monophyletic In systematics, describing a group of organisms that contains all the descendants of a particular single common ancestor. In cladistics such a grouping is called a clade and is the only type of group regarded as valid when constructing classification schemes. Hence the monophyletic grouping Theria contains the marsupial and placental mammals, together with their extinct Mesozoic relatives, all of which share an immediate common ancestor not shared by the more distantly related egg-laying mammals (comprising the Prototheria). Similarly, birds and crocodiles are the living representatives of a monophyletic group, Archosauria, and are more closely related to each other than to other living reptilian descendants. Consequently the grouping `reptiles', used in many modern classification systems, is not monophyletic but paraphyletic, since it excludes the birds (and mammals). Compare polyphyletic.

monophyletic Applied to a group of species that share a common ancestry, being derived from a single interbreeding (or Mendelian) population, as opposed to a polyphyletic group (see POLYPHYLETISM), which is derived from many such populations. If the members of a given taxon are descended from a common ancestor, they are said to be monophyletic; e.g., the families within a class would be monophyletic if they were all descended from the same family or a lower taxonomic unit. Under the strictest definition, they would all have to be descended from a single species.

monophyletic Applied to a group of species that share a common ancestry, being derived from a single interbreeding (or Mendelian) population, as opposed to a polyphyletic group, which is derived from many such populations. If the members of a given taxon are descended from a common ancestor they are said to be monophyletic (e.g. the families within a class would be monophyletic if they were all descended from the same family or a lower taxonomic unit). Under the strictest definition they would all have to be descended from a single species. Monophyly is sometimes used interchangeably with holophyly.

monophyletic If the members of a given taxon are descended from a common ancestor they are said to be ‘monophyletic’, e.g. the families within a class would be monophyletic if they were all descended from the same family or lower taxonomic unit. Under the strictest definition they would all have to be descended from a single species. Compare POLYPHYLETIC.

monophyletic See MONOPHYLY.