The phylogenetic principle of classification is an evolutionary principle: in contrast to the phenetic principle, it classifies species according to how recently they share a common ancestor.
Two species that share a more recent common ancestor will be put in a group at a lower level than two species sharing a more distant common ancestor. As the common ancestor of two species becomes more and more distant, they are grouped further and further apart in the classification. In the end, all species are contained in the category the set of all living things which contains all the descendants of the most distant common ancestor of life..
Cladism and evolutionary classification are the two taxonomic schools which make use of the phylogenetic principle to differing degrees.
For many evolutionary biologists including Michael Donoghue, the phylogenetic principle is the most effective way of classifying species.
Figure: a phylogenetic classification of the main vertebrate groups.