In Tempo and Mode (5), Simpson detailed what he considered to be the most important determinants of evolution. These were (chapter II) variability, mutation rate, character ofmutations, generation length, population size, and natural selection. But missing from this chapter is any indication that extinction plays an important role in evolution. To be sure, chapter II includes occasional mention of specific extinctions, but not as significant drivers of evolution. For example, Simpson suggests that mammals with long generation times (equated with large body size) suffered greater extinction in the latest Pleistocene because natural selection could not operate quickly enough for adaptation to changing climatic conditions. But the implications of this are not developed, and Simpson clearly did not share Darwin's view that extinction is a vital part of the evolutionary process. Elsewhere in Tempo and Mode, however, Simpson noted that major extinctions provide opportunities (space, ecological niches, etc.) for later diversification by the survivors. In sharp contrast to Darwin's view, Simpson saw interspecies competition as only rarely the cause of extinction of species or larger groups. He thought replacement of one group by another was generally passive.
In the history of life it is a striking fact that major changes in the taxonomic groups occupying various ecological positions do not, as a rule, resultfrom direct competition of the groups concerned in each case and the survival of the fittest, as most students would assume a priori. On the contrary, the usual sequence is for one dominant group to die out, leaving the zone empty, before the other group becomes abundant.... (ref. 5, p. 212; emphasis added) As examples of passive replacement, Simpson lists the extinction of the ichthyosaurs millions of years before being replaced by cetaceans, the gap between the extinction of the pterodactyls and diversification of bats, and the fact that dinosaurs died out before the radiation of large terrestrial mammals. In the latter case, he notes the not uncommon observation that the successor group (diverse mammals) had been in existence, albeit not thriving, through much of the dinosaur reign.
Simpson has relatively little to say about whether extinction is sudden or gradual. Mass extinctions are acknowledged, but he follows Darwin in arguing that gaps, saltations, and other abrupt changes in the fossil record should not be taken at face value.
Probably there is always a considerable period of time corresponding with the gap in morphology, taxonomy, and phylogeny. It is impossible to prove that there are no exceptions to this generalization, so that there is some danger that it may represent the statement of an a priori postulate rather than evidence for the postulate; but I believe that this is a valid deduction from the facts. (p. 111)
Taken as a whole, Simpson's treatment of extinction is very different from Darwin's: they differ in their perceptions of the causes of extinction (but not the rates) and, above all, in the role of extinction in evolution.
Despite the foregoing, Simpson made important and lasting contributions to the study of extinction in the fossil record through his use of survivorship analysis above the species level. His now-classic comparison of the slopes of survivorship curves for bivalve mollusks and carnivorous mammals set the stage for development of the technique by Van Valen (6) and many others. Much of our synoptic knowledge of extinction today relies on an expanded use of these techniques.