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Assignment Given the Tree Model: Develop a Standardized Classification Scheme

The two basic functions of biological taxonomy are to (i) provide a universal system for information storage and retrieval, and (ii) encapsulate an evolutionary interpretation of biological diversity (Mayr, 1982). Unfortunately, current biological classifications are grossly nonstandardized because: (i) the species in named taxa are typically united by some unspecified mix of similarity by resemblance and similarity by descent, and (ii) even when the nested taxonomic ranks in a Linnaean hierarchy do register bona-fide nested clades the rankings remain noncomparable across different kinds of organisms (because no serious attempt has ever been made to normalize assayed characters, equilibrate taxonomic assignments, or even adopt any standardized criteria for taxonomic ranking). For example, some taxonomic genera such as Drosophila are an order of magnitude older than others such as Gorilla or Pan, and, because of an apples-versus-oranges problem, a taxonomic rank (such as a genus) shared by fruit flies and primates implies nothing about whether such taxa are similar with respect to genetic, phenotypic, or any other aspect of evolutionary diversity. As noted by de Queiroz and Gauthier (1992), “No scientific enterprise, least of all one that considers the promotion of nomenclatural universality as one of its primary objectives, can accept the inconsistencies and ambiguities current in biological taxonomy.” Or as phrased by Hennig (1966), “If systematics is to be a science it must bow to the self-evident requirement that objects to which the same label is given must be comparable in some way.”

This state of affairs could, in principle, be rectified if systematists were to adopt absolute geological time as the universal evolutionary yardstick against which to standardize taxonomic assignments for extant clades of known age. The basic idea, proposed by Hennig (1966) and elaborated by Avise and Johns (1999), is that extant species that separated from a common ancestor in a specified window of evolutionary time would be assigned a taxonomic rank defined by that temporal band. The boundaries of the temporal windows are arbitrary at the outset and must be ratified by convention, but a proposal that I favor in principle would link each taxonomic rank to a specific geological episode. Serendipitously, there are 17 supraspecific ranks in modern versions of the Linnaean hierarchy (Mayr and Ashlock, 1991) and also 17 primary subdivisions in the traditional geological timescale (Futuyma, 1998), thus affording the possibility of a perfect one-to-one allocation of taxonomic rank to geological episode (Fig. 15.2).

If the field of systematics from its outset had been able to implement a temporal-banding strategy for erecting biological classifications, many of the inconsistencies and ambiguities in current taxonomies could have



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