Self-organized criticality, evolution and the fossil extinction record

Statistical analysis indicates that the fossil extinction record is compatible with a distribution of extinction events whose frequency is related to their size by a power law with exponent tau approximate to 2. This result is in agreement with predictions based on self-organized critical models of extinction, and might well be taken as evidence of critical behaviour in terrestrial evolution. We argue however that there is a much simpler explanation for the appearance of a power law in terms of extinctions caused by stresses (either biotic or abiotic) to which species are subjected by their environment. We give an explicit model of this process and discuss its properties and implications for the interpretation of the fossil record.