Tropical diversity came with time.

Species in the richly diverse tropics don't evolve any faster than do species in temperate zones, researchers report.

Rather, the tropics accumulated its astounding abundance of species largely because life has thrived there so long.

Why the low latitudes teem, flutter, buzz, and slither with so much more diversity than the temperate zones do is a long-standing question. Biologists have proposed a rich abundance of hypotheses, notes John Wiens of the State University of New York at Stony Brook. In the 1990s, several researchers devised versions of what Wiens calls the tropical-conservation hypothesis. It argues that many of the tropics' species-rich lineages originated there and were slow in colonizing the temperate zones that have killer winters. Thus, life in the tropics has had longer to diversify.

Wiens and his colleagues tested the idea in tree frogs. Working with a researcher at San Diego State University, the Stony Brook team constructed and analyzed a family tree of 124 species of tree frogs.

The analysis roots the tree in tropical South America. The researchers found that the longer a lineage lived in any region, the more likely it was to have diversified into lots of species. They also report that tropical lineages didn't branch any faster than the temperate ones. The team's findings appear in the November American Naturalist.