A remarkable feature of the history of life is that so many successful species have died out. Many of the extinctions recorded in the fossil record are of species or large groups of species that were ecologically tolerant and occurred in great numbers in all parts of the world. If these extinctions were caused by slow declines over long periods of time, as Darwin thought, they might be explicable in terms of the cumulative effect of very slight deficiencies or disadvantages. But it is becoming increasingly clear that successful species often die out quickly. This is best documented for the K-T mass extinction because of the extensive field work inspired by the controversy over the cause of that event. Several important biologic groups, including the ammonites and dinosaurs, now appear to have existed at full diversity right up to the K-T boundary (23, 24).
For a species to survive for several million years, as many do, it must be well adapted to the physical and biological stresses normal in its environment. Tree species, for example, that can withstand, or even benefit from, forest fires have presumably evolved this ability because forest fires are common in their environment. It may well be that most species have evolved ways of surviving anything that their environment can throw at them, as long as the stress occurs frequently enough for natural selection to operate. This implies, in turn, that likely causes of extinction of successful species are to be found among stresses that are not experienced on time scales short enough for natural selection to act. The recent Pleistocene glaciation produced very few complete extinctions of species. To be sure, extinction rates during the last deglaciation were high among large mammals and some bird groups but overall, global data (including marine organisms) show the Pleistocene to be on the lefthand tail of the distribution in Fig. 2. A reasonable explanation is that although' the glaciation was associated with marked shifts in climatic regimes, most species were already equipped to cope with the changes by natural physiological tolerance, by having populations in refugia, or by having the ability to migrate to more favorable areas. This appears to be especially true in the marine realm (25). In view of the foregoing, recent hypotheses for extinction caused by the catastrophic effects of extremely rare physical events (e.g., asteroid or comet impact, global volcanism) have great appeal.