Monophyletic group

Definition

A set of taxa comprising an inferred common ancestor and all of its descendants; the only groups considered valid for formal classification in cladistics. Monophyly is diagnosed by shared derived characters (synapomorphies) that originated in the common ancestor and were inherited by all descendants. Groups that exclude descendants (paraphyletic) or combine unrelated ancestors (polyphyletic) are rejected in modern systematics.

Etymology

Greek monos (single, alone) + phyle (tribe, race) + group

Example

Hymenoptera (ants, bees, and wasps) form a monophyletic group diagnosed by a constricted waist (petiole) and two pairs of membranous wings with hooked coupling mechanisms; this clade includes all descendants of their common ancestor, encompassing both the stinging Aculeata and the parasitic parasitoid lineages.

Synonyms

• clade
• natural group
• holophyletic group

Related Terms

• paraphyletic group
• Polyphyletic group
• synapomorphy
• Cladistics
• common ancestor
• crown group
• stem group

Usage Notes

In strict cladistic usage, 'monophyletic' and 'clade' are synonymous; some older literature distinguished 'holophyletic' (all descendants) from 'monophyletic' (any ancestor-descendant set), but this distinction is now obsolete. The term is absolute—a group cannot be 'somewhat monophyletic.' Molecular phylogenetics has revealed many traditionally recognized insect families and orders to be non-monophyletic, prompting taxonomic revision.