1.6 Clades and reptiles
Are the reptiles a proper clade?
No, because despite the reptiles being derived from a common ancestor, two descendent groups – the birds and the mammals – have been removed from them. The reptiles therefore do not include all the descendants of the primordial reptile species and so are not a complete monophyletic taxon. A taxon which thus comprises a single branch from which one or more clades have been removed is called a paraphyletic taxon (as in the left column of Figure 1): the reptiles are therefore paraphyletic. In cladistic classifications, paraphyletic taxa are not recognised, and so the ‘reptiles’ would not be accepted in such a scheme.
So much for the nature of higher taxa, but how are the constituent species grouped together in the first place? The short answer is ‘through comparison of their characters’, but this begs the question of what is a ‘character’. Because organisms are so complex and so highly integrated, the identification of separate aspects to be treated as taxonomic characters has to be arbitrary, and is thus a subjective issue which presents problems whatever the approach to phylogenetic reconstruction. A particular problem with most morphological characters is that the genetic controls on their development are complex and often poorly understood. As with the selection of variates (i.e. shared but variable characters) for the morphometric description and discrimination of closely related species, the choice of characters for phylogenetic analysis is pragmatic. Those features which are reasonably consistent within each species, but which differ sufficiently in expression from species to species so as to permit degrees of similarity between species to be noted, tend to be used. One fundamental consideration must be mentioned here. As noted earlier in reference to the ‘pachyderms’, the similarities of some features will be misleading as an indicator of relationship if they result from evolutionary convergence. Features showing similarities due to convergence are said to be analogous, and a prime objective of modern systematic methods is to avoid the confusion they can cause in classification. Other features, in contrast, are interpreted as being of similar construction because they have been inherited from a common ancestor. These features are said to be homologous.