A group of taxa
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...

 is said to be paraphyletic if the group consists of all the descendants of a hypothetical closest common ancestor minus one or more monophyletic groups of descendants (typically one such group). This term is used in both phylogenetics
In biology, phylogenetics is the study of evolutionary relatedness among groups of organisms , which is discovered through molecular sequencing data and morphological data matrices...

A paraphyletic group is defined in terms of a clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

; that is, the group is the same as the equivalent clade, except that it lacks one or more of the clade's full complement. The concept of the last common ancestor is the same, but it has been expanded to be node-based, branch-based and apomorphy-based. Those terms are defined under Phylogenetic nomenclature.
and linguistics
Linguistics is the scientific study of human language. Linguistics can be broadly broken into three categories or subfields of study: language form, language meaning, and language in context....


Relation to monophyletic groups

Groups that do include all the descendants of the most recent common ancestor are said to be monophyletic. A paraphyletic group is a monophyletic group from which one or more of the clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

s is excluded to form a separate group (as in the paradigmatic example of reptiles and birds, shown in the picture).

A group that is neither monophyletic nor paraphyletic is said to be polyphyletic (Greek πολύς [polys], "many").

These terms were developed during the debates of the 1960s and 70s accompanying the rise of cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...

 (a clade is a term for a monophyletic group).

Examples of paraphyletic groups

Many of the older classifications contain paraphyletic groups, especially the traditional 2–6 kingdom
Kingdom (biology)
In biology, kingdom is a taxonomic rank, which is either the highest rank or in the more recent three-domain system, the rank below domain. Kingdoms are divided into smaller groups called phyla or divisions in botany...

 systems and the classic division of the vertebrate
Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

s. Paraphyletic groups are often erected on the basis of (ancestral similarities) instead of (derived similarities). Examples of well-known paraphyletic groups include:
  • In the flowering plant
    Flowering plant
    The flowering plants , also known as Angiospermae or Magnoliophyta, are the most diverse group of land plants. Angiosperms are seed-producing plants like the gymnosperms and can be distinguished from the gymnosperms by a series of synapomorphies...

    s, Dicotyledon
    The dicotyledons, also known as dicots, are a group of flowering plants whose seed typically has two embryonic leaves or cotyledons. There are around 199,350 species within this group...

    s, in the traditional sense, because they exclude Monocotyledon
    Monocotyledons, also known as monocots, are one of two major groups of flowering plants that are traditionally recognized, the other being dicotyledons, or dicots. Monocot seedlings typically have one cotyledon , in contrast to the two cotyledons typical of dicots...

    s. The former name has not been used as an ICBN classification for decades, but is allowed as a synonym of Magnoliopsida.The history of flowering plant classification can be found under History of the classification of flowering plants. The former angiosperms (Magnoliophyta), or flowering plants, comprised both. Phylogenetic analysis, however, indicates that the monocots are a development from a dicot ancestor. Excluding them from the dicots makes the latter a paraphyletic group.

  • The order Artiodactyla (even-toed ungulate
    Even-toed ungulate
    The even-toed ungulates are ungulates whose weight is borne about equally by the third and fourth toes, rather than mostly or entirely by the third as in odd-toed ungulates such as horses....

    s), because it excludes Cetacea
    The order Cetacea includes the marine mammals commonly known as whales, dolphins, and porpoises. Cetus is Latin and is used in biological names to mean "whale"; its original meaning, "large sea animal", was more general. It comes from Ancient Greek , meaning "whale" or "any huge fish or sea...

    ns (whales, dolphins, etc.). In the ICZN Code, the two taxa are orders of equal rank. Molecular studies, however, have shown that the Cetacea descend from the Artiodactyl ancestors, although the precise phylogeny within the order remains uncertain. Without the Cetacean descendants the Artiodactyls must be paraphyletic.

  • The class Reptilia
    Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...

     as traditionally defined, because it excludes birds (class Aves) and mammals (class Mammalia). In the ICZN Code, the three taxa are classes of equal rank. However, mammals hail from the mammal-like reptiles and birds are descended from the dinosaurs (a group of Diapsida), both of which are classified as reptiles (see the illustration above). Reptiles would be monophyletic if they were defined to include Mammalia and Aves.

