Morphogenesis and Evolution

Abstract

In the preceding chapters we have built up the premises of an argument concerning morphogenesis and evolution. These are as follows: 1.Evolution occurs through processes of introduction and sorting of variation. 2.In a genealogical hierarchy, introduction and sorting of variation can occur at a series of focal levels and it is a property of the hierarchy that there is upward and downward causation between focal levels. 3.The crucial role of developmental processes with respect to evolutionary mechanisms is in the causation of new phenotypes. Phenotypes are always expressed in individual organisms, but the properties of other focal levels in the genealogical hierarchy must also be considered. 4.Development is also essentially hierarchical, involving processes acting at different focal levels. Each level is defined as the place where new gene expression occurs. For simplicity we can divide its hierarchy into stages from early pattern formation to late cytodifferentiation, each including unknown (but very large) numbers of phases of new gene expression. It must be noted that ontogeny of a given individual or given taxon represents a route through the basic hierarchy of developmental stages that, through historical accident or selective bias, may be extremely convoluted and unpredictable. 5.In the systematics of any group, there is a general correlation between taxonomic rank and different grades or ranks of morphological characters. There is a series of levels or grades of generality of phenotypic characters caused at different levels of the morphogenetic hierarchy. 6.The morphogenetic hierarchy that produces the different grades of phenotypic morphology can potentially involve upward but little if any downward causation. 7.The course of evolution appears principally to produce clusters of evolutionarily equivalent species rather than lines of progressive change. At any taxonomic level, diversification within a group has therefore to be distinguished from those rarer phases of progressive evolution leading to establishment of new groups. The two involve quite different processes because they involve the morphogenetic causation of different levels of morphological characters. 8.Phenotypic characters may occur in groups that are linked both in the sense of functional integration of the phenotype itself and/or by virtue of the integration and interdependency of developmental pathways in morphogenesis.