A NEW TEST OF THE PUNCTUATIONAL MODEL USING PRESENTLY RADIATING CLADES OF BIVALVE MOLLUSKS AND MAMMALS

Critical tests using higher taxa with remarkably disparate evolutionary properties, such as bivalve mollusks and mammals, may aid in distinguishing between phyletic gradualism and the punctuational model of evolution in the fossil record. Bivariate regression analysis reveals that median bivalve lifespan (a more widely measured proxy for generation time) and the fractional increase in the number of species per million years, R, are significantly positively correlated for thirteen radiating bivalve families and genera, which is precisely the opposite of the relationship predicted by gradualism. These molluscan stocks generally represent a single adaptive radiation made possible by mantle fusion and siphon formation, which allowed for an astonishing expansion of infaunal life habits during the Mesozoic and Cenozoic Eras. Because the elevated turnover at the end of the Cretaceous Period about 65 MYA essentially interrupted the taxonomic diversification of five siphonate families (Veneridae, Mactridae, Tellinidae, Donacidae, and Teredinidae), their times of first appearance were adjusted via a simple rarefaction technique. In spite of this correction, no inverse correlation is detected within the Bivalvia. Nor is any identified linking median generation time and R among seven radiating mammalian families, again contrary to the gradualistic premise. Inasmuch as these results clearly strengthen the empirical validity of the punctuational model, there are, in fact, multiple variables that determine rates of evolution. Ample abundance data from hundreds of living bivalve and mammalian species can theoretically control for the plausible “overriding” effects of population size and stability, but these estimates could themselves be influenced by several factors, notably sampling intensity of existing populations as well as human-induced activity.