Major climatic and tectonic episodes can drive speciation; rising sea levels can close land bridges, for instance, while a rise in temperature can promote the expansion of grassland at the expense of forests. In the 20 February Sciencexpress, John Mercer and Louise Roth at Duke University, Durham, North Carolina, US, show that major events in the diversification of squirrels are timed with large-scale changes in the Earth's climate (Sciencexpress, DOI:10.1126/science.1079705, February 21, 2003).

Mercer and Roth sequenced three genes from extant squirrel species (family Sciuridae) from African, Asian, and North and South American ground and tree squirrels. Their broad geographic range and early diversification makes these rodents ideal subjects for examining diversification driven by global changes. Using standard molecular phylogeny techniques, they developed a branched tree indicating relatedness and time since divergence for each of the major groups of the squirrel family. They identified a major divergence event that occurred during the Miocene (18-20 million years ago) when a land bridge existed between Asia and Africa and which coincides with the first known squirrel fossils in Africa. In addition, they correlated the explosive diversification of South Pacific squirrels with a time of extremely low sea level approximately 11 million years ago, which opened many previously isolated islands to colonization.

The authors argue the closeness of these correspondences allow inferences to be drawn in both directions — not only can global changes be used to make predictions about phylogeny, but "reciprocally that sciurid diversification studied on a smaller scale can yield inferences about tectonics, sea level, and climate."