In systematics, describing a group of organisms that excludes one or more descendants of a particular single common ancestor. For example, the taxonomic group ‘dinosaurs’ is paraphyletic unless it includes the birds, which share the same common ancestor as extinct dinosaurs. 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. Compare polyphyletic.