The Ontogenetic Method for Determining Character Polarity and its Relevance to Phylogenetic Systematics

Abstract

In an attempt to clarify the relevance of ontogenetic transformations for system­ atics, the ontogenetic method for determining character polarity (the biogenetic law of Nelson, 1978) is analyzed from the perspective of phylogenetic systematics. In phylogenetic systematics, as defined here, the relationships sought are those of common ancestry and, thus, the concept of phylogeny is taken as an axiom from which systematic methods are deduced. This perspective has a number of consequences concerning the role of ontogenetic transformations in system­ atics, among which are the following: (1) Von Baer's second law, which states that less general characters are developed from the most general, is not universally true. (2) The validity of Nelson's biogenetic law (not to be confused with other concepts of similar name) does not depend on the validity of von Baer's law. (3) As a theory about the relationship between on­ togeny and phylogeny, Nelson's biogenetic law can only be tested by known character phylog­ enies. However, outgroup, paleontological, and ontogenetic methods of polarity determination need not be interpreted as scientific theories; instead, they can be interpreted as theorems deduced from the axiom of phylogeny and certain auxiliary assumptions. (4) The usefulness of the ontogenetic method rests on an assumption of ancestral character retention. If ancestral characters are retained in descendant ontogenies, then ancestral characters will be more general than their phylogenetic derivatives. (5) The sequence of ontogenetic transformation is irrelevant to the usefulness of the ontogenetic method; generality is the critical factor. (6) An ontoge­ netic method based on generality may be useful for determining evolutionary polarity when characters are instantaneous morphologies, but ontogenetic transformations rather than instan­ taneous morphologies are more appropriately considered characters when attempting to deter­ mine phylogenetic relationships among organisms. When ontogenetic transformations are viewed as characters, there can be no ontogenetic method for determining evolutionary character po­ larity; however, the comparative phylogenetic method properly involves a comparison of on­ togenetic transformations. (7) Ontogenetic polarities are different than phylogenetic polarities; the two have the relationship of part to whole, respectively. (8) For characters that exhibit ontogenetic transformation, homology is distinct from synapomorphy. (9) Finally, there is no threefold parallelism in phylogenetic systematics. Comparative anatomy, paleontology, and em­ bryology are not three separate disciplines within systematics; rather, the three form a single comparative method unified in the organism by the concept of evolution. (Biogenetic law; von Baer's law; cladistics; character; evolution; generality; homology; ontogenetic method; ontogeny; outgroup method; paedomorphosis; parsimony; paleontological method; phylogenetic system­ atics; phylogeny; polarity; semaphoront; synapomorphy; threefold parallelism.)