Generation, integration, autonomy: three steps in the evolution of homology.

The homology concept harbours implicit assumptions about the evolution of morphological organization. Homologues are natural units in the construction of organismal body plans. Their origin and maintenance should represent a key element of a comprehensive theory of morphological evolution. Therefore, it is necessary to understand the causation of homology and to investigate the mechanisms underlying its origination. The study of this issue cannot be limited to the molecular level, because there appears to exist no strict correspondence between genetic and morphological evolution. It is argued that the establishment of homology follows three distinct (if overlapping) steps: (a) the generation of morphological building elements; (b) the integration of new elements into a body plan; and (c) the autonomization of integrated construction units as lineage-specific homologues of phenotypic evolution. In contrast with traditional views, it is proposed that the mechanistic basis for steps (a) and (b) is largely epigenetic, i.e. a consequence of the inherent propensities of developmental system under changing conditions. Step (c) transcends the proximate mechanisms underlying the establishment of homologues and makes them independent attractors of morphological organization at the phenotypic level.