In thisphylogenetic tree, the green group isparaphyletic; 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) whichdiverged from the green group.
Paraphyly is ataxonomic term describing a grouping that consists of the grouping'slast common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyleticwith respect to the excluded subgroups. In contrast, amonophyletic grouping (aclade) includes a common ancestor andall of its descendants.
The term received currency during the debates of the 1960s and 1970s accompanying the rise ofcladistics, having been coined by zoologistWilli Hennig to apply to well-known taxa like Reptilia (reptiles), which is paraphyletic with respect tobirds. Reptilia contains the last common ancestor of reptiles and all descendants of that ancestor except for birds. Other commonly recognized paraphyletic groups includefish,monkeys, andlizards.[1][page needed]
The termparaphyly, orparaphyletic, derives from the twoAncient Greek wordsπαρά (pará), meaning "beside, near", andφῦλον (phûlon), meaning "genus, species",[2][3] and refers to the situation in which one or several monophyletic subgroups of organisms (e.g., genera, species) areleft apart from all other descendants of a unique common ancestor.
Conversely, the termmonophyly, ormonophyletic, builds on the Ancient Greek prefixμόνος (mónos), meaning "alone, only, unique",[2][3] and refers to the fact that a monophyletic group includes organisms consisting ofall the descendants of aunique common ancestor.
By comparison, the termpolyphyly, orpolyphyletic, uses the Ancient Greek prefixπολύς (polús), meaning "many, a lot of",[2][3] and refers to the fact that a polyphyletic group includes organisms arising frommultiple ancestral sources.
Cladogram of theprimates, showing amonophyly (the simians, in yellow), aparaphyly (the prosimians, in blue, including the red patch), and apolyphyly (the night-active primates, thelorises and thetarsiers, in red).
Groups that include all the descendants of a common ancestor are said to bemonophyletic. A paraphyletic group is a monophyletic group from which one or more subsidiaryclades (monophyletic groups) are excluded to form a separate group. Philosopher of science Marc Ereshefsky has argued that paraphyletic taxa are the result ofanagenesis in the excluded group or groups.[4] A cladistic approach normally does not grant paraphyletic assemblages the status of "groups", nor does it reify them with explanations, as in cladistics they are not seen as the actual products of evolutionary events.[5]
A group whose identifying features evolvedconvergently in two or more lineages ispolyphyletic (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.[citation needed]
These terms were developed during the debates of the 1960s and 1970s accompanying the rise ofcladistics.
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.[6][7] Related terms arestem group,chronospecies, budding cladogenesis, anagenesis, or'grade' groupings. Paraphyletic groups are often relics from outdated hypotheses of phylogenic relationships from before the rise of cladistics.[8]
Wasps are paraphyletic, consisting of thecladeApocrita withoutants andbees, which are not usually considered to be wasps; thesawflies ("Symphyta") too are paraphyletic, as the Apocrita are nested inside the Symphytan clades.
Theprokaryotes (single-celled life forms without cell nuclei) are a paraphyletic grouping, because they exclude theeukaryotes, a descendant group.Bacteria andArchaea are prokaryotes, but archaea and eukaryotes share a common ancestor that is not ancestral to the bacteria. The prokaryote/eukaryote distinction was proposed byEdouard Chatton in 1937[9] and was generally accepted after being adopted by Roger Stanier and C.B. van Niel in 1962. The botanical code (the ICBN, now theICN) abandoned consideration of bacterial nomenclature in 1975; currently, prokaryotic nomenclature is regulated under theICNB with a starting date of 1 January 1980 (in contrast to a 1753 start date under the ICBN/ICN).[10]
Among plants,dicotyledons (in the traditional sense) are paraphyletic because the group excludesmonocotyledons. "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 themonocots are a development from adicot ancestor. Excluding monocots from the dicots makes the latter a paraphyletic group.[11]
Among animals, several familiar groups are not, in fact, clades. The orderArtiodactyla (even-toed ungulates) as traditionally defined is paraphyletic because it excludesCetaceans (whales, dolphins, etc.). Under the ranks of theICZN 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.[12]The classReptilia is paraphyletic because it excludes birds (classAves). Under a traditional classification, these two taxa are separate classes. However birds are sister taxon to a group of dinosaurs (part ofDiapsida), both of which are "reptiles".[13]
Osteichthyes, bony fish, are paraphyletic when circumscribed to include onlyActinopterygii (ray-finned fish) andSarcopterygii (lungfish, etc.), and to excludetetrapods; more recently, Osteichthyes is treated as a clade, including the tetrapods.[14][15]
The "wasps" are paraphyletic, consisting of the narrow-waistedApocrita without theants andbees.[16] The sawflies (Symphyta) are similarly paraphyletic, forming all of theHymenoptera except for the Apocrita, a clade deep within the sawfly tree.[14]Crustaceans are not a clade because theHexapoda (insects) are excluded. The modern clade that spans all of them is thePancrustacea.[17][18][19]
One of the goals of modern taxonomy over the past fifty years has been to eliminate paraphyletic taxa from formal classifications.[20][21] Below is a partial list of obsolete taxa and informal groups that have been found to be paraphyletic.
