Definition: Polyphyly

Polyphyletic Taxa

The Polyphyletic taxon is a group composed of a number of organisms which might bear some similarities, but does not include the most recent common ancestor of all the member organisms (usually because that ancestor lacks some or all of the characteristics of the group). The taxon shares derived characters which originated several times by convergence.

Illustration: it used to be thought (in Victorian times) that all pachydermatous (large stocky hairless thick-skinned herbivores) mammals - e.g. rhinos, hippos, and elephants - were descended from a single large ancestor. It is now known that each of these animals evolved from a separate small ancestor, and the common ancestor of all of them was small and slightly built, with presumably thin skin and fur.

Another Illustration: Some cladists (e.g. Lovtrup and Gardiner) have argued that because warm-blooded birds and mammals share a number of metabolic and anatomical features in common they must have descended from a single warm-blooded Most Recent Common Ancestor and thus constitute the clade Haemothermia. It is now known that endothermy (warm-bloodedness) evolved independently in each group.

Polyphyletic taxa are considered invalid or unnatural groupings, and are not accepted in either the Linnaean/Evolutionary or the Cladistics taxonomies.

Examples:  Pachyderma, Haemothermia, Algae, Vermes (worms).