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.
61/220 page 49
![The techniques and justification of cladism There is no single hierarchy of phenetic resemblance, there are as many of them as there are cluster statistics. Phylogenetic classifica¬ tion should in principle be unambiguous, because there is only one hierarchy of phylogeny. If it can be discovered, it will underwrite the only known objective system of classification; and should therefore be the aim of classification. It should provide what Hennig called the 'general reference system' of biology. A digression on prokaryotes If a set of species does not possess unique phylogeneticrelationships, Hennig's philosophy will not apply to it, and cladistic classification will become ambiguous. All the genes of a set of species with typical Mendelian inheritance have descended through the same phylo¬ genetic hierarchy: the phylogeny of cytochrome genes in vertebrates is the same as the phylogeny of haemoglobins. That is why the phylogenetic classification is unambiguous. If, however, the different genes of a 'species' had experienced different phylogenetic histories, and the species themselves had experienced mixed ancestries, Hennig's argument would break down. The species would not longer possess the unambiguous relations required for them to be 'substituted for Xq, Xj, X2 ... in Figure [4.1]'. Prokaryotes may provide an example. The 'species' of prokaryotes may not have unambiguous ancestries, because of the frequency of 'horizontal' gene transfer. Genes certainly do move between prokaryote species. The phenomenon is well documented. It was discovered independently by two groups of Japanese biologists, led respectively by Akima and Ochiai, in the late 1950s; and became well known elsewhere after their work was reviewed in English in 1963 (see Falkow 1975; Broda 1979). Both groups performed the experiment of brewing Escherichia co7i resistant to a certain antibiotic together with another genus of bacterium. Shigella. Resistant Shigella could later be isolated from the brew. Plasmids called R factors carry the genes conferring resistance between species. Much is now known about the ease of movement of different R factors between different kindof bacteria (Falkow 1975, pp. 78-9), the rates of which vary from 10'^ to less than 10® depending on the species. No one doubts that the plasmid genes of a bacterial species at any one time have experienced multiple ancestry. If bacterial species were defined by their plasmid genes, Hennig's philosophy could not apply to them. But the plasmid genes are actually unimportant. They do not determine much of the bacterial phenotype. Most of the information is contained on the bacterium's larger chromosome of DNA, which is as separate from any plasmids as is the nuclear DNA of a mammal from that of a mammal-dwelling virus. The chromosomal genes are 49](https://iiif.wellcomecollection.org/image/b18021451_0062.JP2/full/800%2C/0/default.jpg)
