Cold periods accelerate evolution

When Charles Darwin visited the Galápagos Islands in 1835, he was struck by the diversity in bill size exhibited by the local finches. Evolutionary biologists attribute such divergences of species to adaptive radiation, which arises when organisms mitigate competition for resources by branching out into ecological niches. Those niches change when Earth’s climate warms or cools, thereby affecting evolution. But shifts in temperature also influence metabolism and, potentially, evolution too. Julien Clavel and Hélène Morlon of the École Normale Supérieure set out to untangle those two temperature-dependent effects. Their starting point was a family tree of 6100 bird species and 3550 mammal species from the Cenozoic era, which began 66 million years ago with the extinction of the dinosaurs and continues, through periods of advancing and retreating glaciation, to the present. Besides ancestries, the database also contained the species’ body size. Clavel and Morlon developed a statistical model for the rate of evolution that incorporated an adjustable temperature dependence. When they fed their model with Cenozoic climate data and compared it with the body sizes in their family tree, they found the best match occurred with a strong negative dependence of evolution on temperature. That is, body size changed fastest when the climate was coldest. Given that the metabolism of birds and mammals slows when the ambient temperature drops, Clavel and Morlon conclude that climate influences evolution primarily through its effect on the environment. (J. Clavel, H. Morlon, Proc. Natl. Acad. Sci. USA 114, 4183, 2017, doi:10.1073/pnas.1606868114 .)