A Truly Taxonomic Revolution ? Numerical Taxonomy 1957 – 1970 Keith Vernon

Abstract

In 1963, the botanist V. H. Heywood suggested that taxonomy might be on the verge of a revolution. He had in mind a truly taxonomic revolution, concerning not merely new information but the fundamental principles of taxonomyalthough he covered himself with the caveat that revolutions had a habit of not quite happening.1 Heywood was referring to the emergence of Numerical Taxonomy, and it was a prescient observation. Numerical Taxonomy (NT) was indeed concerned with principles, and it was potentially revolutionary; but it did not fulfil the potential that some hoped for and others feared. NT proposed that organisms should be classified solely on their overall similarity to each other in the present, measured using many equally weighted characters; that the procedures used to construct the classifications should be clearly set out and logically formulated (in the later term of Numerical Taxonomists, operational);2 and that numerical methods and electronic computers would assist both in manipulating the data and in helping to make the procedures operational and repeatable. This was revolutionary in that the basis of taxonomy for the preceding century had been, ostensibly, evolutionary, and in that it seemed to undermine the professional role of the taxonomist. Although NT attracted considerable interest and generated substantial controversy, it did not succeed as a revolutionary movement. It could not gather enough support against the entrenched resistance of those who supported evolutionary taxonomy. NT also quickly ran into problems of its own, and was overtaken by another, even more revolutionary, system. This paper examines the development of NT from its formulation in the late 1950s to the debates with exponents of the prevailing school, evolutionary systematics, in the mid-1960s; and offers an assessment of the nature and significance of the dispute.3 NT has been the subject of some inquiry, primarily in the work of David Hull, who detailed the controversies in taxonomy during the 1970s and 1980s as evidence for his thesis on the social and conceptual development of science.4 His concerns, however, are not historical, and there is little attempt to explain, in historical terms, how or why the debates came about, followed the course they did, or what their significance was. Nor can we gain a great deal more from Mayr's discussion.5 Mayr, as we shall see, was a central participant in the debate, and this is reflected in his rather superficial account which suggests, simply, that NT was a result of the development of computerswhich is not the caseand that the time had come to re-examine taxonomic principles. In a notable case of simultaneous invention, NT originated with several non-taxonomists who came together in a very short space of time and developed a coherent programme which attracted a vocal, if small, body of followers. In so doing, however, they walked straight into a social and cognitive quagmire that had been festering in taxonomy from the late 1930s.6 During this period, alongside the fashioning of the evolutionary synthesis, two of its principal exponents, Ernst Mayr and G. G. Simpson, had helped to formulate a more overtly evolutionary approach to taxonomy, that of evolutionary systematics.7 As Mayr and Simpson offered the contributions of the field naturalist and the palaeontologist to the synthesis, they also incorporated the synthesis into systematic biology, claiming that the primary goal of taxonomy was to shed light on evolutionary relationships between organisms. Launching a revised image of the modern taxonomist, they presented him as one who worked with natural populations in the wild, the detailed investigations of which revealed evidence of genetic relationships. Far from the old-fashioned, unscientific caricature of popular perception, the taxonomist was, in fact, the biologist best placed to offer insights on population genetics and speciation. This was summarised in the conception of biological species as comprising populations of interbreeding organisms, rather than as a morphological construct from dusty museum specimens. Thus the expert taxonomist, drawing on his experience, assessed the comparative morphology of the organisms, the field evidence on breeding behaviour and fossil evidence on probable phylogenetic pathways to weight, precisely, just those characters that were indicative of evolutionary relationships and produce a classification in accordance with the probable phylogeny. Mayr and Simpson had staked a great deal of the taxonomists' claims to scientific respectability on their ability to contribute to one of the most important biological issues of the day. Evolutionary systematics, however, was not regarded with unalloyed favour throughout the taxonomic community, and a low-level debate had simmered away for twenty years.8 A number of taxonomists argued that evolutionary systematics was virtually impossible in the taxa on which they worked, where the fossil record was negligible and breeding relationships unknown, and that, in practice, morphological criteria were the only kind available. Others argued that taxonomists had their own ends to serve and were not merely handmaids of evolutionary biologists. NT replicated many of these criticisms in a particularly stark and threatening manner and it is no wonder that vigorous debate ensued. To interpret the reception of NT as one of controversy alone, however, is to miss the substantial support and enthusiasm which it enjoyed, particularly among microbiologists and, to a lesser extent, botanists. Very quickly, though, Numerical Taxonomists discovered that putting intuitive taxonomic practices into operational clarity was a great deal more difficult than they had at first supposed, which raised considerable problems for the whole enterprise. Detailed investigation of the problems began to provide some common ground whereby the opposing camps could be reconciled but, in the process, the seeds were sown for yet greater divisiveness. I am not concerned here, though, with the subsequent debates around cladistics. 9 While the NTevolutionary systematics debate rehearsed existing divisions, it also had echoes of much earlier disagreements between taxonomists going back to the late seventeenth century. Accordingly, the paper will conclude with some more general thoughts on the nature of debate in taxonomy.10

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