  • The Prokaryotes (single-celled life forms without cell nuclei), because the Archaea
    The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

     descend from a common ancestor with the Eukaryotes. 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. It was never adopted by any code because by that time the inappropriateness of the ICBN code and the ICZN Code for classifying life forms that are neither plant nor animal was also generally recognized and the ICNB code did not appear until 1975. It did recognize Prokaryotic taxa beginning in 1980. Chatton's system became known as the two-empire system
    Two-empire system
    The two-empire system was the top-level biological classification system in general use before the establishment of the three-domain system. It classified life into Prokaryota and Eukaryota...

     but the latter was replaced by Carl Woese
    Carl Woese
    Carl Richard Woese is an American microbiologist and physicist. Woese is famous for defining the Archaea in 1977 by phylogenetic taxonomy of 16S ribosomal RNA, a technique pioneered by Woese and which is now standard practice. He was also the originator of the RNA world hypothesis in 1977,...

    's three-domain system
    Three-domain system
    The three-domain system is a biological classification introduced by Carl Woese in 1977 that divides cellular life forms into archaea, bacteria, and eukaryote domains. In particular, it emphasizes the separation of prokaryotes into two groups, originally called Eubacteria and Archaebacteria...

     published in 1990. The former Prokaryotes became the Bacteria
    Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

     and the Archaea
    The Archaea are a group of single-celled microorganisms. A single individual or species from this domain is called an archaeon...

    , while the third domain remained the Eukaryotes. Subsequent phylogenetic analysis led to a conclusion that the Archaea and the Eukaryotes share a common ancestor.

  • Agnatha
    Agnatha is a superclass of jawless fish in the phylum Chordata, subphylum Vertebrata. The group excludes all vertebrates with jaws, known as gnathostomes....

    , jawless fish, because of its two significant animal groups, hagfish
    Hagfish, the clade Myxini , are eel-shaped slime-producing marine animals . They are the only living animals that have a skull but not a vertebral column. Along with lampreys, hagfish are jawless and are living fossils whose next nearest relatives include all vertebrates...

     and lamprey
    Lampreys are a family of jawless fish, whose adults are characterized by a toothed, funnel-like sucking mouth. Translated from an admixture of Latin and Greek, lamprey means stone lickers...

    s, the lampreys descend from the stem of the Gnathostomes. In 1806 Duméril
    André Marie Constant Duméril
    André Marie Constant Duméril was a French zoologist. He was professor of anatomy at the Muséum National d'Histoire Naturelle from 1801 to 1812, when he became professor of herpetology and ichthyology...

     united the hagfish with the lampreys under Cyclostomi, which Cope
    Edward Drinker Cope
    Edward Drinker Cope was an American paleontologist and comparative anatomist, as well as a noted herpetologist and ichthyologist. Born to a wealthy Quaker family, Cope distinguished himself as a child prodigy interested in science; he published his first scientific paper at the age of nineteen...

     in 1889 made into Agnatha, as opposed to Gnathostome, the jawed fish. These two taxa became classes or superclasses in the ICZN Code. In the late 20th century phylogenetic analysis using dozens of characters (features) indicated the lampreys came from gnathostome ancestors, but the original agnatha/gnathostome ancestor did not have a jaw. The removal of the lampreys from the Agnatha downgraded the latter to a paraphyletic group.

  • Osteichthyes
    Osteichthyes , also called bony fish, are a taxonomic group of fish that have bony, as opposed to cartilaginous, skeletons. The vast majority of fish are osteichthyes, which is an extremely diverse and abundant group consisting of over 29,000 species...

    , bony fish, are paraphyletic because they include Actinopterygii
    The Actinopterygii or ray-finned fishes constitute a class or sub-class of the bony fishes.The ray-finned fishes are so called because they possess lepidotrichia or "fin rays", their fins being webs of skin supported by bony or horny spines , as opposed to the fleshy, lobed fins that characterize...

     (ray-finned fish) and Sarcopterygii
    The Sarcopterygii or lobe-finned fishes – sometimes considered synonymous with Crossopterygii constitute a clade of the bony fishes, though a strict classification would include the terrestrial vertebrates...

