The concept of homology arose from classical studies of comparative morphology, and took on a new significance with the advent of evolutionary theory. It is currently undergoing another metamorphosis: many developmental geneticists now define homology as shared patterns of gene expression. However, this new usage conflates definition with criteria, and fails to recognize that meaningful assignments of homology must specify a biological level. We argue that although developmental genetic data can help identify homologous structures, they are neither necessary nor sufficient, and do not in any case justify a new definition of homology.
