Concordance and discordance of taxonomic characters in Drosophila classification.
1968; National Institutes of Health; Volume: 17; Issue: 4 Linguagem: Inglês
Autores Tópico(s)
Plant and animal studies
ResumoThe general structure of taxonomic data sets, the alpha and beta elements of this structure, the departures of this structure from randomness, and the rationale by which departures are viewed as historical evidence have been demonstrated. An operational method for selecting character sets is illustrated, an estimate of the genealogy that produced the species is derived from these sets, and this estimate is presented as a phylogeny. In addition, I have demonstrated a method that discloses a stable structure that is relatively insensitive to distortion through addition of new data. Still further, I have shown a procedure that produced clusters of species having phenetic properties that are at least as good as those of clusters produced by direct phenetic methods. It has been demonstrated that different characters have different predictive properties, that different character states of the same character have different predictive properties, and that, in general, derivative states are the sole bearers of valid historical information. Taxonomists cannot, with propriety, weight either characters or character states. They must select and evaluate sets of character states, and this evaluation should be carried out every time new data, breaking established character sets, are incorporated into data tables. Methods for the selection and use of taxonomic characters, and for the evaluation of groups and groupings, are of central importance in taxonomy. Some questions relevant to character use and analysis may seem self-evident to the point of triviality, but they need nonetheless to be disposed of formally. Other aspects of method may be quite ordinary, but still require explication so that a common ground for practice and criticism may be developed. I wish to direct attention to three problems: (1) Are taxonomic characters of equal weight? (2) If taxonomic characters are of unequal weight can they be selected operationally by objective means? (3) Given characters of unequal weight and objective methods for selecting them, can monophyletic groups be detected, and can groupings of taxonomic usefulness be evaluated objectively using operational criteria? These questions and their corollaries will be treated, not necessarily in sequence and not always exhaustively, in what follows. The reader should recognize that I am not considering here the question of phylogenetic method per se. The operations to be described are simply those required to produce unequivocal answers to the questions posed above. They may well be useful as a part of phylogenetic analysis, but they are by no means a complete phylogenetic method and they should not be construed as such. THE INTELLECTUAL ANTECEDENTS The history of taxonomic method, particularly as it relates to Drosophila, is not long. The genus was named in 1823 by Fallen, but Sturtevant (1939, 1942) was the first dipterist to set forth in any detail his methods of analyzing Drosophila characters. Since for all practical purposes Sturtevant brought modern taxonomy to the genus Drosophila, and since the classification of Drosophila bears the strong imprint of his work and of his methods, I will briefly describe what he set out to do, what he did, and what Drosophila classification gained from his attentions. It should become apparent during the description that Sturtevant's work is a most significant contribution to the general methodology of taxonomy. Sturtevant (1939:137) stated his purpose as follows: . . it is desirable that there be available a satisfactory arrangement of
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