Variations on Monophyly: Holophyly, Paraphyly, & Polyphyly

        Loosely, a "monophyletic" group is any group that has a common ancestor. Thus a taxon that comprises A B C D and their common ancestor E is monophyletic sensu lato. ("in the broad sense"). However, because all living organisms have a common ancestor, any conceivable grouping of organisms would qualify as "monophyletic". We can recognize three types of monophyletic taxa, only one of which is "monophyletic" sensu stricto ("in the strict sense") and serves as the basis of modern phylogenetic taxonomy. This is the Holophyletic group, which includes a common ancestor and all of its descendants. In the example, AB and CD are each holophyletic, because each has a common ancestor that is included in the group in gray. The group including [ ((AB) + (CD)) + E ] is also holophyletic, because it includes E and all of its descendants A B C D. Holophyletic taxa are the only ones accepted in modern phylogenetic taxonomy.

        A second type of group formerly called 'monophyletic' is now called 'Paraphyletic.' A paraphyletic group includes a common ancestor and some but not all of its descendants. In the example, the group B C E has a common ancestor E, but excludes A and D. Such groups were formerly considered useful, for example to indicate a judgement that B & C were considered similar to each other, and also to E, whereas A and D were each considered distinctive in their own ways. Groups like B C E have been called Grades, to indicate a similar level of structure. For example, Reptilia, Aves, and Mammalia were classically recognized as three separate taxonomic Classes, marked by scales, feathers, and hair, respectively. With the modern recognition that feathers and hair are both modified scale structures, that birds are closely related to Saurischian dinosaurs, and that mammals are descended from Therapsid reptiles (which are not 'dinosaurs'), the concept of 'Reptilia' as a monophyletic group disappears. Paraphyletic groups are the basis of classical taxonomy, for example the "Seven Classes of Vertebrates". Since the 1960s, the classical system has been replaced by the phylogenetic system, although it persists in popular thinking and some older textbooks.

        Finally, a Polyphyletic group has a common ancestor outside the group, but which was regarded as useful as indicating a grade of structure, or was at one time thought to be monophyletic. Polyphyletic groups are excluded from modern systematics. There are many examples. Linnaeus, the Father of Taxonomy, placed Birds and Bats in the same group as flying creatures, though he did correctly group whales with other mammals. Eared and Earless seals (Otarridae and Phocidae) were at one time believed to be descended from bear- and weasel-like carnivores, respectively, but were placed in the same group Pinnipedia as having similar structure. These two families along with Walruses (Odobenidae) are now recognized as a holophyletic group.