phylogenetic tree, the green group is paraphyletic; it is composed of a common ancestor (the lowest green vertical stem) and some of its descendants, but it excludes the blue group (a monophyletic group) which
diverged from the green group.
The term was coined by
Willi Hennig to apply to well-known taxa like Reptilia (
reptiles) which, as commonly named and traditionally defined, is paraphyletic with respect to
birds. Reptilia contains the last common ancestor of reptiles and all descendants of that ancestor, including all extant reptiles as well as the extinct
synapsids, except for mammals and birds. Other commonly recognized paraphyletic groups include
The term paraphyly, or paraphyletic, derives from the two
Ancient Greek words παρά (pará), meaning "beside, near", and φῦλον (phûlon), meaning "genus, species", and refers to the situation in which one or several monophyletic subgroups of organisms (e.g., genera, species) are left apart from all other descendants of a unique common ancestor.
Conversely, the term monophyly, or monophyletic, builds on the Ancient Greek prefix μόνος (mónos), meaning "alone, only, unique", and refers to the fact that a monophyletic group includes organisms consisting of all the descendants of a unique common ancestor.
By comparison, the term polyphyly, or polyphyletic, uses the Ancient Greek prefix πολύς (polús), meaning "many, a lot of", and refers to the fact that a polyphyletic group includes organisms arising from multiple ancestral sources.
Groups that include all the descendants of a common ancestor are said to be monophyletic. A paraphyletic group is a monophyletic group from which one or more subsidiary
clades (monophyletic groups) are excluded to form a separate group. Philosopher of science Marc Ereshefsky has argued that paraphyletic taxa are the result of
anagenesis in the excluded group or groups. Cladists do not grant paraphyletic assemblages the status of "groups" or reify them with explanations, because they represent evolutionary non-events 
A group whose identifying features evolved
convergently in two or more lineages is polyphyletic (Greek πολύς [polys], "many"). More broadly, any taxon that is not paraphyletic or monophyletic can be called polyphyletic. Empirically, the distinction between polyphyletic groups and paraphyletic groups is rather arbitrary, since the character states of common ancestors are inferences, not observations.
These terms were developed during the debates of the 1960s and 1970s accompanying the rise of
Paraphyletic groupings are considered problematic by many taxonomists, as it is not possible to talk precisely about their phylogenetic relationships, their characteristic traits and literal extinction. Related terms are
chronospecies, budding cladogenesis, anagenesis, or
'grade' groupings. Paraphyletic groups are often relics from outdated hypotheses of phylogenic relationships from before the rise of cladistics.
Wasps are paraphyletic, consisting of the
bees, which are not usually considered to be wasps; the
Symphyta") too are paraphyletic, as the Apocrita are nested inside the Symphytan clades.
prokaryotes (single-celled life forms without cell nuclei) are a paraphyletic grouping, because they exclude the
eukaryotes, a descendant group.
Archaea are prokaryotes, but archaea and eukaryotes share a common ancestor that is not ancestral to the bacteria. The prokaryote/eukaryote distinction was proposed by
Edouard Chatton in 1937 and was generally accepted after being adopted by Roger Stanier and C.B. van Niel in 1962. The botanical code (the ICBN, now the
ICN) abandoned consideration of bacterial nomenclature in 1975; currently, prokaryotic nomenclature is regulated under the
ICNB with a starting date of 1 January 1980 (in contrast to a 1753 start date under the ICBN/ICN).
dicotyledons (in the traditional sense) are paraphyletic because the group excludes
monocotyledons. "Dicotyledon" has not been used as a botanic classification for decades, but is allowed as a synonym of Magnoliopsida.[note 1] Phylogenetic analysis indicates that the
monocots are a development from a
dicot ancestor. Excluding monocots from the dicots makes the latter a paraphyletic group.
Among animals, several familiar groups are not, in fact, clades. The order
even-toed ungulates) as traditionally defined is paraphyletic because it excludes
Cetaceans (whales, dolphins, etc.). Under the ranks of the
ICZN Code, the two taxa are separate orders. Molecular studies, however, have shown that the Cetacea descend from artiodactyl ancestors, although the precise phylogeny within the order remains uncertain. Without the Cetaceans the Artiodactyls are paraphyletic.
Reptilia, as traditionally defined, is paraphyletic because it excludes birds (class
Aves) and mammals. Under the ranks of the
ICZN Code, these three taxa are separate classes. However, mammals hail from the
synapsids (which were once described as "mammal-like reptiles") and birds are sister taxon to a group of dinosaurs (part of
Diapsida), both of which are "reptiles". Alternatively, reptiles are paraphyletic because they gave rise to (only) birds. Birds and reptiles together make
Sauropsids, a clade of
Amniota that is the sister group of the clade that includes mammals.
Species have a special status in systematics as being an observable feature of nature itself and as the basic unit of classification. Some articulations of the
phylogenetic species concept require species to be monophyletic, but paraphyletic species are common in nature, to the extent that they do not have a single common ancestor. Indeed, for sexually reproducing taxa, no species has a "single common ancestor" organism. Paraphyly is common in
speciation, whereby a mother species (a
paraspecies) gives rise to a daughter species without itself becoming extinct. Research indicates as many as 20 percent of all animal species and between 20 and 50 percent of plant species are paraphyletic. Accounting for these facts, some taxonomists argue that paraphyly is a trait of nature that should be acknowledged at higher taxonomic levels.
