paraphyletic

In biological classification, term describing a group of species that includes the most recent common ancestor of all its members but not all of its descendants. For example, the class Reptilia is a paraphyletic group because it includes the reptiles' sauropsid ancestor but not birds, which are also its descendants. Other examples are invertebrates and fish.

If a group consists of all the descendants of a common ancestor plus the ancestor itself, it is called monophyletic; if it includes some or all of the descendants but not their common ancestor, it is called polyphyletic. Distinguishing between such groups is an integral part of the system of classification known as cladistics, which seeks to classify organisms into a hierarchy of monophyletic groups, or clades.