All Caribbean frog species evolved from one South American species
Posted by Miqe on June 7, 2007
Washington, June 7: Nearly all of the 162 land breeding frog species on the Caribbean islands owe their lineage to a single frog species that rafted on a sea voyage from South America about 30-50 million years ago, a new study by Penn State University researchers has revealed.The researchers write in the June 12 issue of the Proceedings of the National Academy of Sciences, that the Central American relatives of these Caribbean frogs also arose from a single species that arrived by raft from South America.
According to Blair Hedges, evolutionary biologist and professor of biology who directed the research, “this discovery is surprising as no previous theories of how the frogs arrived had predicted a single origin for Caribbean terrestrial frogs”.
“Groups of close relatives rarely dominate the fauna of an entire continent or major geographic region. Since land connections among continents allow land-dwelling animals to disperse freely over millions of years, hence the fauna of any one continent is usually a composite of many types of animals,” said Prof. Hedges.
Previously, the anatomy of Caribbean frogs had led to theories that species in Cuba and other western-Caribbean islands were related to different mainland species than were the species on Puerto Rico and other eastern-Caribbean islands.
One prominent theory had proposed that frog species on the large islands of Cuba, Jamaica, Hispaniola, and Puerto Rico had walked there across land bridges that existed when those islands were connected in a geologic arc about 70-to-80-million years ago.
A second major theory proposed that they arrived, instead, by rafting across the Caribbean Sea after the giant asteroid impact near Cuba 65-million years ago, which is widely believed to have exterminated the dinosaurs.
Prof. Hedges said while “both theories acknowledged that the frog faunas must have arrived by rafting over water to the smaller and younger islands, the Lesser Antilles, because they never were connected by land to South America, neither of them proposed that all of the Caribbean island frog species had a single common ancestor”.
As such, “discovering a single origin for all of these species from throughout the Caribbean islands was completely unexpected,” said Prof. Hedges.
Prof. Hedges and coauthor William Duellman, a professor emeritus of the University of Kansas, were involved in much of the fieldwork.
A third co-author of the study, Penn State graduate student Matthew Heinicke, performed DNA sequencing and analyses of nearly 300 species of Caribbean, Central American and South American frogs and used three mitochondrial genes and two nuclear genes to build trees of relationships among the species and timing the divergences of the species with molecular-clock methods.
The DNA research revealed that, while many ocean dispersals might have occurred over time, only two led to the current faunas: one for the Caribbean islands and another for Central America.
“The asteroid impact generated giant waves that devastated the islands, probably eliminating any existing fauna at that time. The original frogs that successfully colonized the Caribbean islands likely hitched a ride on floating mats of vegetation called flotsam, which is the method typically used by land animals to travel across salt water,” said Prof. Hedges.
“Some rafts of flotsam, if they are washed out of rivers during storms and caught in ocean currents, can be more than a mile across and could include plants that trap fresh water and insect food for frogs. It is not likely that the frog species dispersed simply by swimming because frogs dry easily and are not very tolerant of salt water,” he said.