Time to standardize taxonomies.

Taxonomic assignments are grossly nonstandardized in current biological classifications. For example, some genera such as Drosophila are an order of magnitude older than others such as Pan or Gorilla; and, further more, because of an "apples-versus-oranges" problem, a genus-level (or any other) designation for particular assemblages of fruit flies and primates provides almost no information on whether such disparate groups en compass comparable amounts of phenotypic, genetic, or evolutionary variation. Another aspect of nonstandard ization is that some taxa (e.g., Reptilia) are paraphyletic as traditionally delimited, whereas others (e.g., Aves) are monophyletic. Although it is sometimes proclaimed that taxa afforded the same taxonomic rank should in prin ciple be more or less equivalent by some criterion (Van Valen, 1973; Minelli, 2000), what that universal criterion might be and how to implement it have received scant attention (Dubois, 2005). In principle, a "temporal-banding" strategy (Hennig, 1966; Avise and Johns, 1999) for classification could rem edy this situation by moving taxonomy closer to both of its two ideal functions (Mayr, 1982): to provide a univer sal system for information storage and retrieval and to encapsulate an evolutionary interpretation of biological (Jiversity. For any phylogenetic tree with internal nodes reliably dated (from molecular clocks, biogeographic data, fossils, or other evidence), extant species compris ing any clade would be assigned a taxonomic rank de termined by the temporal window (band) in which the basal node for that clade resides. (Although nodal dates can be notoriously difficult to estimate, rapid progress is being made for many clades thanks especially to the availability of extensive molecular sequence data; e.g., Springer et al, 2003; Moreau et al, 2006.) Because clades in any phylogeny are hierarchically arranged, each re sulting classification would also be hierarchical (as un der traditional Linnaean schemes) but with the added benefit of now being calibrated and standardized by a universal yardstick: evolutionary time. Temporal banding could provide a simple way to equilibrate taxonomic ranks across any extant forms of life. The boundaries of the temporal windows to be associated with each rank are inherently arbitrary. However, once formally ratified by a consensus among systematists, they would thereafter provide an objec tive and universal standard for classifying any group of extant organisms for which a well-dated phylogeny is available. A serious problem with the temporal-banding scheme (as formulated above) is that it would necessitate dra matic rank changes for many taxa. Taxonomic stabil ity is also important in biology (Godfray and Knapp, 2004), so any wholesale taxonomic revision would be counterproductive if it complicated more so than fa cilitated effective communication and information re trieval. Here we suggest a simple and straightforward taxonomic tactic by which the epistemological advan tages of temporal banding could be achieved without abandoning tried-and-comfortable Linnaean ranks and nomenclatures. Although we prefer the retention of rank ing hierarchies in biological classifications, our current proposal could also be implemented in rank-free sys tems such as PhyloCode (see de Queiroz and Gauthier, 1992).