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.
36/220 page 24
![Evolution and Classification challenged. It is scientifically difficult, perhaps even impossible, to prove that a character is selectively neutral, because neutrality is a negative or 'null' hypothesis. Null hypotheses can in principle be refuted - they are good hypotheses in the Popperian sense - but that is little help to the scientist.^ Neutrality can be confirmed with respect to a particular environmental variable, or even a series of environmental variables; but the danger will always remain that someone will demonstrate that some other environmental variable is selectively influencing the character. The hypothesis of neutrality will then be refuted. This is as true of embryological and ethological evidence as of molecular. In the wake of the theory of recapitulation came doubts as to whether embryonic characters were any less adaptive than adult ones (Garstang 1922, 1928). Many students of animal signals would doubt whether the relation of form and meaning in signals is arbitrary (e.g. Morton 1977; Hamilton and Zuk 1982). It is at present controversial whether molecular evolution is controlled by selection (Goodman 1981). Many characters that have been thought to be non-adaptive have turned out, on investigation, to be adaptive; and judgements of non-adaptiveness are precarious generally (Cain 1964). Modern evolutionary taxonomy may prefer not to justify its techniques with such a shaky philosophy. It may prefer to rely instead on the second reason why some characters might be more conservative in evolution than others. It is that some characters are adapted to a wider range of environments than are others. Broad and generally adapted characters will not change as much in evolution as will narrower, more specialized characters. The more slowly a character evolves, the more it reveals its ancestry, and the better a taxonomic character it provides. This kind of evolutionary constancy does not result from non- adaptation: it depends instead, as Darwin (1859 [1969 edn, p. 400]) said, 'on such organs having generally been subjected to less change in the adaptation of the species to their conditions of life'. Broad adaptation is a better justification of the taxonomic value of a character than is selective neutrality. It is theoretically more plausible. Many modern biologists are aware this is so, and, indeed, probably often intend it when they describe a character as non- adaptive: the translation is easy (Simpson 1961, pp. 100-1). But biologists do often treat ancestry and adaptation as alternatives. We could just let this pass; but it may be worthwhile to point out one of the reasons for the mistake. Most evidence, and especially crude evidence, of adaptation is provided by the comparative method. If one character repeatedly evolves in one particular environment, we 'Which has not stopped some Popperians from arguing that neutralism is unscientific (Ayala et al. 1974); Maynard Smith (1978, p. 37) points out that 'the neutral theory is a good Popperian one'. 24](https://iiif.wellcomecollection.org/image/b18021451_0037.JP2/full/800%2C/0/default.jpg)
