Wild yet cosmopolitan, squirrels are a diverse bunch. Some are ground based and some live in trees; a few even fly. Evolutionary biologists now have good evidence that shifting continents and global climate changes have helped create the diversity of the 273 species in this family.
The work began when John Mercer and V. Louise Roth of Duke University in Durham, North Carolina, were examining morphological features in different-sized squirrel species and needed to determine how the species were related. That question turned up an unexpected result: The pygmy squirrel they were studying had branched off earlier than expected from the main tree.
So the researchers expanded their analysis, spending the next decade gathering tissue from museum and live specimens representing 50 of the 51 genera. The work confirmed the researchers' initial view that the pygmy squirrel had branched off early. It also revealed surprises--for instance, that flying squirrels all evolved from a single ancestor and not several, as some had thought.
Mercer and Roth calculated when various genera evolved, based on the number of changes in the DNA bases. Next, they looked for correlations among certain global changes--such as shifting continents--and landmarks in squirrel evolution. They found that key branches of the evolutionary tree sprouted in parallel with geologic and climate changes, they report online this week in Science. In addition to the insights into squirrel life in Africa and South America, the study indicates that in Southeast Asia, a rising sea level broke widespread ancestral genera of tree squirrels into isolated populations, which quickly evolved into separate entities.
The results are exciting because they "provide a plausible and provocative scenario for the diversification of squirrels," says Richard Thorington, a mammalogist at the Smithsonian Institution National Museum of Natural History in Washington, D.C. In addition, the work is part of "a clear trend [to investigate] how speciation events may be explained by geological and environmental changes," notes Emmanuel Douzery of the University of Montpellier in France.
Mercer and Roth's Science paper