Natural Classification and the Reality of Higher Taxa

by Jeremy H. Marshall

This document is the abstract of a thesis submitted by Jeremy H. Marshall of St John's College to the University of Oxford in the Trinity Term of 1989 for the degree of Doctor of Philosophy.

The project was supervised by Dr Tom Kemp, Fellow of St John's College, Curator of the Zoology Collections at the Oxford University Museum. The thesis was successfully defended in a viva in Michaelmas Term 1989. The examiners were Dr Rom Harré (Oxford) and Dr Adrian Friday (Cambridge). The original text is deposited in the Bodleian Library. An HTML version is currently in preparation.

Abstract

Many generations of taxonomists have struggled with the self-appointed task of constructing a `natural' classification of biological organisms. Advocates of the cladistic school of classificatory methodology have in recent years made insistent claims that the cladistic method of phylogenetic reconstruction provides the sought-for, unique, natural classification. Especially, they have claimed that the taxa delineated by cladistic methodology are of radically different status from other kinds of taxon. Adopting and extending a philosophical innovation which categorizes biological species as individuals rather than classes, they assert that cladistic taxa are `real entities', and other kinds of taxon merely `subjective' or `convenience' classes whose existence should not be recognized in formal classifications.

Having outlined the present situation as regards rival taxonomic philosophies, and some of its historical background, the thesis examines this attempt to recategorize taxa as individual-like entities, and finds it wanting. The properties of species which render them regardable as individuals do not readily extend to more inclusive levels, or, if they do, are not readily restricted solely to cladistic taxa. Cladistic systematization, in moving away from the notion of a taxon as a class of similar entities, may cease to convey the information expected of a classification system. The practice of biology requires a more flexible and more stable taxonomy than can be provided by strict adherence to cladistic rules, and taxa are better regarded as `historical classes', delineated neither by pure unanalysed similarity nor by logical transformation of hypotheses of phylogenetic relationship, but by a considered pragmatic synthesis of the two, employing the notion of convexity as a criterion of acceptability.

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