Homology and synapomorphy-symplesiomorphy—neither synonymous nor equivalent but different perspectives on the same phenomenon

Abstract

In a recent debate, either synapomorphy and symplesiomorphy or only synapomorphy have been claimed to be synonymous or equivalent to homology. In my view, exactly the same relationship exists between homology supported by a congruence test on the one hand and synapomorphy as well as symplesiomorphy on the other hand. Both conditions become established at the same time with the process of rooting of an unrooted topology. I, however, do not consider the concept of homology equal or synonymous to that of synapomorphy and symplesiomorphy. In my view, they represent different perspectives on the same phenomenon, i.e. correspondence by common origin. Homology has no implication on the direction of transformation, whereas symplesiomorphy as “primitive” condition and synapomorphy as “derived” condition refer directly to phylogenesis, the real historical evolutionary process of speciation and transformation. In addition, synapomorphy and symplesiomorphy might also refer to a character state that refers to the absence of a structure/organ, which creates problems with traditional homology concepts. Hennig's terms synapomorphy and symplesiomorphy are necessary and sufficient for the evolutionary interpretation of character states. For what is corroborated in an unrooted topology as the result of a congruence test, I suggest as a new term “synmorphy” because it can well be applied also to those characters where one state represents the absence of a structure/organ. The place for homology in morphological cladistics, however, is restricted to the characterization of the relationship between different character states of one transformation series (i.e. character).