A model of mass extinction.

In the last few years a number of authors have suggested that evolution may be a so-called self-organized critical phenomenon, and that critical processes might have a significant effect on the dynamics of ecosystems. In particular it has been suggested that mass extinction may arise through a purely biotic mechanism as the result of "coevolutionary avalanches". In this paper we first explore the empirical evidence which has been put forward in favor of this conclusion. The data center principally around the existence of power-law functional forms in the distribution of the sizes of extinction events and other quantities. We then propose a new mathematical model of mass extinction which does not rely on coevolutionary effects and in which extinction is caused entirely by the action of environmental stress on species. In combination with a simple model of species adaption we show that this process can account for all the observed data without the need to invoke coevolution and critical processes. The model also makes some independent predictions, such as the existence of "aftershock" extinctions in the aftermath of large mass extinction events, which should in theory be testable against the fossil record.