Abstract
It is maintained that a classification based solely on phylogeny* is different from and incompatible with the Linnaean system, and the theory of Hennigian classification is flawed. A purely phylogenetic* system has no class concepts, and, if paraphyletic taxa are not allowed, because every descendant taxon has to be of a lower rank than its ancestor and sunk into it, infinite regression occurs and the whole classification telescopes back into its original ancestral taxon. Every genus since the original one must make another genus paraphyletic, and the latter may or may not be extinct. A cladogram, which purports to represent phylogenetic relationships of taxa, is based on a hierarchy different from that of a Linnaean classification, which represents anagenetic relationships. Neither a cladogram nor a phylogenetic tree is a classification, and subjective decisions must always be taken to impose the limits of taxa and their rank. Existing cladistic classifications became possible probably because either the taxa were incompletely known (many ancestral taxa having become extinct), or the data were inadequate (leaving cladograms unresolved), or because the classification was incomplete (not enough ranks being included). Lowering the rank of one taxon to avoid another being paraphyletic generally merely shifts paraphyly to a different rank. A phylogeny and a classification are both desirable but have different functions, and should be allowed to exist side by side, inter-related but not interdependent, to give maximum predictivity. The widespread and increasing disruptiveness of the application of cladistic theory of classification is an additional reason for rejecting it. The criterion of convexity, which is also based on evolutionary history but allows paraphyletic taxa, is a logical possibility and should be adopted in classification. Taxa should be recognisable by characters, not by position in a putative phylogeny. Examples given of the difficulties of Hennigian classification are the cases of Ptycholobium, which makes the currently recognised Tephrosia paraphyletic, and Neuracanthus sect. Leucobarleria which would make sect. Neuracanthus paraphyletic, and other cases mentioned include Lemnaceae which makes Araceae paraphyletic, Cruciferae which makes Capparaceae paraphyletic, and the monocots which make the dicots paraphyletic.
Résumé
On soutient qu’une classification basée uniquement sur la phylogénie est différente du système linnéen et incompatible avec celui-ci, et que la théorie de la classification selon le système de Hennig n’est pas absente de défauts. Un système purement phylogénétique n’a pas de concept de classes et, si l’on n’accepte pas les taxons paraphylétiques pour la raison que chaque taxon descendant doit avoir un rang inférieur à son ancêtre et y être inclus, une régression sans borne s’opère et toute la classification se confond finalement avec le taxon ancestral d’origine. Chaque genre, à partir du genre d’origine, doit produire un autre genre paraphylétique et ce dernier peut ou non avoir disparu. Un cladogramme, qui suppose représenter les relations phylogénétiques des taxons, a pour base une hiérarchie différente de celle de la classification linnéenne, qui représente des relations anagénétiques. Pas plus un cladogramme qu’un arbre phylogénétique ne constitue une classification, et il est toujours nécessaire de prendre des décisions subjectives pour fixer les limites des taxons et leur rang. Des classifications cladistiques existantes sont devenues possibles, probablement parce que soit les taxons étaient incomplètement connus (de nombreux taxons ancestraux ayant disparu), soit les données étaient inadéquates (laissant des cladogrammes non résolus), soit parce que la classification était incomplète (pas assez de rangs n’étant inclus). L’abaissement de rang d’un taxon pour éviter qu’un autre ne devienne paraphylétique ne fait généralement que déplacer la paraphylie à un autre rang. Une phylogénie et une classification sont toutes deux désirables mais ont des fonctions différentes et on peut concevoir qu’elles existent côte à côte, interconnectées mais non interdépendantes, afin d’obtenir un maximum d’information prévisible. Le bouleversement croissant et étendu de l’application de la théorie cladistique de la classification est une raison supplémentaire pour la rejeter. Le critère de convexité, qui se base également sur l’historique de l’évolution mais accepte des taxons paraphylétiques, est une possibilité logique et pourrait être adopté dans une classification. Les taxons devraient pouvoir se reconnaître par des caractères et non par leur emplacement dans une phylogénie hypothétique. Comme exemples des difficultés de la classification selon Hennig, on notera le cas de Ptycholobium, qui rend paraphylétique Tephrosia, genre couramment accepté, tout comme celui de la section Leucobarleria de Neuracanthus, la section Neuracanthus devenant paraphylétique; d’autres cas évoqués se rapportent aux Lemnaceae qui donnent aux Araceae un statut paraphylétique, aux Cruciferae qui donnent aux Capparaceae un statut paraphylétique, ainsi qu’aux Monocotylédones qui donnent aux Dicotylédones un statut paraphylétique.
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Brummitt, R.K. (1996). In defence of paraphyletic taxa. In: van der Maesen, L.J.G., van der Burgt, X.M., van Medenbach de Rooy, J.M. (eds) The Biodiversity of African Plants. Springer, Dordrecht. https://doi.org/10.1007/978-94-009-0285-5_49
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