ABSTRACT
Though controversy over the rôle of stasis in the total evolutionary pattern continues, most workers now accept that many species persisted for long periods of time with little morphological change. Many extant species are relatively invariant over their geographical ranges. These two phenomena, which may be regarded as species-level integrity in time and space, have been considered to be the result of either ‘stabilising selection’ or ‘constraint’. In the former explanation the environment plays the dominant rôle, but in the latter, intrinsic characteristics are dominant. It is now argued that these two mechanisms act in concert and that various genetic and developmental constraints drive and mediate stabilising selection.
