Notes on the rates and patterns of size change in evolution

Abstract

The rates and patterns of phyletic body size increase and decrease are discussed on the assumption that size change proceeds toward some limit due to selective advantage. Because the selection pressure must decrease as the average body size of an evolving population approaches the limit, some sigmoidal curve should be regarded as more appropriate for the model of size increase than an exponential curve as in the case of population growth. For phyletic size decrease, the predicted pattern is similar to a radiometric decay curve. The conventional unit of morphological rate, darwin, has been used on the assumption that the change is exponential, but actual lineages in the fossil record may represent only fractions of such sigmoidal curves. Some actual data on size increases in ceratopsian dinosaurs and Jurassic bivalves are examined, and it is concluded that sigmoidal size increase seems also empirically to be a more widespread pattern than exponential.