2005]: ‘Outline of an Explanatory Account of Cladistic Practice

Abstract. A naturalistic account of the strengths and limitations of cladistic practice is offered. The success of cladistics is claimed to be largely rooted in the parsimony-implementing congruence test. Cladists may use the congruence test to iteratively refine assessments of homology, and thereby increase the odds of reliable phylogenetic inference under parsimony. This explanation challenges alternative views which tend to ignore the effects of parsimony on the process of character individuation in systematics. In a related theme, the concept of homeostatic property cluster natural kinds is used to explain why cladistics is well suited to provide a traditional, verbal reference system for the evolutionary properties of species and clades. The advantages of more explicitly probabilistic approaches to phylogenetic inference appear to manifest themselves in situations where evolutionary homeostasis has for the most part broken down, and predictive classifications are no longer possible. Why use parsimony in systematics? In this paper I present certain elements of cladistic practice in a naturalistic philosophical context. There are two major and interrelated themes. On the one hand, my intent is to explain how a particular implementation of the parsimony criterion -the so-called congruence test -is used iteratively to achieve (more) phylogenetically informative observations of similarity. To this end, I interpret statements of homology as natural kind terms in the Quinean sense. The congruence test is portrayed as a means to inductively assess and increase their projectibility. I contend that parsimony considerations affect the process of character selection in individual analyses, and also contribute to the long-term entrenchment of a phylogenetic terminology. This position challenges other accounts that are primarily concerned with the performance of parsimony algorithms vis-a`-vis a range of evolutionary process model