Other Taxonomic Priorities

Two conflicting priorities for taxonomy have been mentioned previously: hierarchical information content, and stability in the entities to which names refer. There are, unquestionably, other priorities that can be considered key in assigning names to groups of taxa. In particular, some nomina have been viewed as bearing an implied significance that extends beyond their correct application according to taxonomic rules. A good example of this is the genus name Homo. Workers wanting to apply this name have generally sought some standard of humanity by which to judge the appropriateness of referring a taxon to our own genus. Louis Leakey and coworkers, for example, considered the ability to make tools key to recognition of human status when they named the species Homo habilis in 1964. Wood and Collard (1999) have sought to recognize a major adaptive shift with membership in the genus Homo, requiring that taxa designated with this nomen show such features as a human-like pattern of development and postcranial features associated with obligate bipedalism. Proponents of this view require that taxa not only be evolutionarily congruous, but also adaptively coherent. Of course, this approach is not terribly helpful to designating the taxonomic position of forms that fall outside of the key taxon in question. Wood and Collard, for example, while arguing for the removal of Homo habilis and Homo rudolfensis from the genus Homo, suggest only that these species be transferred to Australopithecus, making that genus even more paraphyletic, and poorly defined adaptively, than it is likely to be in the context of its commonly accepted composition. Such an approach is fine if you are only interested in the very tip of the phylogenetic tree, but is manifestly inappropriate if the goal is to actually place the "end taxa" in an evolutionary context. The relationships among the members of the genus Australopithecus are important to understanding the evolutionary background against which features of Homo must be assessed, in the same way the relationships of archontans (or euar-chontans) that are not definitively primates are vital to an understanding of the adaptive sequence that went into building primates of modern aspect.

Another set of taxonomic priorities must also be recognized: stability and continuity through time. Although phylogenetic taxonomy considers stability a key priority, the type of stability that it fosters is more metaphysical than practical (Nixon and Carpenter, 2000). In fact, De Queiroz and Gauthier (1990: 312) explicitly state: "The use of phylogenetic definitions will effectively initiate a new era in biological taxonomy. In this new era there will be, in one sense, no existing taxa (named entities), for the names have not yet been tied explicitly to the entities through phylogenetic definitions" (emphasis added). Some authors, upon recognizing that traditionally applied taxo-nomic labels may refer to paraphyletic taxa, have nonetheless consciously retained them, prioritizing historical stability above other considerations (e.g., Silcox, 2001; Silcox et al., 2001; Van Valen, 1994). Such paraphyletic taxa have been called "metataxa" by Archibald (1994), and "natural paraphyletic groups" by Van Valen (1994), and often exist as a series of primitive branches off the stem leading to some cohesive monophyletic group. As such "natural paraphyletic groups" can be recognizable and diagnosable, if only by primitive or intermediate traits. The carpolestid genus Elphidotarsius, for example, includes a cluster of animals that can be differentiated from non-carpolestid by the presence of a mitten-shaped, blade-like P4, and from more derived carpolestids by fewer apical cuspules on the P4 and a less expanded P3 and P4 (Rose, 1975; Silcox, 2001; Silcox et al. 2001).

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