Polyphyly

Polyphyly is a concept in taxonomy and phylogenetics describing a group of organisms that does not include their most recent common ancestor. In a polyphyletic group, the members arise from multiple evolutionary origins, and the group's defining traits are often the result of convergent evolution or other non-homologous similarities. This stands in contrast to monophyly, in which a group consists of a common ancestor and all of its descendants (a clade), and paraphyly, which includes the common ancestor and some but not all descendants.

Examples commonly cited include informal groupings such as “flying animals,” which includes birds, bats, and extinct pterosaurs, because flight arose independently in those lineages. Likewise, terms like “warm-blooded animals” are considered polyphyletic because endothermy evolved in birds and mammals separately. In practice, many historical groupings were polyphyletic or paraphyletic, and modern systematics seeks to redefine groups as monophyletic.

Determining polyphyly involves phylogenetic analyses that integrate molecular and morphological data to reconstruct evolutionary relationships. When a group is found to be polyphyletic, taxonomists typically revise its composition or split it into monophyletic subgroups to better reflect shared ancestry. Understanding polyphyly helps avoid misinterpretations of evolutionary history and improves the consistency of classifications across biology.