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.
44/220 page 32
![Evolution and Classification meaning. It does not define groups that are monophyletic in Hennig's sense. With the broader meaning a group can be called monophyletic even if it lacks some of the descendants of its common ancestor: the Reptilia can be defined and termed monophyletic. It is defined, moreover, by ancestral homologies. Ancestral characters reveal w^hether a group shared a common ancestor: it is hardly surprising that evolutionary taxonomists put ancestral homologies to use. Evolutionary taxonomy has practically interpreted its philosophy of representing evolution as the aim to classify monophyletic groups, recognized by homologies. Homologies define monophyletic groups in the sense that they share a common ancestor that possessed the homologies; but if some of the descendants of that ancestor then lost the homologies, they will not be included in the group. The interpretation leads to a compromise among aspects of evolution, but a principled compromise, of a particular kind: phenetic resemblance can override the pattern of splitting; but only for phenetic resemblance in ancestral homologies. The kind of phenetic informa¬ tion that the evolutionary taxonomist will not admit (if we ignore Simpson's definition of monophyly) is convergent similarity. Convergent characters do not reveal common ancestry: they therefore must be excluded. But in the original treatises that I cited, the conflict between rates of divergence and order of splitting was not taken to be a crucial issue. It was noticed as a difficulty, but not as one of great importance. They were content with some uncertainty as to whether splitting or phenetic divergence should be preferred in particular cases. Since then, however, the cladistic school has appeared. This has forced evolutionary taxonomists to state their preferences more boldly, and both Mayr(1974,1981) and Simpson (1975, also 1978, pp. 271,275), as well as others (e.g. Johnson 1970, p. 233; Bock 1974; Gould 1977b; Hecht and Edwards 1977; Szalay 1977; Halstead, White, and Maclntyre 1979; Ashlock 1980; Martin 1981; Charig 1981, 1982) have done so. They have interpreted their philosophy of monophyly and homology decidedly in favour of representing rapid phenetic divergence in classification. They would not, however, always call it rapid phenetic divergence. According to Mayr (1974), birds are not simply classified by their phenetic divergence from reptiles. The evolution of birds, he stresses, was by means of an adaptive innovation: the change was not simply phenetic, it was adaptive; and the evolution of a new adapted type should be classified as a new 'grade' (Huxley 1958). Thus: 'Rensch, [etc.] ... have particularly emphasized the importance of these levels of adaptation, designated by Huxley as grades. All members of a grade are characterized by a well-integrated adaptive complex' (Mayr 1974 [1976, p. 450]). And 'In the history of the vertebrates we know many 32](https://iiif.wellcomecollection.org/image/b18021451_0045.JP2/full/800%2C/0/default.jpg)
