Relationship between rates of speciation and phyletic evolution: Stratophenetic data on pelagic conodont chordates and benthic ostracods

Abstract

In the fossil record, distinction between the two basic aspects of evolution, i.e. branching of evolutionarytrees (cladogenesis) and evolutionary transformations along lineages (phyletic evolution, “anagenesis”), is especially well visible. The process of allopatric speciation, presumably the most common, if not the only, way to split a lineage, cannot be recognised directly in any single geological section, as it proceeds in a geographic dimension. It has to be inferred from data preserved in many places, which makes the inference vulnerable to all the limitations of rock correlation. On the contrary, the course of phyletic evolution is potentially observable in a single geological section, with application of the Steno's rule of superposition as the only guide to the time distribution of evolutionary events. However, only rarely and in extremely stable environments can a complete time range of a morphologically recognisable segment of the evolution (chronospecies) be established. The stratophenetically documented phyletic evolution of both pelagic (Ordovician and Carboniferous conodonts) and benthic (Ordovician ostracods) organisms from such unusual sites provides evidence that ranges of chronospecies observed in particular sections represent generally only a minor part of their actual durations. As a result, rates of evolution estimated by counting reported ranges of taxa must appear much higher than they really were. Moreover, biometrically proven rates of evolution invariably appear to be much slower than expected. Ranges of fossils have thus little to do with the evolution of species they represent. In the studied cases, with a good geological time control, there is no evidence of any direct relationship between the speciation rate (or migration events) and the rate of morphological change. Generally, numerous slowly evolving lineages may develop in a varied environment, and a uniform environment may remain inhabited by a few, although fast evolving, lineages. Rates of speciation and extinction mostly reflect features of the physical environment and its transformations, being thus rather unreliable measures of the rate of biological evolution. Perhaps they may offer a measure of the evolution of complexity of ecosystems and/or effectiveness of their biocoenoses in exploiting resources. Nevertheless, they can hardly be used in empirical studies as neither speciations nor extinctions are directly observable in the fossil record and their identification requires a complex inference based on not obvious assumptions.