The fact that phenotypic evolution can be studied on a character by character basis suggests that the body is composed of locally integrated units. These units can be considered as modular parts of the body which integrate functionally related characters into units of evolutionary 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 explain the differential suppression of pleiotropic effects between different modules and the augmentation of pleiotropic effects among the elements within the module. Four scenarios are discussed: selection for adaptation rate, constructional selection, stabilizing selection and a combination of directional and stabilizing selection. It is concluded that a combination of directional and stabilizing selection is a prevalent mode of selection and a likely explanation for the evolution of modularity.