HOMOLOGY, HOX GENES, AND DEVELOPMENTAL INTEGRATION

The establishment and inheritance of individualized structural units i s a key feature of morphological evolution, embodied in the concept of homology. In current debates, homology is often equated with identica l genetic encoding. The empirical evidence for this assumption is ambi guous. Genetic identity can indicate morphological identity in some ca ses, but several examples show that gene expression patterns and regul atory systems of development may be highly conserved while morphologic al characters undergo dramatic evolutionary innovation. This indicates some independence of structural homology from its genetic and develop mental makeup. It is proposed that phenotypic evolution depends strong ly on the epigenetic context in which genetic redundancy becomes avail able for the control of new developmental interactions. The integrated character of developmental systems may represent an important factor in the origin and identity of morphological characters and can stabili ze incipient structures before their full genetic integration, The ori gin of the autopod section of the tetrapod limb is an example which su ggests that novel homologues can arise in evolution as a consequence o f changing the epigenetic context of conserved gene function.