Paraphyletic

Definition

Describing a taxonomic group that contains the most recent common ancestor of all members but excludes one or more descendant lineages that also share that ancestor. Paraphyletic groups are incomplete clades: they capture ancestry but not full descent, often because some descendants have been reclassified into separate groups based on divergent traits.

Etymology

From Greek para- (beside, near) + phylon (tribe, race) + -ic, referring to a grouping that lies alongside but does not fully contain a clade.

Example

In traditional classifications, 'Protura' (tetrapods excluding amniotes) is paraphyletic because it contains the ancestor of amphibians, reptiles, birds, and mammals but excludes the amniote descendants. In entomology, 'Apterygota' (primitively wingless insects) is paraphyletic if it excludes the winged insects (Pterygota) that evolved from within this grade, as molecular phylogenetics now supports.

Related Terms

• monophyletic
• polyphyletic
• clade
• grade
• stem group
• crown group
• phylogenetic systematics

Usage Notes

Paraphyletic groups are rejected in cladistic taxonomy but remain common in traditional and practical classifications where they capture useful morphological grades (e.g., 'worms,' 'algae,' 'invertebrates'). The term contrasts with monophyletic (holophyletic) groups, which include all descendants, and polyphyletic groups, which lack the common ancestor entirely. In arthropod systematics, many historically recognized groups—such as 'Crustacea' excluding Hexapoda, or 'Myriapoda' as traditionally defined—have been revealed as paraphyletic through molecular phylogenetics, prompting taxonomic revision.