Paraphyly

A cladogram showing a hypothetical descent from an ancestor species of the clade vertebrata. Note that cladograms do not necessarily correspond to taxonomic classifications. In this one, Sauropsida are a paraphyletic group. It can be made monophyletic by including the birds (Aves).

Comparison of phylogenetic groups, showing a monophyly (all descendants of the first reptiles), a paraphyly (descendants of reptiles, minus birds), and a polyphyly (warm-blooded animals: mammals and birds)

A group of taxa 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 and linguistics.

Phylogenetics

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 clades 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 (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 systems and the classic division of the vertebrates. Paraphyletic groups are often erected on the basis of (sym)plesiomorphies (ancestral similarities) instead of (syn)apomorphies (derived similarities). Examples of well-known paraphyletic groups include:

• In the flowering plants, Dicotyledons, in the traditional sense, because they exclude Monocotyledons. The former name has not been used as an ICBN classification for decades, but is allowed as a synonym of Magnoliopsida. 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 ungulates), because it excludes Cetaceans (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 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 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 but the latter was replaced by Carl Woese's three-domain system published in 1990. The former Prokaryotes became the Bacteria and the Archaea, 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, jawless fish, because of its two significant animal groups, hagfish and lampreys, the lampreys descend from the stem of the Gnathostomes. In 1806 Duméril united the hagfish with the lampreys under Cyclostomi, which Cope 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, bony fish, are paraphyletic because they include Actinopterygii (ray-finned fish) and Sarcopterygii (lungfish, etc.). However, tetrapods 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 sponges, which together are not monophyletic.^{[citation needed]}

• Recently Crustaceans has been defined as paraphyletic group by molecular phylogenetic study, so Hexapods would be evolved from a subfamily of this group.^{[citation needed]}

The following table shows some paraphyletic groups.

Paraphyletic Taxon	Excluded Clades	Corresponding Monophyletic Taxon
Dicotyledonae	Monocotyledoneae	Angiospermae
Artiodactyla	Cetacea	Cetartiodactyla
Reptilia	Aves	Sauropsida
Prokaryota	Eukaryota	Cellular Organisms
Agnatha	Gnathostomata	Vertebrata
Osteichthyes	Tetrapoda	Euteleostomi
Monkeys	Hominoidea	Simians
Invertebrata	Vertebrata	Animalia
Gymnospermae	Angiospermae	Plantae
Fish	Tetrapoda	Vertebrata

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, such as a stem group, 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.

For instance, the Prokaryote 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 is sometimes used for such groups.

Linguistics

Main article: Tree model

The concept of paraphyly has also been applied to historical linguistics, where the methods of cladistics have found some utility in comparing languages. For instance, the Formosan languages form a paraphyletic group of the Austronesian languages as the term refers to the nine branches of the Austronesian family that are not Malayo-Polynesian and restricted to the island of Taiwan.

Bibliography

• Simpson, Michael George (2006). Plant systematics. Burlington; San Diego; London: Elsevier Academic Press. ISBN 0126444609. 

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

External links

• Funk, D. J.; Omland, K. E. (2003). "Species-level paraphyly and polyphyly: Frequency, cause and consequences, with insights from animal mitochondrial DNA". Annual Review of Ecology, Evolution and Systematics 34: 397–423. doi:10.1146/annurev.ecolsys.34.011802.132421. http://www.umbc.edu/biosci/Faculty/OmlandLabWebpage/NewPages/papers/FunkOmlandARev.pdf.