Bridge species drive tropical engine of biodiversity

Although scientists have known since the middle of the 19th century that the tropics are teeming with species while the poles harbor relatively few, the origin of the most dramatic and pervasive biodiversity on Earth has never been clear. New research sheds light on how that pattern came about. Furthermore, it confirms that the tropics have been and continue to be the Earth's engine of biodiversity. By examining marine bivalves (two-shelled mollusks, including scallops, cockles and oysters), a model system for large-scale ecological and evolutionary analysis, the study shows that most evolutionary lineages started in the tropics and expanded outward. "This 'out of the tropics' dynamic is the major process that shapes the latitudinal pattern of biodiversity that we see today on land and sea," said lead author David Jablonski, the William R. Kenan Jr. Professor in Geophysical Sciences at the University of Chicago. His team focuses on marine bivalves because they combine a wealth of important biological patterns with a large but manageable number of living species (about 8,000) and a rich fossil record.