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...

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