Defining sameness: historical, biological, and generative homology

Current debate concerning homology arises from three different research int erests-phylogenetics, character evolution, and generative pathways. Phyloge netic homology focuses on descent of the character from a common ancestor. Biological homology addresses character evolution and diversification. Exce ptions to the general case complicate these two approaches: historically an d biologically homologous characters may be produced by different generativ e pathways, and minutely similar characters produced by the same generative pathways may have a sporadic phylogenetic distribution. We suggest that fo r studies of comparative developmental biology, new descriptive terms are n eeded to distinguish similar structures that result from the same generativ e pathways from those that result from different generative pathways. The t erms syngeny, meaning "same genesis", and allogeny, meaning "different gene sis", allow the acknowledgement of sameness at the generative level and can be used in combination with the terminology of historical homology and bio logical homology to describe any given character. BioEssays 22:846-853, 200 0. (C) 2000 John Wiley & Sons, Inc.