• There are two main principles, and three main schools, of biological classification: phenetic and phylogenetic principles, and phenetic, cladistic, and evolutionary schools. The schools differ in how (if at all) they represent evolution in classification.

• Phenetic classification ignores evolutionary relations and classifies species by their similarity in appearance; cladism ignores phenetic relations and classifies species by their recency of common ancestry; evolutionary taxonomy includes both phenetic and phylogenetic relationships.

• Phenetic classification is ambiguous because there is more than one way of measuring phenetic similarity and the different measures can disagree. Cladism, however, is unambiguous because there is only one phylogenetic tree of all living things.

• Evolutionary taxonomy avoids some of the extraordinary properties of cladism. But it suffers from the ambiguity of phenetic taxonomy, and its argument for excluding one kind of phenetic relation (convergence) works equally well against the kind of phenetic relation (differential divergence) that it includes.