Phylogenetíc Approaches to Classification and Nomenclature, and the History of Taxonomy (An Alternative Interpretation)

Herpetological Review has recently featured a series of papers debating the merits of recent developments in the theory and practice of systematic biology, using herpetological examples. In the latest and longest paper in this series, Pritchard (1994) passionately argued against many of these "cladistic" developments. Here I address three issues raised in his paper that bear directly on my own writings concerning systematic theory and practice. These are: 1) the abandonment of paraphyletic higher taxa, 2) the replacement of the current ("Linnean") system of biological nomenclature, and 3) the significance of these proposals in the context of taxonomic history. Contrary to Pritchard's implications, the proposals in question are not based on arbitrary or illogical cladistic dogma but on well-established evolutionary principles. Moreover, these proposals are not passing fads but manifestations of an important trend in the historical development of biological taxonomy. Paraphyly.•^Pritchard opposes the proposal that groups each consisting of a common ancestor and all of its descendants (monophyletic groups, clades) should be recognized as higher taxa, but that groups each consisting of a common ancestor and only some of its descendants (paraphyletic groups) should not. He argues in favor of recognizing paraphyletic higher taxa•for example, a Reptilia that does not include birds•on the grounds that first, they agree with commonsense vernacular concepts, and second, they are congruent with a particular model of speciation, which he illustrates using a human socioeconomic analogy. All of his arguments, however, overlook some important considerations. Pritchard's first argument for recognizing paraphyletic higher taxa overlooks the fact that much of scientific progress has consisted of replacing commonsense vernacular concepts with concepts derived from corroborated scientific theories. A few examples of commonsense vernacular concepts that were widely accepted in previous times but have largely been replaced in the face of accumulating data are a flat earth, a geocentric universe, solid matter, fixed continents, immutable species, and a single taxon for both amphibians and reptiles. Evolution is often said to be the unifying theory of biology, and most biologists accept the proposition that taxonomy is to be based on evolution. A fundamental component of the principle of evolution is the proliferation of species from common ancestors. Therefore, equating higher taxa with groups of species sharing a unique and exclusive common ancestry constitutes an evolutionary concept of higher taxa (Hennig 1966). The lack of correspondence between these clades or monophyletic groups and commonsense vernacular concepts of taxa is a poor reason for retaining the latter, and it might even be considered resisting scientific progress. Pritchard also favors the recognition of paraphyletic taxa based on a model of speciation in which an ancestral species remains unchanged while giving rise to a modified descendant species. He argues that the unmodified ancestral species deserves the same taxonomic designation before and after giving rise to the modified descendant, which deserves recognition as a separate taxon (thus rendering the ancestral species paraphyletic). Pritchard's argument suffers from implicitly extending to higher taxa a model that applies properly to species alone. That is to say, although there are accepted evolutionary processes through which species give rise to other species, there is no known evolutionary process through which higher taxa give rise to other higher taxa of the same or greater rank in the Linnean hierarchy (e.g., Wiley 1979). It is this inflated assignment of categorical ranks that results in paraphyly. For example, assigning both Amphisbaenia and Lacertilia to the rank of suborder renders Lacertilia paraphyletic because it implies that Amphisbaenia is entirely separate from Lacertilia rather than being a subgroup ofthat taxon. Categorical assignments are made by humans. Therefore, paraphyletic higher taxa owe their existence as much to the human mental process of assigning ranks as to any evolutionary process. Systematists who advocate the elimination of paraphyletic higher taxa do so because they wish to recognize as taxa only those entities resulting entirely from evolutionary processes. Pritchard uses a sociological analogy to illustrate the supposed rationality of recognizing paraphyletic taxa and, at the same time, the supposed irrationality of cladistic practice. He describes a situation in which a lineage of peasants•let us call it the Bauer clan ("Bauer" is German for "peasant")•produces a daughter who becomes educated and gives rise to a lineage of urban professionals. He argues that cladists would insist nonsensically that the urban professionals be called peasants because they are descended from peasants. This characterization of the cladistic position is erroneous and results from Pritchard's failure to distinguish between socioeconomic classes and genealogical lineages (or more generally, between classes and systems, see de Queiroz 1988). Everyone who makes this distinction, including cladists, will classify the descendants of the educated daughter as urban professionals according to socioeconomic criteria, but they will also realize that the descendants' change in socioeconomic status does not remove them from the Bauer clan. An analogous distinction appUes to taxonomy. Consider a situation in which a clade composed of insectivores ancestrally•Squamata, for example•produces a descendant lineage that enters a new adaptive zone and gives rise to a clade of herbivores. As long as a distinction is made between classes based on diet and monophyletic entities, there will be no problem classifying the modified descendants as herbivores while at the same time recognizing that they are part of the squamatan clade. Nomenclatural Systems.•Pritchard also criticizes a recent proposal to replace the current system of biological nomenclature with one based on evolutionary principles (de Queiroz and Gauthier 1990, 1992, 1994). In his view, this as an example of "contortions forced on cladists by...their own arbitrary rules," which represents "iconoclasm that is unlikely to appeal to anyone except highly theoretical cladists" (p. 105). Furthermore, Pritchard implies that the current system promotes nomenclatural stability and that adoption of the proposed alternative would result in "total sacrifice" (p. 105) ofthat stability. Although adoption of a phylogenetic system of nomenclature would be radical in some important respects, it would not, contrary to the impression given by Pritchard, produce utter nomenclatural chaos. Indeed, adopting a phylogenetic approach to nomenclature would largely solve problems of nomenclatural ambiguity and instability caused by the current system. Pritchard blames cladistic doctrine for problems stemming from the nonevolutionary nature of the current nomenclatural system, using recent proposals concerning the nomenclature of acrodontan lizards to illustrate his points. Frost and Etheridge (1989) proposed to unite the families Agamidae and Chamaeleonidae into a single family as a result of their conclusions concerning paraphyly of the family Agamidae. Because "Chamaeleonidae" is the older name in the family group, it becomes the name of the new family-level taxon. As a consequence, many species that were formerly members of the family Agamidae will now belong to the family Chamaeleonidae, and the group of species formerly rec-