Cladism is natural

In a natural classification, the members of a group resemble one another for non-defining as well as defining characters.

The cladistic classification should be natural for all shared derived characters: the shared derived characters not formally used to define the classification should also fall into the same groups, because all evolutionary changes occur in the same phylogenetic tree. If two sets of shared derived characters seems to fall into contradictory groups, then at least one of them must have been mistakenly analyzed: it is impossible for correctly analyzed shared derived characters to contradict each other.

However, shared ancestral and convergent characters need not show the same pattern, and the cladistic classification will therefore not be natural with respect to them.

Cladistic classifications are therefore natural for cladistic characters, but may be artificial for non-cladistic characters.

Figure: there is a simple relationship between the phylogenetic (cladistic) classification of a group of species and their phylogenetic tree. (a) The phylogenetic tree of seven species. (b) Their cladistic classification. (c) The formal Linnaean classification, for species five as an example.