Monophylum

Monophylum is a taxonomic concept used to describe a monophyletic group—a lineage that includes a common ancestor and all of its descendants. The term comes from Greek monos ("single") and phylon ("tribe" or "race"). In many current discussions, monophylum is treated as essentially synonymous with a monophyletic group, though monophyly and related terms like clade are more common in contemporary cladistics.

In a phylogenetic tree, a monophylum corresponds to a single clade: a node and all of its descendant branches. Such groups are distinguished from paraphyletic groups, which include the common ancestor and some, but not all, descendants, and polyphyletic groups, whose members do not share an immediate common ancestor within the group.

Determining whether a group is a monophylum relies on phylogenetic analyses based on morphology, molecular data, or both. The goal is to recover the most recent common ancestor of the included taxa and to include all descendants arising from that ancestor.

Examples often cited include the clade Mammalia, which includes all living mammals and their extinct relatives; more broadly, any clade defined by a single ancestral lineage that has not been broken by excluded descendants. Monophyla are central to many taxonomic and evolutionary discussions, as they reflect true genealogical relationships rather than superficial similarity.