Polyphyletic group

Definition

A taxonomic assemblage whose members are derived from two or more distinct evolutionary lineages but whose most recent common ancestor is excluded from the group. Such groups are defined by convergent similarities (homoplasies) rather than shared ancestry, and they are rejected in cladistic classification because they do not reflect true evolutionary relationships.

Etymology

Greek polys (many) + phylon (tribe, race), referring to multiple separate origins.

Example

The traditional grouping 'worms'—including annelids, nematodes, and flatworms—is polyphyletic because these phyla arose independently and their last common ancestor was not a worm; similarly, 'jellyfish' unites scyphozoans and hydrozoans that converged on medusa body plans but exclude their shared ancestor with other cnidarians.

Synonyms

• polyphyly (arrangement)

Related Terms

• Monophyletic group
• paraphyletic group
• Convergent evolution
• Homoplasy
• clade
• grade
• phylogenetic systematics

Usage Notes

Polyphyletic groups are artificial in evolutionary terms and are actively dismantled during phylogenetic revision; they persist only as informal grades or vernacular categories. Contrast with paraphyly, where the common ancestor is included but some descendants are excluded. In entomology, historical groupings such as 'Homoptera' (aphids, scale insects, cicadas, leafhoppers) proved polyphyletic once molecular data showed Sternorrhyncha and Auchenorrhyncha arose separately within Hemiptera, leading to their dissolution.