Speciation patterns and trends in the fossil record

Abstract

Phyletic trends in the fossil record can be explained by alternative models involving anagenesis andcladogenesis, the latter being favoured by the propounders of punctuated equilibrium theory. To render fossil material suitable for evolutionary analysis of morphospecies it is necessary to make allowance for a full range of populational variation. A number of studies suggest that fossil morphospecies are sufficiently representative of biological species to allow valid generalisation about speciation patterns. Punctuated equilibrium, with long periods of stasis interrupted by relatively brief episodes of change, appears to be the dominant mode, but the evidence for producing trends by species selection or sorting is weak. Instead, most of the trends described can be readily attributable either to a combination of anagenesis and heterochrony, or to phyletic size increase. Examples are given from Jurassic ammonites and bivalves, and an attempt made to relate key changes to an ecological model involving changing adaptive strategies related to environmental fluctuations.