Classes and Cladists

Beatty, J. (Department of the History of Science, Harvard University, Cambridge, Massachusetts 02138) 1982. Classes and cladists. Syst. Zool., 31:25-34.-Disillusions concerning evolutionary theorizing have forced a split among cladists. The disillusioned group of "pattern" cladists seeks an evolutionarily neutral brand of cladistics. But pattern cladistics is not, after all, evolutionarily neutral. Rather, it is at odds with evolutionary theorizing. [Classes; cladistics; evolution; phylogenetics; Popper.] Hull (1976, 1978) has argued that if species are interpreted as classes, as is traditional, then a little problem arisesnamely, species cannot evolve. But according to our best accounts, they do. Thus there is an inconsistency in our understanding of species. In Section 1, I reformulate the difficulty in order to reemphasize the problem. My ultimate purpose, however, is to discuss the differences between two ever more distinguishable groups of cladists in terms of the difficulty that this problem poses for each. In Section 2, I discuss the background to the widening split between phylogenetic and pattern cladists. Finally, in Section 3, I argue that pattern cladism has conceptual drawbacks that stem from the problem outlined in Section 1. 1. Evolutionary Theory vs. a School of Systematics 1.1The sort of interpretation of species that Hull finds most objectionable from the viewpoint of evolutionary biology is one according to which a species name is defined in terms of the set of properties operationally employed to recognize and distinguish members of that.species from other species. Thus, we recognize and distinguish polar bears (Ursus maritimus) from brown bears (Ursus arctos) by, among other traits, their white coats. Consequently, we might include the differentiating property "white coat" in the definition of "Ursus maritimus." We would also include the other properties that distinguish maritimus from other species of Ursus, as well as those properties that distinguish Ursus from other genera of its family, and the properties that distinguish its family from other families of its order, and so on. To say that species so designated are "classes" is just to say that they are collections of objects that share the defining properties of the species name. Two kinds of classes, hence two kinds of class interpretations of species, are commonly distinguished (e.g., Beckner, 1959, Chapter 5; Caplan, 1980). The name of an "Aristotelian class" is defined in terms of properties that are collectively necessary and sufficient for membership in the group. Members of a "cluster class," on the other hand, need only satisfy most of the defining properties of the class name. So any defining property of the name of an Aristotelian class is present in 100% of the members of that class, while any defining property of a cluster class is present in a high proportion of members of the class. One can imagine still other sorts of class interpretations of species. For example, the defining properties of a species name need not be the usual sorts of properties operationally used to recognize and distinguish members of different species. They might instead be relational properties like interbreeding and ancestor-descendant relations specified with regard to model members of the group. Alternative class interpretations like the latter can be worked out. But it is the interpretation in terms of the usual recognition properties that is at issue in Hull's work and in the recent division