Philosophy and the Transformation of Cladistics

Abstract

Platnick, N. I. (Department of Entomology, The American Museum of Natural History, New York, New York 10024) 1979. Philosophy and the transformation of cladistics. Syst. Zool. 28:537-546.-Although Hennig presented cladistic methods by referring to a model of the evolutionary process, neither the value nor the success of the methods is limited by the value or success of that evolutionary model. Dichotomous cladograms can be preferred simply on the basis of their maximal information content, without reference to speciation mechanisms. Because only the interrelationships of diagnosable taxa (those with unique sets of apomorphic characters) can be investigated, questions about whether speciation can occur without branching, or whether species become extinct at branching points, are irrelevant to cladistic practice. The distinction between plesiomorphic and apomorphic character states depends not on the reconstruction of actual evolutionary history, but on the discrimination of more general from less general characters; groups based on plesiomorphy are defined by the absence of characters and are therefore artificial. Hence cladistic methods are not the methods of phylogenetics per se, but the methods of natural classification in general; phylogenetic conclusions are an extrapolation from hypotheses about natural order. [Cladistics; phylogenetics; characters; natural classification.] At least two students of current systematic theory have suggested that there has been in recent years a transformation within phylogenetic systematics or cladistics. David Hull, in a paper delivered at the 1977 Toronto meeting of the Society of Systematic Zoology (Hull, 1979), distinguished between early views of cladistics as a means of reconstructing evolutionary history and later, more general views of cladistics as a means of discerning natural order in any system that involves some sort of descent with modification. A somewhat different division within the ranks of cladists was subsequently postulated by George Simpson (1978a) in a review of the recent NATO symposium on vertebrate evolution. Simpson distinguished between what he called canonical and non-canonical versions of cladistics on the basis of whether their proponents do or do not regard cladograms as being equivalent to phylogenetic trees. The first question that comes to mind, of course, is a historical one: has such a transformation actually occurred? In this regard, it's interesting that even though Hull and Simpson distinguished their respective groups of cladists at least partly on different grounds (or characters), both classifications do cluster many of the same workers together. On the basis of this congruence, the two groups of cladists might be regarded as natural ones, and hence as real reflections of history. At any rate, the possible extent of the differences between classical and modern cladistics is highlighted in another statement made by Simpson in his recently published autobiography (1978b: 271): The main principles of the Hennigian system are: first, that the basic process of organic evolution (phylogeny) is the splitting (dichotomy) of an ancestral species into two descendant species; second, that each dichotomy should be taken as marking the origin of two new units (taxa) of classification; and third, that the hierarchic level of such units (whether species, genera, families, etc.) should be determined by the geological time when the dichotomy occurred, the earlier the time, the higher the level. Having thus discovered what the main principles of the Hennigian system are, Simpson goes on, of course, to demolish them: I will just say that the first principle, as given above, an apparent statement of fact, is not true and that the second and third principles, statements of opinion, are inane.