Homologues and homology and their related terms in phylogenetic systematics

Abstract

In the field of phylogenetic systematics, the terms homology and homologue and their relationship to cladistic terms such as character, character state, synapomorphy and symplesiomorphy, as well as their relationships to each other, have been and are still discussed frequently. A recent re-emergence of concepts of homology/homologue free of any reference to explanatory hypotheses prompted us to explore these concepts and their relationships to each other as well as to the concept of morpheme, as introduced recently. All concepts are examined with regard to their ontological status and their bearing in the epistemological process in morphology and phylogenetic systematics. To us, morphemes, homologues and in partem character states refer to things (concrete objects in the ontological sense). However, although morphemes are exclusively descriptive, the latter two represent objects of causal explanations. Homologue always refers to the things themselves, yet a character state also can be a property or the absence of a thing. In this context, a character as a transformation series of character states does not represent a thing but a natural kind. Character states of one character are connected by homology relationships, i.e. common descent. Synapomorphy and symplesiomorphy represent different states of a single transformation series. A nonexplanatory, purely descriptive, concept of homologues is contradictory to its original as well as the post-Darwinian, evolutionary, concept which refers to causal relationships between parts of organisms and their correspondences in the archetype or ancestor, respectively. Character states, homologues and synapomorphies/symplesiomorphies can only be approximated in the form of hypotheses. We argue that the high value of the concept of homology and its related concepts for evolutionary biology should be maintained by acknowledging their explanatory nature and that dilution with nonexplanatory (even idealistic) definitions should be avoided.