MACROEVOLUTION AND THE FOSSIL RECORD.

The past few years have been marked by enlivened controversy about the tempo of evolution, the legitimacy of the concept of macroevolution, and the history of these and related questions. At the core of the debate is what has become known as punctuational evolution. I have suggested that the punctuational model be defined as the assertion that most evolution is associated with speciation by way of localized populations (Stanley, 1979 p. 16). The traditional, gradualistic model represents the alternative position, that most evolution occurs within established species. The known fossil record, in failing to document important gradual evolutionary transitions, has long been recognized to have a punctuational appearance. Darwin (1859) claimed that the record itself is at fault here, rather than the gradualistic scheme of evolution that he proposed, and Darwin's view prevailed for more than a century. A decade ago, the punctuational model attracted attention through the advocacy of Eldredge (1971) and Eldredge and Gould (1972), who favored it as being most compatible with biological concepts of speciation. Certainly it is not compatible with many (and I would argue most) traditional concepts of speciation. Dobzhansky (1972) referred to the slow divergence of new species from their ancestors as the "usual and by now orthodox view," and earlier Huxley (1942 p. 389) wrote: "Species formation constitutes one aspect of evolution; but a large fraction of it is in a sense an accident, a biological luxury, without bearing on the major and continuing trends of the evolutionary process. " In fact, it has been Mayr (1954, 1963, 1970) who since mid-century has argued most forcefully for the importance of rapidly divergent speciation events. Mayr (1954) suggested that sudden appearances of evolutionary novelties in the fossil record have resulted from rapid transitions in local areas. Rapid transformation of entire species after long periods of stasis is an alternative to rapidly divergent speciation, and in this paper I will also examine this alternative, which some might include within the punctuational model. My interest in the punctuational question was stimulated by statements of earlier workers that the fossil record's testimony is equivocal. Believing that the analysis of fossil data could indeed resolve the issue, I proposed formal tests, whose outcomes favored the punctuational model (Stanley, 1975). The most general of these tests, more fully developed in a later contribution (Stanley, 1979), relates to the great longevity of species in the fossil record. This test is labeled the test of adaptive radiation because its conclusion is that during a typical adaptive radiation, major evolutionary transitions have occurred during geological intervals that have been brief relative to intervals over which species produced during the radiation have survived almost without change. Established species are evolving so slowly that major transitions between genera and higher taxa must be occurring within small, rapidly evolving populations that leave no legible fossil record. In the present contribution I will describe new evidence that strengthens the verdict of the test of adaptive radiation in favor of the punctuational model.