  We present a model for evolution and extinction in large ecosystems. The
model incorporates the effects of interactions between species and the
influences of abiotic environmental factors. We study the properties of the
model by approximate analytic solution and also by numerical simulation, and
use it to make predictions about the distribution of extinctions and species
lifetimes that we would expect to see in real ecosystems. It should be possible
to test these predictions against the fossil record. The model indicates that a
possible mechanism for mass extinction is the coincidence of a large
coevolutionary avalanche in the ecosystem with a severe environmental
disturbance.
