The Biological Homology Concept

Two structures are called homologous if they represent corresponding parts of organisms which are built according to the same body plan (23, 33). The existence of corresponding structures in different species is explained by derivation from a common ancestor that had the same structure as the two species compared (25, 35). The eye of a cow is homologous to the eye of a fish but not to the eye of a squid. Homology is assessed regardless of shape or function. Only morphological equivalence in terms of relative position, struc­ ture, and connections with nerves and blood vessels counts. Among evolutionary biologists, homology has a firm reputation as an elusive concept (27, 44). Nevertheless, homology is still the basic concept of comparative anatomy and has been used successfully in reconstructions of phylogenetic history. A large number of characters are certainly derived from the same structure in a common ancestor and are therefore undoubtedly homologous. One simply cannot escape the conclusion that the brain of a rat and a human are actually the "same" in spite of their obvious differences. However, there are also quite problematic aspects of the current homology concept, which has been in use since the time of Darwin. This is here called the historical homology concept, since it is defined by historical continuity of descent from a common ancestor. The historical homology concept explicitly ignores iterative homology (12, 25), i.e. the correspondence between parts of the same organism, (e.g. the correspondence between two segments of an annelid). More importantly a large body of developmental data seems to contradict certain implications of the current homology concept (see below). This chapter reviews the open biological questions associated with the homology concept. In addition, the different attempts to establish a biological