Defining sameness: historical, biological, and generative homology.

Current debate concerning homology arises from three different research interests-phylogenetics, character evolution, and generative pathways. Phylogenetic homology focuses on descent of the character from a common ancestor. Biological homology addresses character evolution and diversification. Exceptions to the general case complicate these two approaches: historically and biologically homologous characters may be produced by different generative pathways, and minutely similar characters produced by the same generative pathways may have a sporadic phylogenetic distribution. We suggest that for studies of comparative developmental biology, new descriptive terms are needed to distinguish similar structures that result from the same generative pathways from those that result from different generative pathways. The terms syngeny, meaning "same genesis", and allogeny, meaning "different genesis", allow the acknowledgement of sameness at the generative level and can be used in combination with the terminology of historical homology and biological homology to describe any given character.