The Role of Extinction in Evolution

Extinction plays an important role in evolution. From the fossil record we know that many species have become extinct. Raup (1991) estimates that only one in a thousand species that have ever lived on earth still exist today. He concludes that all species have a relatively low risk of extinction most of the time, but that rare intervals of a vastly higher risk of extinction occur throughout evolutionary history. These so-called mass extinctions, in which a large percentage of the thenexisting species disappears, seem to be separated by periods with lengths of the order of 108 years. Raup argues that extinctions of widespread species probably involve extreme global environmental conditions not normally experienced by species (i.e., to which they are not adapted). Mass extinctions involve huge environmental stresses that cut across ecological lines.

During the past 100 years many species have become extinct. Although extinction apparently is an inescapable factor of life on earth, many recent extinctions probably involve human-related causes. Some people have even voiced the opinion that we may be heading for a new mass extinction because of human interference (see Lawton and May 1995). To avoid extinction, it is important to be able to estimate the vulnerability of species and to have some impression as to which factors contribute to extinction risk and what can be done about them.

On a less dramatic but nevertheless important scale, there are local extinctions of species or loss of genetic diversity from local populations.