In phylogenetics, a grouping of organisms is said to be paraphyletic (Greek para = near and phyle = race) if the group contains the most recent common ancestor of its members, but the group does not include all the descendants of this common ancestor. In biology, Phylogenetics (Greek: phylon = race and genetic = birth) is the taxonomical classification of organisms based on how closely they are related in terms of evolutionary differences. ... An ancestor is a parent or (recursively) the parent of an ancestor. ...

Reptiles are a paraphyletic group. The group can be made monophyletic by including the birds (aves).

Groups which include all the descendants of the most recent common ancestor are commonly termed monophyletic or holophyletic. The former is more common, but sometimes paraphyletic groups are also considered monophyletic, in which case the latter is used. A paraphyletic group can be fixed by expanding it, and including other clades. A class which does not contain the most recent common ancestor of its members is called polyphyletic. Image:Paraphyletic. ... Image:Paraphyletic. ... In phylogenetics, a group is monophyletic (Greek: of one stem) if all organisms in that group are known to have developed from a common ancestral form, and all descendants of that form are included in the group. ... A clade is group of organisms which share a common ancestor and which includes all decendents of that ancestor. ... In biology, a taxon is polyphyletic if it is descended from more than one root form (in Greek poly = many and phyletic = racial). ...

Many of the older classifications contain paraphyletic groups, especially the traditional 2-6 kingdom systems and the classic division of the vertebrates. For example, the class Reptilia as traditionally defined is paraphyletic because that class does not include birds (class Aves), which are descended from reptiles. Paraphyletic groups are often erected on the basis of plesiomorphies (ancestral similarities) instead of upon apomorphies (derived similarities). In biology, a kingdom or regnum is the top-level, or nearly the top-level, taxon of organisms in scientific classification. ... Groups Conodonta Hyperoartia Petromyzontidae (lampreys) Pteraspidomorphi (early jawless fish) Thelodonti Anaspida Cephalaspidomorphi (early jawless fish) Galeaspida Pituriaspida Osteostraci Gnathostomata (jawed vertebrates) Placodermi Chondrichthyes (cartilaginous fish) Acanthodii Osteichthyes (bony fish) Actinopterygii (ray-finned fish) Sarcopterygii (lobe-finned fish) Actinistia (coelacanths) Dipnoi (lungfish) Tetrapoda Amphibia Amniota Sauropsida/(Reptiles) Aves (Birds) Synapsida Mammalia... Orders  Crocodilia - Crocodilians  Rhynchocephalia - Tuataras  Squamata   Suborder Sauria - Lizards   Suborder Serpentes - Snakes Testudines - Turtles Superorder Dinosauria  Saurischia  Ornithischia The reptiles are a group of vertebrate animals. ... For other meanings of bird, see bird (disambiguation). ...

In most cladistics-based schools of taxonomy, the existence of paraphyletic groups in a classification are regarded as errors. Some groups in currently accepted taxonomies may later turn out to be paraphyletic, in which case the classifications may be revised to eliminate them. Some, however, feel that having paraphyletic groups is an acceptable sacrifice if it makes the taxonomy more understandable. Others argue that paraphyletic groups are necessary to have a comprehensive classification including extinct groups, since each species, genus, and so forth necessarily originates from part of another. It has been suggested that paraphyletic groups should be allowed but clearly marked as such, for instance in the form Reptilia*. The term "evolutionary grade" is sometimes used for such groups. This cladogram shows the relationship among various insect groups. ...

References

• Colin Tudge (2000). The Variety of Life. Oxford University Press. ISBN 0198604262.