Cladists advocate a phylogenetic species concept  that does not consider species to exhibit the properties of monophyly or paraphyly, concepts under that perspective which apply only to groups of species. They consider Zander's extension of the "paraphyletic species" argument to higher taxa to represent a
Uses for paraphyletic groups
When the appearance of significant traits has led a subclade on an evolutionary path very divergent from that of a more inclusive clade, it often makes sense to study the paraphyletic group that remains without considering the larger clade. For example, the
Neogene evolution of the
Artiodactyla (even-toed ungulates, like deer, cows, pigs and hippopotamuses - note that
Hippopotamidae, the families that contain these various artiodactyls, are all monophyletic groups) has taken place in environments so different from that of the
Cetacea (whales, dolphins, and porpoises) that the Artiodactyla are often studied in isolation even though the cetaceans are a descendant group. The
prokaryote group is another example; it is paraphyletic because it is composed of two Domains (Eubacteria and Archaea) and excludes (the
eukaryotes). It is very useful because it has a clearly defined and significant distinction (absence of a cell nucleus, a
plesiomorphy) from its excluded descendants.
Also, some systematists recognize paraphyletic groups as being involved in evolutionary transitions, the development of the first tetrapods from their ancestors for example. Any name given to these hypothetical ancestors to distinguish them from tetrapods—"fish", for example—necessarily picks out a paraphyletic group, because the descendant tetrapods are not included. Other systematists consider reification of paraphyletic groups to obscure inferred patterns of evolutionary history.
Current phylogenetic hypotheses of tetrapod relationships imply that
viviparity, the production of offspring without the external laying of a fertilized egg, developed
independently in the lineages that led to humans (Homo sapiens) and
southern water skinks (Eulampus tympanum, a kind of lizard). Put another way, viviparity is a synapomorphy for
Theria within mammals, and an autapomorphy for Eulamprus tympanum (or perhaps a synapomorphy, if other Eulamprus species are also viviparous).
"Groups" based on independently-developed traits such as these examples of viviparity represent examples of
polyphyly, not paraphyly.
Flightless birds are polyphyletic because they independently (in parallel) lost the ability to fly.
Animals with a
dorsal fin are not paraphyletic, even though their last common ancestor may have had such a fin, because the
Mesozoic ancestors of porpoises did not have such a fin, whereas pre-Mesozoic fish did have one.
Quadrupedalarchosaurs are not a paraphyletic group.
Bipedal dinosaurs like Eoraptor, ancestral to quadrupedal ones, were descendants of the last common ancestor of quadrupedal dinosaurs and other quadrupedal archosaurs like the crocodilians.
Non-exhaustive list of paraphyletic groups
The following list recapitulates a number of paraphyletic groups proposed in the literature, and provides the corresponding monophyletic taxa.
^Stackebrabdt, E.; Tindell, B.; Ludwig, W.; Goodfellow, M. (1999). "Prokaryotic Diversity and Systematics". In Lengeler, Joseph W.; Drews, Gerhart; Schlegel, Hans Günter (eds.). Biology of the prokaryotes. Stuttgart: Georg Thieme Verlag. p. 679.
abSimpson 2006, pp. 139–140 harvnb error: no target: CITEREFSimpson2006 (
help). "It is now thought that the possession of two cotyledons is an ancestral feature for the taxa of the flowering plants and not an apomorphy for any group within. The 'dicots' ... are paraphyletic ...."
^Romer, A. S. & Parsons, T. S. (1985): The Vertebrate Body. (6th ed.) Saunders, Philadelphia.
abcSharkey, M. J. (2007).
"Phylogeny and classification of Hymenoptera"(PDF). Zootaxa. 1668: 521–548.
10.11646/zootaxa.1668.1.25. Symphyta and Apocrita have long been considered as suborders of Hymenoptera but since recognition of the paraphyletic nature of the Symphyta (Köningsmann 1977, Rasnitsyn 1988) and the advent of cladistic methods the subordinal classification should be avoided. Likewise the woodwasps are thought to be non-monophyletic, forming a grade that is ancestral relative to Apocrita and Orussidae. The traditional hymenopteran classification is faulty, by cladistic criteria,in the same way as pre-cladistic vertebrate classifications in which groups sharing plesiomorphic characterswere recognized as natural, e.g., fishes were once grouped together as 'Pisces', which excluded tetrapods.
^New data on Kimberella, the Vendian mollusc-like organism (White sea region, Russia): palaeoecological and evolutionary implications (2007), "Fedonkin, M.A.; Simonetta, A; Ivantsov, A.Y.", in Vickers-Rich, Patricia; Komarower, Patricia (eds.), The Rise and Fall of the Ediacaran Biota, Special publications, vol. 286, London: Geological Society, pp. 157–179,
citation}}: CS1 maint: uses authors parameter (
^Martindale, Mark; Finnerty, J.R.; Henry, J.Q. (September 2002). "The Radiata and the evolutionary origins of the bilaterian body plan". Molecular Phylogenetics and Evolution. 24 (3): 358–365.
^Webster, Bonnie L.; Copley, Richard R.; Jenner, Ronald A.; Mackenzie-Dodds, Jacqueline A.; Bourlat, Sarah J.; Rota-Stabelli, Omar; Littlewood, D. T. J.; Telford, Maximilian J. (November 2006). "Mitogenomics and phylogenomics reveal priapulid worms as extant models of the ancestral Ecdysozoan". Evolution & Development. 8 (6): 502–510.
^Greenhill, Simon J. and Russell D. Gray. (2009.) "Austronesian Language and Phylogenies: Myths and Misconceptions About Bayesian Computational Methods," in Austronesian Historical Linguistics and Culture History: a Festschrift for Robert Blust, edited by Alexander Adelaar and Andrew Pawley. Canberra: Pacific Linguistics, Research School of Pacific and Asian Studies, The
Australian National University.