George Cuvier not only established extinction as fact, he was also the first to recognize that mass extinctions have occurred at the end of what we now call the Paleozoic and Mesozoic eras. Extinction is the total disappearance of a species and is represented by the contraction of a species geographical range and population to the number zero. This contraction is governed by limiting factors. Limiting factors include the physical environment, competition, predation, and chance factors. Climate is one of the most important environmental factors. During short geological periods of time, 2-10 million years, the Earth has experienced the repeated extinction of a large number of species or mass extinctions.

In 1973, Leigh Van Valen published a study that compared the duration of certain groups of organisms against the number that survived. He found that species do not become better at avoiding extinction as they persist through time; old species have the same probability of becoming extinct as young ones. Van Valen called this the Red Queen Hypothesis. The Red Queen in Lewis Carrol’s Alice Through the Looking Glass told Alice that she must keep running to stay in the same place. Thus species must constantly evolve to avoid extinction.

In 1982 David Raup and John Sepkoski, American paleontologists, plotted the number of extinctions in marine invertebrate families per million years. They discovered a steady background rate of 2 to 4 family extinctions per million years. However, five intervals stood out in which 10 to 20 families became extinct.

What is the nature of these background extinctions? These mass extinctions are fundamentally different from normal background extinction. In 1986 Jablonski found that during mass extinctions organisms with extinction-resistance qualities, such as wide geographic distributions, were just as likely to become extinct than those without these properties.