DOES THE FOSSIL RECORD SUPPORT GRADUALISTIC OR RECTANGULAR EVOLUTION? DATA FROM GENERATION TIMES AND TAXONOMIC RATES

One of the expectations of phyletic gradualism is that the generation times of organisms are inversely correlated with large-scale evolution. Although this relationship is consistent with the fundamental principles of population genetics, evidence gathered from various clades appears to decisively reject such an association. Among the Mammalia, for example, the large, slowly-maturing elephants evolved far more rapidly than small rodents with much shorter generation times. No correlation is presumed, however, within the alternative framework of the rectangular or punctuational model, in which rates of evolution are primarily controlled by the frequency of speciation. The present work compares neontological data on generation times or roughly equivalent life history traits with rates of adaptive radiation, speciation, extinction, and turnover for a total of 18 higher taxa. Bivariate regression analysis using the reduced major axis method demonstrated a lack of significant correlation between log-transformed generation times and taxonomic rates of evolution. These results are upheld despite simultaneously adjusting for both termination of lineages and pseudoextinction (i.e., the phyletic transition between chronospecies in phylogeny) at incidences ranging from 20–70%. Exclusion of the extinct trilobites and ammonites, whose generation times were approximated by those of their closest living relatives (the horseshoe crab Limulus and the coleoid cephalopods, respectively), also failed to appreciably change the preceding findings. While suggestive of the rectangular model, this preliminary study cannot conclusively discount gradualism because of the potential influence of population size and intensity of natural selection.