Homology in the nervous system: of characters, embryology and levels of analysis.

The establishment of homologies is critically dependent upon the process of character identification. Valid characters must reliably appear in many individuals and be delimitable from other characters. They are not defined by any essential attributes, but rather by the formation of distinct clusters in a multidimensional morphospace. Features in two or more species can be considered possible homologues only if they are identifiable as the same character, for it would be nonsensical to homologize them as different characters. In order to confirm that a character is indeed homologous between species, one must examine its phylogenetic distribution to determine that it is unlikely to have evolved several times independently in the taxa being compared. This method of homologue identification can be applied to embryonic as well as adult characters and to characters at various levels of organization, including cell types and cellular aggregates. Difficulties arise, however, when one attempts to link the homology of adult characters to that of their embryonic precursors, or the homology of cellular aggregates to that of their constituent cell types. These efforts are misguided because different characters cannot be homologized to each other (as different characters). This perspective suggests that many neural characters may lack homologues, and therefore be truly novel, in other taxa.