Describing a group of organisms that contains the descendants of two or more different ancestors, while excluding other descendants of a single ultimate common ancestor. Such a group may be constructed on the basis of certain shared traits, which may have evolved convergently (see convergent evolution), but it does not necessarily reflect any close evolutionary relationship, and is therefore rejected as a basis for phylogenetic classification. For example, mammals and birds share the trait of endothermy (‘warm-bloodedness’), but the grouping ‘endotherms’ is polyphyletic because its members have quite different immediate ancestors and the defining trait evolved independently in each member. Compare monophyletic; paraphyletic.