The relation of taxonomic method to an explanation of organic evolution

Abstract

THE impact of Charles Darwin's classic, The Origin of Species, has been incalculable. The concept of evolution that he made explicit, and cogently documented, now permeates and gives direction to all biology. Yet, paradoxically, the orientation of his argument, directed as it was toward an explanation of the origin of species, has provided an impediment to study of the causal factors involved in evolution.Species or specific types had been the focus of attention since the beginning of the Greek development of biology. Within the span of man's memory each kind seemed immutable, and their permanence, accorded religious sanction, had scarcely been challenged. In view of the evidence he had adduced, which could be adequately understood only in terms of an evolutionary hypothesis, it is not surprising that Darwin should have emphasised his refutation of the fixity of species. His monumental contribution, however, was his thesis that evolution proceeds from the natural selection of individual variants. Being handicapped by the lack of genetic evidence, he was forced to frame his argument in taxonomic terminology. He presented extensive data to show that characters which are individual in some instances are subspecific in others, and specific in still others. He also pointed to parallel variation in higher categories. He inferred from this evidence that specific and other typical character results ultimately from the particular differences of individual adaptedness.Darwin's successors were no more able than he to explain the adaptive process involved in the formation of typical character. Imbued with taxonomic concepts, and impressed by the role of isolation in maintaining specific character for long periods, they regarded the species not only as a taxonomic category but also as an evolutionary unit. This view resulted from the failure to distinguish potential exchange of genetic materials within a species from the actual exchange which alone can be effective in evolution. As a consequence their efforts were directed toward description of the succession of organic types. The origin of a type was explained in terms of change of an earlier type rather than in terms of causally effective individual differences.This taxonomic conception of evolution still persists. It persists, for example, in the use of categories, such as species, ecotypes, and biotypes, to denote the effective vehicles or so-called dynamic units 313

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