polyphyletic

Polyphyletic describes a group of organisms that are classified together based on similarities that arose independently in separate lineages, rather than from a shared recent ancestor. In contrast, a monophyletic group (a clade) includes an ancestor and all of its descendants, and a paraphyletic group includes the ancestor and some, but not all, descendants. A polyphyletic group does not include the most recent common ancestor of all its members.

Polyphyly often results from grouping by convergent traits—features evolved independently in different evolutionary lineages—rather than by shared ancestry. It can also arise from incomplete sampling or biased interpretation of data. Modern phylogenetic methods, especially molecular data such as DNA, RNA, and protein sequences, help identify true evolutionary relationships and reveal polyphyletic assemblages.

Examples commonly cited in discussions of polyphyly include informal groupings like “flying vertebrates,” which may include birds and bats (and sometimes extinct pterosaurs) because flight evolved independently in these lineages. Another debated example is organizing by endothermy (warm-bloodedness), which has evolved in mammals and birds separately; a trait-based group defined this way is considered polyphyletic.

Taxonomic implications: when a group is identified as polyphyletic, taxonomists typically restructure classifications to reflect monophyletic relationships. This often involves splitting the group into separate lineages or reassigning members to different groups so that each unit represents a true clade. The term remains a diagnostic concept in systematics to explain why certain historical classifications are not scientifically valid.