Caryopteris (Lamiaceae) and the Conflict between Phylogenetic and Pragmatic Considerations in Botanical Nomenclature

Abstract

The delimitation of Caryopteris (Lamiaceae) exemplifies the conflict between the representation of monophyletic groups and pragmatic concerns in Linnaean classification. Cladistic analyses of nonmolecular (mainly morphological) data and chloroplast DNA (rbcL and ndhF) sequences were performed to test the monophyly of the eastern Asian genus Caryopteris. The results corroborate earlier studies indicating that Caryopteris is either para-or polyphyletic. If paraphyletic, other genera whose sister groups lie within the currently accepted limits of Caryopteris include Trichostema, Rubiteucris, and probably Ajuga, Schnabelia, and Amethystea. Because phylogenetic resolution is too poor to refer all of the species of Caryopteris to well supported clades, a choice must be made between recognizing a paraphyletic genus or several monotypic ones, three of which would comprise morphologically quite similar species. A compromise solution is adopted in which three new genera are described (Pseudocaryopteris, Discretitheca, and Tripora), the latter two monotypic, and four species of Caryopteris are transferred to other genera, one of which may not be monophyletic. Nine new combinations are provided: Discretitheca nepalensis, Pseudocaryopteris bicolor, Pseudocaryopteris foetida, Pseudocaryopteris paniculata, Rubiteucris siccanea, Schnabelia aureoglandulosa, Schnabelia nepetifolia, Schnabelia terniflora, and Tripora divaricata. For comparative purposes, an alternative classification is provided that employs de Queiroz and Gauthier's phylogenetic system of nomenclature. Using this system, no new names are required, and all supraspecific taxa are both monophyletic and easily recognizable. For more than a decade, plant taxonomy has experienced a controversy over whether to accept paraphyletic groups in classification (e.g., Cronquist 1987; Donoghue and Cantino 1988; Brummitt 1997; Sosef 1997; Brummitt and Sosef 1998; Freudenstein 1998, Schander 1998; Welzen 1998). The opposing viewpoints, which are argued with heartfelt conviction systematists on both sides, reflect a fundamental conflict between two imperatives of modern systematics, which we will call the phylogenetic and pragmatic imperatives. The phylogenetic imperative dictates that all supraspecific taxa should be monophyletic (i.e., each should comprise a common ancestor and all of its descendants). This principle is central to the professional world view of many phylogenetic systematists. It stems (for us and, we suspect, for many others) from a deep conviction that formally named taxa, the most concrete products of the science of systematics, should be objective natural entities that are discovered rather than created. Monophyletic taxa qualify, as do species if they are conceived of as segments of population level evolutionary lineages (de Queiroz, 1998), but paraphyletic and polyphyletic supraspecific taxa are subjective human constructs. The pragmatic imperative, which is also a central maxim of many taxonomists (including many phylogenetic systematists), requires that named taxa be recognizable; that is, there should be a large enough phenetic gap between taxa so that they can be identified by means (Cronquist 1978). What constitutes ordinary means will vary for different groups of organisms, but we suspect that most people would agree that a taxon distinguished only on the basis of gene sequence variations is not a practical unit in a general-purpose classification. There also is an expectation that genera will be more easily distinguished (i.e., more divergent morphologically) than are species within the same family. The arguments for and against the acceptance of paraphyletic taxa have emphasized theoretical issues, but the intensity of feeling generated the controversy may stem largely from disagreement