Linnaeus and the PhyloCode : where are the differences?

On stability Phylogenetic nomenclature (PN) was introduced in a series of papers by de Queiroz & Gauthier (1990, 1992, 1994), and has developed into an unofficial code of biological nomenclature, the PhyloCode (Cantino & de Queiroz, 2000). Yet, PN has been recently criticised by some authors (Benton, 2000; Nixon & Carpenter, 2000; see also Forey, 2001), because of its supposed instability if compared to the traditional (“Linnean”) system. Indeed, when “stability” is measured in terms of “taxa included in a group referred to by a name” (Nixon & Carpenter, 2000: 301), PN is highly unstable, since a taxon whose name is defined under the PN system can present great differences in inclusiveness depending on which phylogenetic hypothesis is adopted as a template (see examples in Dominguez & Wheeler, 1997; Benton, 2000). However, for de Queiroz & Gauthier (1994), “stability” means that “a name should not designate different taxa, nor a taxon be designated by different names”. Central to the differences between Nixon & Carpenter’s and de Queiroz & Gauthier’s concepts of stability are the notions of “taxon name definition” and “taxon circumscription” (de Queiroz, 1992; Stuessy, 2000). As stated by de Queiroz (1992), the definition of a taxon name simply serves the purpose of “specifying the meaning of a symbol” (see also Ghiselin, 1966; Løvtrup, 1987; Härlin & Sundberg, 1998). Taxonomic circumscription, on the other hand, is the determination of which biological entities are included in a given taxon (de Queiroz, 2000; Stuessy, 2000). Accordingly, a taxon is first given its name (which is typified and given a definition), followed by circumscription based on aggregation of the entities that conform to that definition. In the present paper, following de Queiroz (1992), taxa (or taxonomic) definitions are used solely to describe the first of those procedures. The “stability” of de Queiroz & Gauthier refers to the definition of taxon names, whereas that of Nixon & Carpenter refers to the circumscription of a taxon. Certainly, Nixon & Carpenter dismissed the stability of de Queiroz & Gauthier as metaphysical, and so it is. It refers to definitions, which have “nothing whatever to do with the practical problem of identifying the things that might happen to fit the definition” (Ghiselin, 1966). In fact, as discussed by Bryant (1997), only in the presence of a classification hypothesis does the definition of a taxon name specify the composition of that taxon. Accordingly, because they “say nothing empirical” (Ghiselin, 1966), taxonomic definitions are per se always stable. The correlation between a taxon and its name is always that which was defined, regardless of which class of definition that is. Using the recurrent example of birds, the concept of Aves is stable, whether it is defined by the presence of contour feathers or as “the clade