Instead of hopping from Central and South America, frogs arrived in the Caribbean islands by raft, the authors of a new genetic study argue. Frogs made the trip only 50 million years ago, the researchers report, and as the land bridges to the mainland had disappeared by then, the little amphibians must have traveled by sea, most likely on floating mats of vegetation.
Most of the world's frog species are not green, pond-dwelling creatures but centimeters-long, colorful jumpers that live in isolated jungle mountaintops. Biologists have lumped about 800 species from Central America and the Caribbean into one genus--Eleutherodactylus--making it the most diverse vertebrate genus. But because such frogs are so rarely spotted, scientists have never before traced their evolutionary history or determined how each geographically isolated species is related.
After smearing on copious bug repellent, biologist S. Blair Hedges and colleagues at Pennsylvania State University in State College collected tissue from almost 300 kinds of frogs in this group. By analyzing five genes, they found that the frogs in Central America are in a distinct part of the family tree from those on Caribbean islands. And the frogs on the islands are more closely related to each other than to frogs anywhere else, meaning they all evolved from one ancestral population, which rafted from island to island and developed into all the various species. Previously, scientists assumed that frogs on different islands evolved at different times from separate South American emigrants.
Even more surprising was that the speciation occurred only 50 million years ago, more recently than the existence of a land bridge between any Caribbean islands and Central and South America. This means frogs must have traversed the ocean to get there, the researchers report today in the Proceedings of the National Academy of Sciences. Swimming was not an option for the freshwater frogs, Hedges says: "They would have dried out and died." Rather, he says, they likely floated on bundles of decaying plants, reeds, and roots, which can stretch for a mile and can surf along the ocean currents. Either eggs or a population of adult frogs could have hitched a ride, Hedges says, perhaps in orchidlike plants that hold rainwater or in a rotten log.
Most scientists think that frogs rarely cross large geographical barriers, but the new finding changes the picture, says Franky Bossuyt, a specialist in amphibian evolution at the Free University of Brussels in Belgium. Amphibian biologists now face the challenge of more fully explaining how the frogs evolved from island to island. Evidence from other plant and animal groups that may have coevolved with frogs can help solidify the timeline, Bossuyt says.