It is usually believed that Darwin's theory leads to a smooth gradual
evolution, so that mass extinctions must be caused by external shocks. However,
it has recently been argued that mass extinctions arise from the intrinsic
dynamics of Darwinian evolution. Species become extinct when swept by
intermittent avalanches propagating through the global ecology. These ideas are
made concrete through studies of simple mathematical models of coevolving
species. The models exhibit self-organized criticality and describe some
general features of the extinction pattern in the fossil record.Comment: 17 pages uuencoded with style file lamuphys.sty. 9 figures not
included but can be obtained via [email protected]. to appear in ``Physics
of Biological Systems'' Lecture Notes in Physics (Springer-Verlag, Heidelberg
, 1996
