The Definitions of Species and Clade Names: A Reply to Ghiselin

Ghiselin (1993) has criticized my proposal (de Queiroz 1992) that phylogenetic definitions of taxon names (de Queiroz and Gauthier 1990) provide a means of reconciling the position that taxa are individuals (composite wholes) with the traditional method of defining taxon names in terms of necessary and sufficient properties (intensional definitions), which implies that taxa are classes. As Ghiselin points out, he and I agree as to how systematists are to define the names of species and clades, that is, biological taxa. We also seem to agree that the definitions of taxon names are interpretable as ostensive definitions, and that many of the properties possessed by individual organisms, species, and clades are not defining properties. Where we differ concerns whether the definitions of species and clade names can be considered to be stated in terms of logically necessary (defining) properties, and thus whether they represent a means reconciling traditional and individualist views on the nature of taxa and the definitions of taxon names. Here I will argue 1) that the definitions of species and clade names are indeed stated in terms of defining properties, 2) that Ghiselin's conclusion to the contrary is based on a misinterpretation of what are the defining properties, and 3) that Ghiselin's conclusion concerning the irreconcilability of traditional and individualist views results from his maintaining false dichotomies between intensional and ostensive definitions and between classes and individuals. Ghiselin believes that the properties I considered defining are not logically necessary but simply matters of contingent fact. Although I agree with his proposition that mere use of a property in a definition does not necessarily mean that the property is defining in the sense of being logically necessary, I maintain that the properties I identified are in fact logically necessary. The property I identified as defining the name of a higher taxon, considered as a whole, is being a particular clade. Because of their composite nature, taxa can also be considered in terms of their parts. Thus, the defining property of the name of a