Point of View The phylogenetic approach to biological taxonomy: practical aspects

The increasing rejection of traditional, character-based definitions of taxon names in favour of ancestor-based definitions represents one of the more important recent trends in biological taxonomy (de Queiroz, 1994). In the former approach, taxa are atemporal classes defined by essential characters (e.g. all organisms with feathers), while in the latter approach, taxa are historical entities defined ostensively, by pointing to a particular portion of a phylogeny. de Queiroz & Gauthier (1990, 1992), elaborating the earlier work of Hennig (1966), have recently argued strongly for the latter approach, which they term “phylogenetic taxonomy”. They have attempted to devise a simple yet rigorous protocol for attaching taxon names to monophyletic groups of organisms (clades). L i d h & Oxelman ((1996): L&O) have recently criticised these proposals, and reject phylogenetic taxonomy in favour of an approach they term the “current system”. Their objections are not directed at the rationale behind phylogenetic taxonomy: rather, they are directed at practical aspects of it. This is an important consideration: no matter how philosophically sound a methodology might be, it is still of dubious utility if it cannot be readily applied in the real world. In view of the increasing use of phylogenetic definitions of higher taxon names (e.g. Rowe, 1988; Lee, 1995; Laurin & Reisz, 1995), it is important that L&O’s critique be carefully evaluated. The issues raised are therefore discussed below. (1) Although L&O advocate the “current nomenclatural system” (p. 183) over phylogenetic taxonomy, they do not describe their interpretation of the “current system” and indeed, how it differs from phylogenetic taxonomy. They quote with approval Linnaeus’ statement about “characters not making a taxon” (p. 183), and thus reject character-based definitions of taxon names. This is one of the tenets of phylogenetic taxonomy. They also agree that “the central role of taxonomy [is] communication of phylogeny” (p. 183). Again, this is one of the central proposals of phylogenetic taxonomy. They further agree that taxa are “equivalent to named clades” and that the aim of taxonomy is “to select well-established categories and names for well-supported clades” (p. 183). Apart from the comment about mandatory categories (see point 5), this yet again perfectly matches the goals of phylogenetic taxonomy. Presumably, the way that L&O’s interpretation of the “current system” differs from phylogenetic taxonomy must reside in the way clades are delineated. In phylogenetic taxonomy, clades can be identified on the basis of node-based, stem-based and apomorphy-based definitions (see de Queiroz & Gauthier, 1990, 1992). L&O do not state how named clades are delineated in their “current system”: they merely state that the criteria are “informal” and “subjective” (p. 183). Apart from the three methods ju5t discussed, it might be argued that another possible method to delineate a clade is by simple listing of the known included taxa. However, this is inadequate, since there will almost always be taxa discovered in the future which lie within the clade, and the definition of the