     (lungfish, etc.). However, tetrapod
    Tetrapods are vertebrate animals having four limbs. Amphibians, reptiles, birds and mammals are all tetrapods; even snakes and other limbless reptiles and amphibians are tetrapods by descent. The earliest tetrapods evolved from the lobe-finned fishes in the Devonian...

    s are descendants of the nearest common ancestor of Actinopterygii and Sarcopterygii, and tetrapods are not in Osteichthyes, hence Osteichthyes is paraphyletic.

  • Porifera are now divided into siliceous sponges and calcareous sponge
    Calcareous sponge
    The calcareous sponges of class Calcarea are members of the animal phylum Porifera, the cellular sponges. They are characterized by spicules made out of calcium carbonate in the form of calcite or aragonite...

    s, which together are not monophyletic.

  • Recently Crustaceans has been defined as paraphyletic group by molecular phylogenetic study, so Hexapods
    The subphylum Hexapoda constitutes the largest grouping of arthropods and includes the insects as well as three much smaller groups of wingless arthropods: Collembola, Protura, and Diplura . The Collembola are very abundant in terrestrial environments...

     would be evolved from a subfamily of this group.

Uses for paraphyletic groups

Advocates of cladistics have several arguments in favor of clades over polyphyletic groupings of organisms.
These arguments apply to a lesser extent against paraphyletic groups.
Others argue that paraphyletic groups of one clade minus one highly divergent descendant clade are necessary for a comprehensive classification including extinct groups, since each species, genus, and so forth necessarily originates from part of another. Ereshefsky notes that paraphyletic taxa are the result of anagenesis
Anagenesis, also known as "phyletic change," is the evolution of species involving an entire population rather than a branching event, as in cladogenesis. When enough mutations have occurred and become stable in a population so that it is significantly differentiated from an ancestral population,...


For instance, the Prokaryote
The prokaryotes are a group of organisms that lack a cell nucleus , or any other membrane-bound organelles. The organisms that have a cell nucleus are called eukaryotes. Most prokaryotes are unicellular, but a few such as myxobacteria have multicellular stages in their life cycles...

 group is paraphyletic because it excludes many of its descendent organisms (the Eukaryotes), yet the Prokaryote group is very useful because it has a clearly defined and significant distinction (no cell nucleus) from its excluded descendants. So, even though Prokaryotes are not a clade, the term is still useful.

It has been suggested that paraphyletic groups be clearly marked to distinguish them from clades, for instance with asterisks: Reptilia*. The term evolutionary grade
Evolutionary grade
In alpha taxonomy, a grade refers to a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a strictly phylogenetic unit.-Definition:...

is sometimes used for such groups.


The concept of paraphyly has also been applied to historical linguistics
Historical linguistics
Historical linguistics is the study of language change. It has five main concerns:* to describe and account for observed changes in particular languages...

, where the methods of cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...

 have found some utility in comparing languages. For instance, the Formosan languages
Formosan languages
The Formosan languages are the languages of the indigenous peoples of Taiwan. Taiwanese aborigines currently comprise about 2% of the island's population. However, far fewer can still speak their ancestral language, after centuries of language shift...

 form a paraphyletic group of the Austronesian languages
Austronesian languages
The Austronesian languages are a language family widely dispersed throughout the islands of Southeast Asia and the Pacific, with a few members spoken on continental Asia that are spoken by about 386 million people. It is on par with Indo-European, Niger-Congo, Afroasiatic and Uralic as one of the...

 as the term refers to the nine branches of the Austronesian family that are not Malayo-Polynesian
Malayo-Polynesian languages
The Malayo-Polynesian languages are a subgroup of the Austronesian languages, with approximately 385.5 million speakers. These are widely dispersed throughout the island nations of Southeast Asia and the Pacific Ocean, with a smaller number in continental Asia...

 and restricted to the island of Taiwan
Taiwan , also known, especially in the past, as Formosa , is the largest island of the same-named island group of East Asia in the western Pacific Ocean and located off the southeastern coast of mainland China. The island forms over 99% of the current territory of the Republic of China following...


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