monophyly

Monophyly is a term in phylogenetics describing a group of organisms that includes an ancestral population and all of its descendants. Such a group is called a clade. Monophyletic groups can be represented on a phylogenetic tree by a single branch that includes the most recent common ancestor and every lineage that descends from it. The concept applies to living and extinct taxa, and is central to modern cladistic approaches to taxonomy. The term derives from Greek monos “alone” and phylon “race” or “tribe.”

Crown groups and stem groups are useful distinctions within monophyly. A crown group comprises the last common ancestor of all living members of a clade and all of its descendants. A stem group includes extinct lineages that are more closely related to the crown group than to any other living lineage but are not themselves part of the crown.

Monophyly is contrasted with paraphyly and polyphyly. A paraphyletic group contains the most recent common ancestor but excludes one or more of its descendants. Traditional Reptilia is often described as paraphyletic because birds—the descendants of archosaurs—are not always included. A polyphyletic group gathers organisms without including their most recent common ancestor, typically because the grouping is based on convergent traits rather than shared ancestry, as in some “warm-blooded” groupings that unite birds and mammals.

In taxonomy, the recognition of monophyletic groups guides classification and nomenclature. Molecular data (DNA, RNA, proteins) and morphological data are used to infer phylogenies and assess whether a proposed group is monophyletic.