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"Taxonomy is often undervalued as a glorified form of filing-with each species in its prescribed place in an album; but taxonomy is a fundamental and dynamic science, dedicated to exploring the causes of relationships and similarities among organisms. Classifications are theories about the basis of natural order, not dull catalogues compiled only to avoid chaos." (S. J. Gould 1990, p.98)
"Our classifications will come to be, as far as they can be so made, genealogies . . . The rules for classifying will no doubt become simpler when we have a definite object in view." (Darwin 1859, p. 486)
Rank-based Taxonomy
In 1735, Swedish botanist Carl Linnaeus (also known as Carl von Linné; 1707-1778) provided the familiar system of classification that is still used today, consisting of a two-part name (genus and species) to uniquely identify each species of organism, and hierarchical categories (kingdom, phylum, class, order, family, genus and species) for grouping them. Linnaeus' assignments were based on the non-evolutionary concept of a type species, whereby individuals were considered members of a given species if they possessed certain "important" fixed properties characteristic of that ideal or type. Since Darwin's The Origin of Species was published in 1859, the classification of species assumed new significance. Systematists began working towards discovering and describing the evolutionary relationships among species in addition to naming and classifying them. In 1950, the late German entomologist Willi Hennig revolutionized the practice of systematics by providing a method for inferring the true branching pattern of evolutionary history (phylogenetic systematics), and argued that classification should reflect only phylogenetic relationships, not overall similarity.
While Hennig's method of phylogenetic systematics sparked a fundamental shift in the estimation of phylogenetic relationships, his recommendation that classification should explicitly represent phylogenetic relationships has not yet been realized. Although many of Linnaeus' original assignments have been changed, his pre-Darwinian system of nomenclature is still in use today. The International Code of Zoological Nomenclature (ICZN), which provides a formal set of rules governing the naming of animal taxa (similar codes exist for plants and bacteria), uses this subjective, non-evolutionary system of classification to assign the names of organisms to particular hierarchical ranks. This current rank-based code is of limited use because it does not make explicit reference to phylogeny (and thus evolutionary history). Species, along with groups of organisms that share uniquely derived character states (clades), comprise the tree of life, but the current code does not provide an unambiguous means of naming such entities. Under ICZN rules, the application of ranks to supraspecific taxa (including genera) is subjective; consequently, different authors may use different binomial names for the same species (Hillis, in press). Additionally, because species names change whenever they are assigned to different genera, a rank-based system is unstable over time, making information storage and retrieval difficult.
Advantages of a Phylogenetic System of Taxonomy
In response to the widely accepted recognition that a rank-based system of classification is subjective, unstable, and contains limited biological information (de Queiroz and Gauthier 1990; de Queiroz and Gauthier 1992; de Queiroz and Gauthier 1994), the International Code of Phylogenetic Nomenclature (PhyloCode) was developed. Unlike the current rank-based system of nomenclature, PhyloCode provides a logical, unambiguous framework for naming species and clades through explicit reference to phylogeny. The advantage of such a system of classification is the information about evolutionary history it provides, not only for systematists, who are accustomed to interpreting phylogenetic information, but for the myriad biologists who use phylogeny only to the degree that it is rendered in a classification (Hillis, in press).
A New Taxonomy for Amphibians?
Recently, Frost et al. (2006) published a phylogeny of living amphibians and proposed major taxonomic changes, arguing that the "currently accepted classification of living amphibians is shown not to warrant the degree of authority conferred on it by use and tradition." (Frost et al., 2006; p. 8). While some have already chosen to adopt this new taxonomy, we argue that there are clear advantages to maintaining the traditional names for many of these taxa. A fundamental goal of AmphibiaWeb is to promote clear communication among biologists through efficient information storage and retrieval. The majority of the new names proposed by Frost et al. (2006) are disconnected from the vast amount of previous amphibian research that has taken place, and thus adopting this new taxonomy hinders effective dissemination of information. The literature dealing with frogs of the northern hemisphere is enormous, in particular with reference to such familiar genera as Bufo and Rana, for example.
Nothing in the ICZN requires taxonomic recommendations such as those of Frost et al. to be followed. For example, although the changes proposed by Frost et al. for the genera Bufo and Rana were made at the generic level, biologists who wish to recognize the subdivisions of these genera, but maintain the stability of familiar species names and still follow the rules of the ICZN, may choose to recognize newly created subdivisions of these genera as subgenera (e.g., (Smith and Chiszar 2006); but see Hillis, in press). Under ICZN rules, the subgenus category may follow the genus name in parentheses, e.g., Bufo (Chaunus) marinus or Rana (Lithobates) pipiens. At some point, when the hypotheses of relationships in Frost et al. have been tested with additional data, better taxon sampling, and more realistic analytical methods, changes may be in order. At least for the time being however, we have elected to retain the traditional names for amphibian taxa pending publication of a phylogenetic taxonomy for additional branches of the living amphibian portion of the Tree of Life (e.g., Hillis et al., 2001; Hillis & Wilcox, 2005; Ron et al., 2006). Until the ICZN amends its rules to allow for naming supraspecific clades, a compromise must be reached between the two alternative systems of classification currently in use (Hillis, in press).
Literature Cited
de Queiroz, K., and J. Gauthier. 1990. Phylogeny as a central principle in taxonomy - phylogenetic definitions of taxon names. Systematic Zoology 39:307-322.
de Queiroz, K., and J. Gauthier. 1992. Phylogenetic taxonomy. Annual Review of Ecology and Systematics 23:449-480.
de Queiroz, K., and J. Gauthier. 1994. Toward a phylogenetic system of biological nomenclature. Trends in Ecology & Evolution 9:27-31.
Frost, D. R., T. Grant, J. Faivovich, R. H. Bain, A. Haas, C. F. B. Haddad, R. O. De Sá, A. Channing, M. Wilkinson, S. C. Donnellan, C. J. Raxworthy, J. A. Campbell, B. L. Blotto, P. Moler, R. C. Drewes, R. A. Nussbaum, J. D. Lynch, D. M. Green, and W. C. Wheeler. 2006. The Amphibian Tree of Life. Bulletin of the American Museum of Natural History:370.
Hillis, D. M. Constraints in naming parts of the Tree of Life. Molecular Phylogenetics and Evolution In Press
Hillis, D. M., D. A. Chamberlain, T. P. Wilcox, and P. T. Chippindale. 2001. A new species of subterranean blind salamander (Plethodontidae : Hemidactyliini : Eurycea : Typhlomolge) from Austin, Texas, and a systematic revision of central Texas paedomorphic salamanders. Herpetologica 57:266-280.
Hillis, D. M., and T. P. Wilcox. 2005. Phylogeny of the New World true frogs (Rana). Molecular Phylogenetics and Evolution 34:299-314.
Ron, S. R., J. C. Santos, and D. C. Cannatella. 2006. Phylogeny of the tungara frog genus Engystomops (=Physalaemus pustulosus species group; Anura: Leptodactylidae). Molecular Phylogenetics and Evolution 39:392-403.
Smith, H. M., and D. Chiszar. 2006. Dilemma of name-recognition: Why and when to use new combinations of scientific names. Herpetological Conservation and Biology 1:6-8.