Paraphyly

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A group of taxa is said to be paraphyletic if the group consists of all the descendants of a possibly hypothetical closest common ancestor minus one or more monophyletic groups of descendants (typically one such group). This term is used in both phylogenetics[note 1] and linguistics.

Contents

Phylogenetics

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 is neither monophyletic nor paraphyletic is said to be polyphyletic (Greek πολύς [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).

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:

  • In the flowering plants, Dicotyledons, in the traditional sense, because they exclude Monocotyledons. The former name has not been used as an ICBN classification for decades, but is allowed as a synonym of Magnoliopsida.[note 2] The former angiosperms (Magnoliophyta), or flowering plants, comprised both. Phylogenetic analysis, however, indicates that the monocots are a development from a dicot ancestor. Excluding them from the dicots makes the latter a paraphyletic group.[2]

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