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.
33/220 page 21
![The techniques and justification of evolutionary taxonomy elaborate the most modern techniques later. We do not need to know their primitive forms, but should give evolutionary taxonomy some credit for the techniques to be discussed in Chapters 3 and 4. For now, let us consider two kinds of technique to reveal convergent characters, by theory and by observation. By theoretical analysis, I mean the application of the theory of evolution to estimate the probability that each character is homologous or analogous. Two kinds of criteria have been used. Both were discussed by Darwin in the Origin (1859 [1969 edn, pp. 399-403]). According to the first criterion, non-adaptive characters are more likely to be homologies than are adaptive characters; and according to the second, characters uniformly adapted to a broad range of environments are more likely to be homologies than are characters that fickly change, among a limited number of states, with every small change in the environment. Let us take the first criterion first. Convergence is caused by the natural selection of adaptations; the obvious method of avoiding convergent characters, therefore, is to avoid adaptations. Groups should be defined by non-adaptive or, more precisely, selectively neutral characters. Whatever its merits, the idea has certainly been influential. Nearly every enthusiasm that has swept through evolutionary taxonomy has been inspired by the doctrine of selective neutrality. Developmental characters and molecular sequences are two outstanding instances. In the late nineteenth century, a biologist would have classified his material by one (or both) of two main kinds of character - adult characters and developmental characters. Different biologists held different opinions about which of the two should be preferred, and the preferences changed with time. The golden age of the embryo- logical criterion began in about 1870, and ran for more than two decades. Embryologists made hay in that long summer undisturbed by any murmuring sceptics. The doubters did come, of course, but they do not matter to us (see Gould 1977a; Ridley 1986). What we should ask is why biologists in 1870 held embryological evidence in such high esteem. Let us turn to the principal paper of its principal British advocate: the paper on the 'genealogical classification of animals', by Edwin Ray Lankester (1873). As Lankester observed the classifications of his time, he saw, in an updated form, the methods and the results of Cuvier; species were classified according to the comparative anatomy of adults. Lankester disapproved of what he saw. He preferred the method of embryology, and It would not be surprising if the facts of development were to lead to another primary grouping of the animal kingdom than that indicated in the four Cuvierian types or the six or seven types now generally adopted ... They are confessedly 21](https://iiif.wellcomecollection.org/image/b18021451_0034.JP2/full/800%2C/0/default.jpg)
