DEVELOPMENTAL GENETICS AND TRADITIONAL HOMOLOGY

The concept of homology arose from classical studies of comparative mo rphology, and took on a new significance with the advent of evolutiona ry theory. It is currently undergoing another metamorphosis: many deve lopmental geneticists now define homology as shared patterns of gene e xpression. However, this new usage conflates definition with criteria, and fails to recognize that meaningful assignments of homology must s pecify a biological level. We argue that although developmental geneti c data can help identify homologous structures, they are neither neces sary nor sufficient, and do not in any case justify a new definition o f homology.