Evolutionary relationships of insects

Abstract

In approaching the study of the evolutionary relationships among insects and their close allies (or, for that matter, among any group of organisms), it is necessary to indicate some of the principles on which phylogenetic inferences should be based. In the past these have differed more or less radically from one group of biologists to another and it is unlikely that general agreement will be reached in the near future. However, there is growing acceptance of the view, first clearly enunciated and developed by Hennig during the 1950s, that monophyletic groups (i.e. those consisting of all the descendants of a common ancestor) are best recognized by the fact that their members share a number of specialized characteristics (synapomorphies). Shared primitive features on the other hand (symplesiomorphies) are not convincing proof of a common origin, since such characteristics may simply have been retained from more remote ancestral forms. It follows that attempts to infer phylogeny without distinguishing between primitive (plesiomorphic) and specialized (apomorphic) features are unsatisfactory in principle. How far synapomorphic characters can be identified and how they may be distinguished from the resemblances due to convergent evolution is, however, not always easy to decide in practice, with the result that even those who accept Hennig’s general principle may differ considerably from one another in the schemes of evolutionary relationship that they devise.