Classifying species versus naming clades

The February 2004 issue of Taxon included two articles and an opinion piece that either (Jorgensen, 2004) opposed development of the PhyloCode (Cantino & de Queiroz, 2003) or (Barkley & al., 2004a, b) argued that it was unnecessary to meet the needs of phylogenetic classification. Barkley & al. (2004a: 159) conducted "an article by article vetting" of the ICBN and concluded that it "does not contain any rules that actually prevent phylogenetic classification" but that clarification of a few articles is needed. To this end, Moore & al. (2004) proposed adding notes to Articles 3.1, 22.3, and 26.3 of the ICBN (Greuter & al., 2000). However, a major problem that motivated development of the PhyloCode-the inability of rank-based nomenclature to provide stable, unambiguous names for clades-was not given serious consideration by Barkley & al. (2004a, b), and the ICBN rules that cause the greatest problems (e.g., Articles 11.2, 11.3 and the various articles that mandate terminations at particular ranks) were completely ignored. In my view, the primary goals of modem systematics are to document biological diversity and place this diversity in a phylogenetic context. Stated simply, we are trying to reconstruct the tree of life and provide names for the parts of the tree to facilitate communication and information retrieval. The "parts" that are of interest to phylogenetic systematists are species and clades. It is important to discover, characterize, and name both of these kinds of entities, and the names we provide for them should be stable and unambiguous. I consider nomenclatural clarity to be just as important for clades as it is for species. While other PhyloCode supporters may have different reasons for preferring phylogenetic to rank-based nomenclature (the "Linnaean" nomenclature of Barkley & al., 2004a, b), my rejection of the latter is due largely to practical considerations: its inability to provide stable, unambiguous clade names, and the difficulty of naming clades as they are discovered without developing or revising a classification. These aspects of rank-based nomenclature discourage systematists from naming clades and are an impediment to communication about phylogeny (Hibbett & Donoghue, 1998; Cantino, 2000). The problems with rank-based nomenclature. The instability of clade names governed by rank-based nomenclature has been pointed out repeatedly (de Queiroz & Gauthier, 1994; Cantino & al., 1997; de Queiroz, 1997; Kron, 1997; Hibbett & Donoghue, 1998; de Queiroz & Cantino, 2001; Pleijel & Rouse, 2003). To cite just one example, the subclade of Lamiaceae that is named Ajugoideae when ranked as a subfamily must be named Teucrieae if ranked as a tribe due to application of priority within rank (ICBN Art. 11.2); the oldest name at the tribal rank based on a type within this clade is Teucrieae Dumort., while the oldest such name at the subfamilial rank is Ajugoideae Kostel. (Index Nominum Supragenericorum Plantarum Vascularium Project DataBase; http://matrix.nal.usda.gov: 8080/star/supra genericname.html). Nothing about the clade has changed other than its arbitrary rank assignment, and the same information is provided by the name regardless of its rank. If comparably unnecessary changes were occurring in species names, the taxonomic community would not stand for it and the nomenclatural system would be changed to eliminate the instability. Not only may a clade have different names under rank-based nomenclature, but the same name is frequently applied to different clades. This may occur because different systematists choose to apply the same name to different nodes of a cladogram. For example, Urticaceae sensu Zomlefer (1994) applied to a more inclusive clade than Urticaceae sensu Judd & al. (2002), though there was no apparent disagreement about the phylogeny (Bryant & Cantino, 2002). It may also occur because ICBN Arts. 19.4 and 22.1 mandate the names that must