The contemporary concepts of homology in biology: A theoretical review

Abstract

A brief review of the contemporary theoretical concepts of homology being developed basically in systematics and phylogenetics as well as in developmental biology is presented. Ontologically, both homology and analogy represent a kind of correspondence considered from the standpoint of nominalism, realism, and conceptualism. According to their nominalistic treatment, both are described by a set-theory approximation which makes them classes (in the logical sense). The realistic treatment provides their holistic view according to which a homologue is an anatomical or evolutionary singular while analogue remains a class. The conceptualistic treatment means that there are real (objective) correspondences existing among real (objective) entities while fixation of any of them is based on certain theoretical presumptions adopted by a researcher; homology as a natural kind (including homeostatic property cluster) seems to be most consistent with such a treatment. Realistic view of homology makes it "absolute", while two others make discrimination of homology and analogy strictly relative. Two basic general homology concepts have been developed in recent literature--taxic and transformational ones; the first considers respective correspondences as structure relations, the second as process relations. The taxic homology is nearly the same as classical typological one (Owen), while transformational homology unites all its phylogenetic, ontogenetic (developmental) and transformation-typological definitions. Process-structuralistic approach seems to unite both taxic and transformational ones. The latter makes it possible to apply general homology concept not only to structures but to processes as well. It is stressed that homology is not identical to the similarity, the latter being just the means for revealing the former. Some closer consideration is given to phylogenetic, ontogenetic and genetic treatments of homology; significant uncertainty is shown to exist between them which causes the "homology problem". Epistemologically, any homology statement has a status of hypothesis which makes such a statement theory-dependent according to the hypothetic-deductive argumentation scheme. This dependence allows to stress once more the relative nature of homology and analogy correspondences. Some questions concerning operational concepts and criteria of homology are considered. A hierarchical concept of homology seems to be the most promising prospect of future development of the "homology problem".