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( 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. 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.
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