(Gr. para: near; phyletikos: bout a tribe) Term applied to a group of organisms which includes the most recent common ancestor of all of its members, but not all of the descendants of that most recent common ancestor. For example, the taxonomic group `reptiles` used in evolutionary systematics is paraphyletic because it excludes birds and mammals, which share the same common ancestor as the reptiles. In cladistics such groups are regarded as invalid when constructing classification schemes, since cladists allow only monophyletic groups, or clades, as a basis for taxonomic groupings. However, in evolutionary systematics paraphyletic groups, or evolutionary grades, are sometimes permitted in order to reflect biological similarities. Thus `reptiles` is used as a group because its members are closely related to each other and they share certain essential characteristics, such as ectothermy (`cold-bloodedness`), that differ markedly from their fellow clade members, the birds and mammals. Cf. polyphyletic.