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Corpus Systématique Végétale

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A botanical critique of cladism

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  • Arthur Cronquist
  • The New York Botanical Garden, 10458, Bronx, New York

Clade versus grade is an old question in taxonomy, going back as far as Darwin himself. Taxonomists have long believed that both must be taken into account in the formation of a general-purpose system. Recently clade has been elevated to a position of total dominance by a group of taxonomists who take their inspiration from Willi Hennig. Mayr has dubbed this approach cladism, and its exponents cladists. Cladistic theory is being vigorously developed and propounded by Hennig’s disputatious disciples, and much of the present-day theory would scarcely be recognized by the founder. I here address myself to what I consider the core features of present-day cladism. The essential distinctive feature of cladism, and its fatal flaw, is that a group is considered to be monophyletic, and thus taxonomically acceptable, only if it includesall the descendants from the most recent common ancestor. The traditional taxonomic view has been that a group can still be considered monophyletic (and thus taxonomically acceptable) after some of its more divergent branches have been trimmed off. This simple and seemingly innocuous difference has profound consequences to the taxonomic system. In Hennigian classification, organisms are ranked entirely on the basis of recency of common descent, that is, on the basis of the sequence of dichotomies in the inferred phylogeny. Theamount of divergence scarcely enters into the picture. This procedure represents an effort to capture taxonomy for a narrowly limited special purpose, at the expense of the important and necessary function of providing a general-purpose system that can be used by all who are concerned with similarities and differences among organisms. The first corollary of the Hennigian concept of phylogenetic taxonomy is that no existing taxon can be ancestral to any other existing taxon. The descendant must be included in the same taxon as its ancestor. At the level of species this is palpably false. The ancestral species often continues to exist for an indefinite time after giving rise to one or more descendants. At the higher taxonomic levels adherence to the principle often requires excessive lumping or excessive splitting to avoid paraphyletic groups (i.e., groups that do not include all of their own descendants), and it forbids the taxonomic recognition of many conceptually useful groups. Neither the prokaryotes nor the dicotyledons form a cladistically acceptable taxon, since both are paraphyletic. The prokaryotes are putatively ancestral to the eukaryotes, and the dicotyledons are putatively ancestral to the monocotyledons. Many other traditional and readily recognizable taxa would have to be abandoned, without being replaced by conceptually useful groups. Fossils present a special problem, because the whole concept of cladistic classification depends on the absence of taxa at the branch points of the cladogram. Presumably all of these branch points were at some time in the past represented by actual taxa, which under cladistic theory can neither be assigned to one of their descendants nor treated as paraphyletic taxa. The difficulty is mitigated somewhat by the gaps in the known fossil record. Once it is admitted that paraphyletic as well as holophyletic groups are taxonomically acceptable, there is much value in cladistic methodology. Formal outgroup comparison for the establisment of polarity, and the emphasis on synapomorphies in the construction of a cladogram can both be usefully incorporated into taxonomic theory and practice. These require no revolution in taxonomic thought. There are unresolved problems, however, in how to gather and manipulate the data, and how to interpret the cladogram produced by computers. In any complex group, the computer may produce several or many cladograms of equal or nearly equal parsimony. This is particularly true in angiosperms, among which the extensive evolutionary parallelism casts doubt on the importance of parsimony and may lead to the production of hundreds of such cladograms for a single group. Despite the claims of objectivity and repeatability in cladistic taxonomy, the necessity for some subjective decisions remains. The Wagner groundplan-divergence method has most of the advantages of formal cladism without the most important disadvantages. Wagner accepts paraphyletic taxa in principle, and he casts a wider net for data bearing on the polarity of characters. In complex groups consisting of many taxa, however, both methods retain a strong subjective component in the computer manipulation and in the degree of reliance on absolute parsimony.

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  • 1 - Life Sciences ; 2 - Agricultural and Biological Sciences ; 3 - Ecology, Evolution, Behavior and Systematics
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  • 1 - science ; 2 - plant sciences
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The Botanical Review

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