Paraphyly

In phylogenetics, a group of organisms is said to be paraphyletic (Greek para = near and phyle = race) if the group contains its most recent common ancestor, but does not contain all the descendants of that ancestor.

Reptiles are a paraphyletic group. The group can be made monophyletic by including the birds (Aves).

Groups that do include all the descendants of the most recent common ancestor - are said to be monophyletic.

Sometimes the term holophyletic is used instead of monophyletic. Technically these two terms are not equivalent: originally a monophyletic group was simply one including the most recent common ancestor of its members (Greek monos = one) and would thus be either monophyletic or paraphyletic in the modern sense; while a holophyletic group included all descendants of the most recent common ancestor (Greek holos = whole), thus being monophyletic in the modern sense. However, in actual practice monophyletic has lost this original meaning and has displaced holophyletic which has mostly dropped out of use.

Technically, in the original meaning of the words, a paraphyletic group is a monophyletic group from which one of the clades is excluded to form a separate group (as in the paradigmatic example of reptiles and birds, shown in the picture). A paraphyletic group can be fixed either by expanding it, and including the missing clade, or by splitting it into its component parts.

A group that does not contain the most recent common ancestor of its members is said to be polyphyletic (Greek polys = many).

These formulations were developed during the debates of the 1960s and 70s accompanying the rise of cladistics (a clade is a term for a monophyletic group). Before that period the distinction between mono- and polyphyletic groups was based on the inclusion or exclusion of the most recent common ancestor. It was shown, however, that the inclusion of ancestors in the classification leads to unavoidable logical inconsistencies, and, in some schools of taxonomy, the phylogenetic pattern is described exclusively in terms of nested patterns of the sister group relationships between the known representatives of taxa without referring to the ancestor-descendant relationships.

Examples of Paraphyletic Groups

Many of the older classifications contain paraphyletic groups, especially the traditional 2–6 kingdom systems and the classic division of the vertebrates. Paraphyletic groups are often erected on the basis of (sym)-plesiomorphies (ancestral similarities) instead of (syn)apomorphies (derived similarities). Examples of well-known paraphyletic groups includes:

• The class Reptilia as traditionally defined is paraphyletic because that class excludes birds (class Aves), which are descended from reptiles (see the illustration above). Reptiles would be monophyletic if Reptiles were defined to include Aves.

• The Prokaryotes (organisms without cell nuclei) consisting of bacteria and archaea. This group is paraphyletic because the clade containing the nearest common ancestor also includes all eukaryotes (organisms with cell nuclei).

• Agnatha, jawless fish. This group contains two significant animal groups, hagfish and lampreys. yet their nearest common ancestor is the ancestor of all vertebrates, so Agnatha is paraphyletic.

• Osteichthyes, bony fish, are paraphyletic because they include Actinopterygii (ray-finned fish) and Sarcopterygii (lungfish, etc). However, tetrapods are descendents of the nearest common ancestor of Actinopterygii and Sarcopterygii, and tetrapods are not in Osteichthyes, hence Osteichthyes is paraphyletic.

Cladistics Generally Discourage Paraphyletic Groups

In most cladistics-based schools of taxonomy, the existence of paraphyletic groups (as well as polyphyletic 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 organisims, for the following reaons:

• Clades are simple to define: a typical clade definition is "All descendents 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 Synapsid clade, minus the Aves clade". Or "Warm-blooded animals are the Aves clade plus the Mammals clade". Because polyphyletic and paraphyletic groups are defined in terms of clades, they are considered less important than clades.

• For a given evolutionary tree of, say, N nodes, there are exactly N clades (one per node). However, the number of paraphyletic groups and polyphyletic groups is exponentially larger than that, on the order of N². Yet only a small fraction of the paraphyletic groups are given names or discussed.

• Paraphyletic 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 paraphyletic, rather than monophyletic, then such a group looses its original significance.

Reasons Why Paraphyletic Groups Are Sometimes Useful

Others argue that paraphyletic groups are necessary for a comprehensive classification including extinct groups, since each species, genus, and so forth necessarily originates from part of another.

For instance, the Prokaryote group is paraphyletic because it excludes many of its descendent organisms (such as Eukaryotes), yet the Prokaryotegroup is very useful because it has a clearly-defined and significant distinction (no cell nucleus) from its excluded descendents. So, even though Prokaryotes are not a clade, the term is still useful.

It has been suggested that paraphyletic groups be clearly marked to distinguish them from clades, for instance with asterisks: Reptilia*. The term evolutionary grade is sometimes used for such groups.

References

• Tudge, Colin (2000). The Variety of Life. Oxford University Press. ISBN 0-19-860426-2.