Evolution and classification : the reformation of cladism / Mark Ridley.
- Mark Ridley
- Date:
- [1986]
Licence: Attribution-NonCommercial 4.0 International (CC BY-NC 4.0)
Credit: Evolution and classification : the reformation of cladism / Mark Ridley. Source: Wellcome Collection.
71/220 page 59
![The techniques and justification of cladism It is in just such cases that evolutionary taxonomy prefers to recognize groups defined by shared ancestral characters. Reptiles are an example. Reptiles are defined by ancestral homologies, and the cladist unhesitatingly splits off the crocodiles to form a monophyletic group (which Mayr would call holophyletic) together with the birds. Mayr (1974 [1976, p. 436]) thinks he is criticizing cladism, and defending evolutionary classification, as he remarks 'the number of evolutionary statements and predictions that can be made for many holophyletic groups (like birds and crocodiles) is often quite minimal'. He thinks it is a criticism because 'there has long been agreement among the theoreticians of classification that in most cases those classifications are best' ' which allow the greatest number of conclusions and predictions' (ibid.) and he even quotes Mill in support of this naive phenetic criterion. As criticism, Mayr's remark has two defects. One is that it is a defence of phenetic, not evolutionary classification. The most phenetically natural classifica¬ tion will be a phenetic one: the main difference between phenetic and evolutionary taxonomy is that the latter excludes convergence; but phenetic taxonomy only admits convergence when it makes a classification more natural. When an evolutionary taxonomist excludes the kind of convergence recognized in a phenetic classifica¬ tion, they reduce the 'number of conclusions and predictions' they can make, and contradict Mayr's just quoted philosophy. The second defect is that Mayr's criterion of the 'best' classification is as subjective as it is flagrantly phenetic. There is no single most natural classification; the criterion of 'greatest number of conclusions and predictions' allows innumerable contradictory classifications. Cladism has entirely rejected phenetic criteria because they do not determine one 'best' kind of classification. It has moved instead to an objective, and justified philosophy. The philosophy will in some cases imply classifications less natural in the phenetic sense than the evolutionary and phenetic classifications; but the cladist regards that not as a defect but as an intention successfully fulfilled. Cladism concentrates on shared derived characters because it is exclusively interested in the branching pattern of phylogeny. It does not attempt to represent all aspects of evolution; it does not attempt to represent differential rates of evolutionary divergence among groups: it only seeks to represent the hierarchy of recency of common ancestry. This is the hierarchy it is after, and this is the hierarchy it needs techniques to reveal. It could not concentrate so exclusively on shared derived characters if it were trying to represent phenetic similarity as well as branching. But it is not. It has philosophically rejected phenetic similarity. I therefore also disagree with Mayr' when he says 'it is evident that the cladist reveals great ambivalence in the 'Mayr (1974 [1976, p. 449]); reveals' is inieiided lo reflect upon the ( ladist - reveals in himself - and not the ironical double entendre I should prefer - reveals in the other classificatory sc hools. 59](https://iiif.wellcomecollection.org/image/b18021451_0072.JP2/full/800%2C/0/default.jpg)
