Homology, Hox Genes, and Developmental Integration

The establishment and inheritance of individualized structuralunits is a key feature of morphological evolution, embodiedin the concept of homology. In current debates, homology isoften equated with identical genetic encoding. The empiricalevidence for this assumption is ambiguous. Genetic identitycan indicate morphological identity in some cases, but severalexamples show that gene expression patterns and regulatory systemsof development may be highly conserved while morphological charactersundergo dramatic evolutionary innovation. This indicates someindependence of structural homology from its genetic and developmentalmakeup. It is proposed that phenotypic evolution depends stronglyon the epigenetic context in which genetic redundancy becomesavailable for the control of new developmental interactions.The integrated character of developmental systems may representan important factor in the origin and identity of morphologicalcharacters and can stabilize incipient structures before theirfull genetic integration. The origin of the autopod sectionof the tetrapod limb is an example which suggests that novelhomologues can arise in evolution as a consequence of changingthe epigenetic context of conserved gene function.