HOMOLOGS, NATURAL KINDS AND THE EVOLUTION OF MODULARITY

The fact that phenotypic evolution can be studied on a character by ch aracter basis suggests that the body is composed of locally integrated units. These units can be considered as modular parts of the body whi ch integrate functionally related characters into units of evolutionar y transformation. These units may either emerge spontaneously by self- organization, or may be the product of natural selection. A selection scenario that could explain the origin of modular units needs to expla in the differential suppression of pleiotropic effects between differe nt modules and the augmentation of pleiotropic effects among the eleme nts within the module. Four scenarios are discussed: selection for ada ptation rate, constructional selection, stabilizing selection and a co mbination of directional and stabilizing selection. It is concluded th at a combination of directional and stabilizing selection is a prevale nt mode of selection and a likely explanation for the evolution of mod ularity.