Replacement of an Essentialistic Perspective on Taxonomic Definitions as Exemplified by the Definition of "Mammalia"

Abstract

?Current controversies about the definitions of taxon names reflect different under? lying philosophical perspectives concerning the nature of definitions. The antithetical perspec? tives, called methodological essentialism and methodological nominalism (Popper, 1966, The open society and its enemies, Princeton Univ. Press, Princeton, NJ), are exemplified by alternative views a controversy surrounding the definition of the name Mammalia. Traditional per? spectives on the definition of Mammalia are essentialistic that the definition takes the form of a description stated terms of the traits of individual organisms, thus implying that taxa are abstract categories, i.e., that taxa have essences. In addition, the extension of the defined term (the set of species or organisms to which the name applies) is logically prior to its intension or defining formula (the property or properties that a species or organism must have to be designated by the name). Consequently, the name is treated as if it had a designation proper to it, which conforms with the essentialistic view that the name is an abbreviated description of the essence. An alternative perspective is manifested the redefinition of Mammalia as the name of the monotreme and therian crown dade. This perspective contrasts with the traditional one that the definition is stated terms of common ancestry relationships rather than organismal traits, thus implying that taxa are concrete composite wholes rather than abstract categories. In addition, the defining formula is logically prior to the extension of the defined term, and thus the name is treated only as a convenient label or shorthand symbol for the defining formula that has no proper designation. The replacement of an essentialistic perspective on the definitions of Mam? malia and other taxon names by a more nominalistic one is associated with the development of a phylogenetic perspective on biological nomenclature and represents an important step the development of a more broadly scientific approach to that subject. [Definition; essentialism; nominalism; Mammalia; nomenclature; philosophy; taxonomy; Karl R. Popper.] It has been said that in no other science is definition as important as it is tax? onomy (Hull, 1965:315). Recently, the im? portance of definitions taxonomy has been highlighted by the realization that explicitly phylogenetic methods for defin? ing the names of taxa are central to the development of a phylogenetic system of biological nomenclature (de Queiroz and Gauthier, 1990, 1992, 1994). But many of the taxon names current use, particu? larly the oldest and most familiar, were coined long before the advent of phylo? genetic definitions. Indeed, many were coined before widespread acceptance of the concept of phylogeny itself. Consequently, it should not be surprising that definitions formulated the context of the phylo? genetic system are coming into conflict with older definitions of the same names. An example of this conflict concerns the name Mammalia, which has been de? f ned phylogenetically as the crown dade stemming from the most recent common ancestor of monotremes and therians (Gau? thier et al., 1988a, 1988b, 1988c, 1989; Rowe, 1988; de Queiroz and Gauthier, 1992). This definition has been contested by system? atists adopting a view that has become tra? ditional among paleontologists, according o which Mammalia designates a taxon including not only the monotreme and therian crown dade but also several of its extinct outgroups (Miao, 1991; Lucas, 1992; for further discussion see Rowe and Gau? thier, 1992; Bryant, 1994). On one level, the disagreement is nothing more than a dif? ference preferences concerning the as? sociation of a taxon name with one dade or another. Here, I argue that it also reflects