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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.
Additional recommended knowledge
Relation to Monophyletic Groups
Groups that do include all the descendants of the most recent common ancestor are said to be monophyletic. A paraphyletic group is a monophyletic group from which one or more of the clades is excluded to form a separate group (as in the paradigmatic example of reptiles and birds, shown in the picture).
A group that does not contain the most recent common ancestor of its members is said to be polyphyletic (Greek polys = many).
These terms 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 term paraphyly may be used in any system in which genetic descent modeled by trees is useful. For instance, in linguistics, the Formosan languages form a paraphyletic group of the Austronesian languages as the term refers to the nine branches of the Autronesian family that are not Malayo-Polynesian and restricted to the island of Taiwan.
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:
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 Prokaryote group 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.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Paraphyly". A list of authors is available in Wikipedia.|