Eugene Weber/CalPhotos

Sailing the seas.
Many bivalves, such as this oyster, first appeared in tropical zones before dispersing elsewhere.

Tropical Beginnings

In the tropics, which wrap around swaths of nearly every continent, animal and plant diversity is abundant, and it declines as one moves toward the North and South poles. Why this difference? Many evolutionary biologists favor one of two explanations: Either the tropics are where most new species originate, or species are less likely to go extinct there than elsewhere. New research suggests that both are true. The finding indicates that the tropics drive biodiversity and that extinctions there may have especially devastating effects.

University of Chicago evolutionary biologist David Jablonski and colleagues reached those conclusions after 10 years studying fossils of bivalves, marine and freshwater mollusks with two-part hinged shells. The researchers mapped the global geographic distribution and evolution of the shellfish over the past 11 million years. They broke the world into 10-degree bands they called "latitudinal bins" and then tracked how 147 lineages of bivalves deployed in each band over time. "We tracked back through geological time," Jablonski says, "looking at ... where they started and where they migrated to."

They found that the tropics are the source of most of the world's bivalve biodiversity. More than two-thirds made their first appearance there. Many then migrated to other regions. The tropics "are net exporters of biodiversity, so damaging biological diversity in the tropics cuts biological evolution off at its roots," Jablonski says. Their paper appears in the 6 October issue of Science.

The work "cuts to the very core of our knowledge about evolution," says Stuart Pimm, an evolutionary biologist at Duke University in Durham, North Carolina. The results of this study have practical as well as theoretical implications, Pimm says. "When you start hurting the tropics, you are cutting off the source of species for other places."

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Posted in Evolution