THE ALLOPATRIC MODEL AND PHYLOGENY IN PALEOZOIC INVERTEBRATES

Time is the one element of paleontological data which mitigates, to a degree, the disadvantages inherent in the fossilization process. Addition of this fourth dimension to evolutionary biology has greatly sharpened out perspective of both rates and modes of evolutionary processes. Paleontologists have understandably emphasized the importance of time in the elaboration of evolutionary models, and have made particularly important contributions in the general area of the origin of higher taxa. The concept of gradualism, an important aspect of geological thinking (see Simpson, 1970), has permeated paleontologic thought to the extent that all phylogenetic change is generally conceived to occur by small increments over vast periods of time. This dominantly phyletic model of transformation, stressing the importance of time and the aggregation of large numbers of small steps of morphological change, has underlain most paleontological discussions of the origin of new taxa, including species. In fact, this phyletic model applies not only to strict cases of phyletic transformation (i.e., linear trends in which arbitrary segments are viewed as "new" taxa), but also to most discussions of divergence, where new branches in a phylogenetic tree are considered as gradually diverging stocks from a parent group. At the species level, the only such level in the taxonomic hierarchy where a taxon can actually be said to exist in nature, such a gradualistic view of the origin of new taxa is, in one sense at least, clearly at odds with currently accepted views of 'speciation derived from studies of the recent biota. On the one hand, while it cannot be denied that a gradual, strictly phyletic, progressive change in a speciesstock has eventually led to populations of individuals sufficiently distinct phenotypically (and probably genotypically) to warrant recognition of a "new" species, a model which would allow single or multiple splittings of the lineage into new species during the same time span is the more satisfactory for the explanation of the diversity of life since the Cambrian. On probabalistic grounds alone, we must conclude that the overwhelming majority of metazoan species that have appeared on the earth's surface arose through some process of splitting. On the other hand, paleontological analyses of lineage splitting have generally dealt only with morphological divergence and do not fully correspond to neontological discussions of speciation. A biological species concept has been incorporated into paleontology only relatively recently (see general discussions by Newell, 1956; Imbrie, 1957, and two excellent recent studies by Waller, 1969; Gould, 1969). Since, in favorable circumstances, paleontologists have been successful in recognizing true "biospecies" on criteria which are as valid and complete as those used to differentiate the majority of recent species, a reappraisal of paleontological models of speciation is called for. Of the various models of speciation proposed and discussed over the past forty years, the allopatric model has gained nearly total acceptance among current evolutionary biologists. I would suggest that the allopatric model (geographic speciation) be substituted in the minds of pale-