Relating Rates of Global Change, Evolutionary Adaptation, and Extinction

It is widely assumed that extinction occurs when environmental change outpaces a species' capacity to adapt. However, this hypothesis lacks support at the scale of global change, in part because the distribution of adaptation rates is unknown. Here, we test this idea by formulating a general model that predicts the distribution of adaptation rates across species. By assuming that species go extinct when they adapt too slowly, we derive a precise sigmoidal relationship between the rate of extinction and the rate of environmental change. We then show that above-background extinction rates in the fossil record follow the same sigmoidal response to global carbon-cycle change, indicating that the adaptation-rate distribution is effectively a distribution of critical thresholds. The inferred range of adaptation rates is similar to the spread of extreme rates of environmental change. This suggests that macroevolution may align the diversity of adaptation rates with environmental forcing, thereby setting the biosphere's sensitivity to global change. When rescaled to the slow rates of the geologic past, modern rates of environmental change appear to be below, but near, the point of maximal extinction susceptibility.