Despite many uncertainties, we can formulate a reasonable statement of the probable role of extinction, containing the following elements.
(i) Extinction of a widespread species, or a widespread group of species, requires an environmental shock (physical or biological) which is not normally encountered during the geological life spans of such species or groups, and the shock must be applied rapidly enough over a broad geographic area to prevent adaptation by natural selection or escape by migration. If the most effective extinction mechanisms are beyond the experience of the victims, a high degree of apparent randomness should be expected. Survivors are most likely to be those organisms which are fortuitously preadapted to an "unexpected" stress (26).
(ii) The most intense episodes of extinction, like the Big Five, produce major restructuring of the biosphere. Threequarters, or more, of the standing diversity is removed, and diversification of the surviving lineages yields a global biosphere very different from that before the extinctions. Previously successful clades are lost, and unlikely survivors expand. Although the extinction does not, by itself, make a creative contribution to the evolution of complex structures such as wings or limbs, it may be decisive in sustaining or eliminating such structures.
The pterosaurs died out in the latest Cretaceous, and reptiles never again achieved powered flight. Did this foster the Tertiary radiation ofbats? What further adaptations might pterosaurs have evolved had they survived? Seen in this light, the major extinctions have a profound influence on the future course of evolution, sometimes constructive and sometimes destructive.
(iii) At lower levels of extinction intensity, Darwin-style selectivity may be relatively common, but except for a few spectacular cases, including Jablonski's studies of the effects of larval development and geographic range (17), we do not have enough hard evidence to claim that low-level extinction has anything approaching the importance given it by Darwin. Further studies in this area, under the rubric of species selection, are sorely needed.
I thank David Jablonski for many helpful discussions and for his helpful comments on the manuscript. This research was supported by National Aeronautics and Space Administration Grants NAGW-1508 and NAGW-1527.