The role of theories in biological systematics

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Abstract

The role of scientific theories in classifying plants and animals is traced from Hennig's phylogenetics and the evolutionary taxonomy of Simpson and Mayr, through numerical phenetics, to present-day cladistics. Hennig limited biological classification to sister groups so that this one relation can be expressed unambiguously in classifications. Simpson and Mayr were willing to sacrifice precision in representation in order to include additional features of evolution in the construction of classifications. In order to make classifications more objective, precise and quantitative, numerical pheneticists limited themselves to representing degrees of phenetic similarity. Finally, present-day cladists can be separated into phylogenetic cladists, who retain much of Hennig's theory of classification, and pattern cladists, who have stripped Hennig's system down to its bare essentials.

Section snippets

Hennig and Phylogenetic Systematics

In Germany the chief exponents of the view that biological classifications should be theory-neutral compendia of observations can be found among ideal morphologists. Not all ideal morphologists rejected evolution, but they did decry its influence on the classification of plants and animals (see, for example, Otto Heinrich Schindewolf, 1950). Ideal morphologists are often termed ‘typologists’, and for good reason. According to ideal morphologists, discrete patterns or ‘archetypes’ exist out

The New Systematics

Science is supposed to be international, but Hennig's views did not influence systematics in the English-speaking world for another twenty years. Because ideal morphology had never been especially popular in the English-speaking world, early contributors to the new systematics did not feel obligated to argue against it at any great length, but periodically voices were raised against the intrusion of ideas about phylogeny and/or the evolutionary process into biological classifications. The chief

Numerical/Phenetic Taxonomy

As the new systematics continued its triumphant march through the systematics community, two new features were added to the mix—computers and molecular data. When systematists decide that a particular group needs reworking, they must study not only the specimens that they themselves have collected but also available specimens used in previous classifications. Because systematics has such a long history, any systematist who proposes to rework a group is usually confronted with massive

Cladistic Analysis

The initial point at issue in this literature was the role of fossils in classification. Hennig did not think that fossils were all that much help in reconstructing phylogeny. At most the presence of a fossil in a particular stratum implied that the taxon to which the specimen belonged existed while this stratum was being formed. The taxon could have existed much earlier and may have persisted long afterwards. Given the scanty nature of the fossil record and the relatively few characters

Pattern Cladism

The epistemological differences between latter-day pheneticists and cladists are a good deal more problematic. In the early days, Sokal and Sneath settled on the variable weighting of characters as the most serious weakness of traditional taxonomy. At the outset all characters must be weighted equally. More recently, Sokal (1986, p. 426) characterized the phenetic position on this score as follows:

As originally formulated, numerical phenetics based its estimates of similarity on many characters

To the Present

In the preceding discussion I have distinguished several schools of systematics. Although none of these schools were totally internally homogeneous, they do warrant being distinguished. Ideal morphologists of varying stripes classified on the basis of the essential morphological characteristics of each taxon. Process theories and phylogeny played no role in their enterprise. Advocates of the new systematics wanted to make biological classification more scientific. Toward this end they urged the

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