Polyphyly

A polyphyletic (Greek for "of many races") group is one whose members' last common ancestor is not a member of the group.

For example, the group consisting of warm-blooded animals is polyphyletic, because it contains both mammals and birds, but the most recent common ancestor of mammals and birds was cold-blooded. Warm-bloodedness evolved separately in the ancestors of mammals and the ancestors of birds.

Scientific classification aims to group species together such that every group is descended from a single common ancestor and therefore it is frequently a goal to eliminate groups that are found to be polyphyletic. This is often the stimulus for major revisions of the classification schemes. A polyphyletic group can be "fixed" either by excluding clades or by adding the common ancestor.

Opinions differ as to whether valid groups need to contain all the descendants of a common ancestor. Groups that do so are called monophyletic, and according to cladistics it should be the aim of classification to ensure that all groups have this property. However, many other taxonomists would argue that there is a valid place for groups that are paraphyletic, i.e. contains its most recent common ancestor but does not contain all the descendants of that ancestor.

Cladistics generally discourages polyphyletic groups
In most cladistics-based schools of taxonomy, the existence of polyphyletic groups (as well as paraphyletic groups) in a classification is discouraged. Monophyletic groups (that is, clades) are considered by these schools of thought to be the most important grouping of organisms, for the following reasons:


 * Clades are simple to define: a typical clade definition is "All descendants of the nearest common ancestor of species X and Y". On the other hand, polyphyletic and paraphyletic groups are always defined in terms of clades, for example "reptiles are the amniote clade, minus the bird and mammal clades". Or "Warm-blooded animals are the Aves clade plus the Mammals clade". Because polyphyletic and paraphyletic groups are defined in terms of clades plus or minus other clades, they are considered less fundamental than monophyletic (single, whole) clades.


 * For a given evolutionary tree of, say, N nodes, there are exactly N clades, one per node. However, the number of paraphyletic groups of one clade minus a descendant clade is N2, one for each child of each node. The total number of polyphyletic groups is exponentially larger than that, a power set on the order of 2N. Only a small fraction of the paraphyletic or polyphyletic groups are given names or discussed.


 * Polyphyletic groups often have their origin in traditional taxonomy, based on similar morphological characteristics. The original perception may have been that the group was entirely descended from a single ancestor. If such a group is later discovered (for instance, due to convergent evolution) to be polyphyletic, rather than monophyletic, then such a group loses its original significance.