Species have a special status in systematics as being an observable feature of nature itself and as the basic unit of classification.[47] Some articulations of thephylogenetic 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 inspeciation, whereby a mother species (aparaspecies) gives rise to a daughter species without itself becoming extinct.[48] Research indicates as many as 20 percent of all animal species and between 20 and 50 percent of plant species are paraphyletic.[49][50] Accounting for these facts, some taxonomists argue that paraphyly is a trait of nature that should be acknowledged at higher taxonomic levels.[51][52]
Cladists advocate a phylogenetic species concept[53] that does not consider species to exhibit the properties of monophyly or paraphyly, concepts under that perspective which apply only to groups of species.[54] They consider Zander's extension of the "paraphyletic species" argument to higher taxa to represent acategory error[55]
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, theNeogene evolution of theArtiodactyla (even-toed ungulates, like deer, cows, pigs and hippopotamuses -Cervidae,Bovidae,Suidae andHippopotamidae, the families that contain these various artiodactyls, are all monophyletic groups) has taken place in environments so different from that of theCetacea (whales, dolphins, and porpoises) that the Artiodactyla are often studied in isolation even though the cetaceans are a descendant group. Theprokaryote group is another example; it is paraphyletic because it is composed of two Domains (Eubacteria and Archaea) and excludes (theeukaryotes). It is very useful because it has a clearly defined and significant distinction (absence of a cell nucleus, aplesiomorphy) from its excluded descendants.[citation needed]
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.[56] Other systematists consider reification of paraphyletic groups to obscure inferred patterns of evolutionary history.[57]
The term "evolutionary grade" is sometimes used for paraphyletic groups.[58]Moreover, the concepts ofmonophyly, paraphyly, andpolyphyly have been used in deducing key genes forbarcoding of diverse group of species.[59]
^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.doi: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.
^abZrzavý, J.; Štys, P. (May 1997). "The basic body plan of arthropods: insights from evolutionary morphology and developmental biology".Journal of Evolutionary Biology.10 (3):353–367.doi:10.1046/j.1420-9101.1997.10030353.x.
^Berg, Linda (2008).Introductory Botany: Plants, People, and the Environment (2nd ed.). Belmont CA: Thomson Corporation. p. 360.ISBN978-0-03-075453-1.
^Schlegel, Martin; Hülsmann, Norbert (2 August 2007). "Protists – A textbook example for a paraphyletic taxon".Organisms Diversity & Evolution.7 (2):166–172.doi:10.1016/j.ode.2006.11.001.ISSN1439-6092.
^Thewissen, J. G. M.; Williams, E. M. (2002). "The Early Radiations of Cetacea (Mammalia): Evolutionary Pattern and Developmental Correlations".Annual Review of Ecology and Systematics.33:73–90.doi:10.1146/annurev.ecolsys.33.020602.095426.OCLC4656321698.
^Marques, Antonio C.; Allen G. Collins (March 2004). "Cladistic analysis of Medusozoa and cnidarian evolution".Invertebrate Biology.123 (1):23–42.doi:10.1111/j.1744-7410.2004.tb00139.x.
^Ross, Howard A. (July 2014). "The incidence of species-level paraphyly in animals: A re-assessment".Molecular Phylogenetics and Evolution.76:10–17.doi:10.1016/j.ympev.2014.02.021.PMID24583289.
^Aubert, D. (2015). "A formal analysis of phylogenetic terminology: Towards a reconsideration of the current paradigm in systematics".Phytoneuron.66:1–54.
^Brower, Andrew V. Z.; Schuh, Randall T. (2021).Biological Systematics: principles and applications (3rd ed.). Ithaca, New York: Cornell University Press.ISBN978-1-5017-5277-3.
^Parhi, J.; Tripathy, P.S.; Priyadarshi, H.; Mandal S.C.; Pandey P.K. (2019). "Diagnosis of mitogenome for robust phylogeny: A case of Cypriniformes fish group".Gene.713: 143967.doi:10.1016/j.gene.2019.143967.PMID31279710.S2CID195828782.
^Greenhill, Simon J. and Russell D. Gray. (2009.) "Austronesian Language and Phylogenies: Myths and Misconceptions About Bayesian Computational Methods", inAustronesian 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, TheAustralian National University.
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