Evolutionary Radiations and Extinctions

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Evolutionary turnover is pervasive in the fossil record. Species appear, persist for some interval of time (with or without change), and then become extinct. Mean durations in the Phanerozoic fossil record vary from 1–10 Ma (6); the mean residence time of species is therefore short, relative to geologic time. The frequency distribution of durations is highly skewed; short-lived taxa are far more common than long-lived taxa. Since many short duration species are not preserved, the actual mean duration is probably considerably shorter than the measured estimates. As Van Valen (13) and others have shown, the extinction of species is often rather similar to the decay of radioactive isotopes; that is, the proportion of species that become extinct in successive time intervals of equal length is essentially constant. Major exceptions to this pattern occur as a consequence of mass extinction events.