles, Losos showed that the diversity of lizard species is primarily a result of the evolutionary process of speciation, rather than the ecological processes of colonization and extinction.

Losos, and co-author Dolph Schluter, Ph.D., professor of biology at the University of British Columbia, published these results in the Dec. 14, 2000 issue of Nature. The study is an important and novel extension of a 33 year old theory on the genesis of biological diversity.

"When you focus on the larger islands, the rate of speciation is a function of island area," said Losos. "A large island equals more speciation events. At some level this is intuitive, but it has never been demonstrated before that differences in the rate of speciation, of evolution, can produce the species-area relationship."

Losos and Schluter’s results complement the well-known "Equilibrium Theory of Island Biogeography," proposed in 1967 by the late Robert MacArthur of Princeton University and E.O. Wilson of Harvard University. MacArthur and Wilson’s ecological theory proposed that the number of species on any island reflects a balance between the rate at which new species colonize it and the rate at which populations of established species become extinct.

An "island" in this sense is not strictly an island in a stream or ocean, but any ecosystem, say a forest, surrounded by barriers. A major component of this theory is that the rate of extinction of most species on large islands is lower than on small islands, and if everything else is equal, then a relationship is observed between the area of an island and the number of species occurring on that island. But Losos and Schluter have shown that evolution can be just as important as colonization and extinction in producing the species-area relationship.

MacArthur and Wilson were unable to address the role that evolution plays in producing the species-area relationship because the appropriate data were unavailable until recently. In order to address such evolutionary questions, Losos and Schluter used the Caribbean lizard phylogeny, or genetic evolutionary family tree, of Caribbean lizards to estimate the number of immigration and speciation events on the islands.

"The phylogeny is indispensable," Losos said. "Only with the understanding of evolutionary relationships of the species can we address these sorts of questions. The critical step was the work done in Allan Larson’s laboratory here at Washington University identifying the evolutionary relationships among these lizards."

Todd Jackman, Ph.D., assistant professor of biology at Villanova University, reconstructed the phylogeny while he was a postdoctoral fellow working in the laboratory of Allan Larson, Ph.D., professor of biology in Arts and Sciences at Washington University.

Given that a species-area relationship exists in the Caribbean, Losos and Schluter provide an explanation for why larger islands have more speciation events.

"There is simply more opportunity for isolation to occur and for species to diverge on larger islands," said Losos.

The classic explanation of how speciation occurs is that one species gets separated into two or more geographically isolated groups, between which there is no genetic contact. They are therefore geographically and genetically isolated; they are not interbreeding. Over time, the groups diverge so that even if the geographic barrier that caused the isolation was removed, they are now separate species and cannot interbreed.

An unexpected finding reported by Losos and Schluter is the existence of an island area threshold of 3000 square kilometers (roughly 1,800 square miles), below which speciation does not occur. It is only on the four largest Caribbean islands that there is evidence of speciation. This finding, according to Losos, has never been convincingly demonstrated until now, and, as is often the case in science, raises more questions than it answers.

"We don’t know why the threshold is there," said Losos. "The islands of Guadeloupe and Martinique are quite large and vegetationally diverse, there are plenty of habitats for lizards to exploit, and yet speciation has not occurred there.

"This new work fleshes out the ecological theories of MacArthur and Wilson and others," Losos added. "And it gives a fuller, richer understanding of what causes species diversity."