Paraphyly is a characteristic of some groups of organisms and families of languages. In phylogenetics (a subfield of biology) and in linguistics, a group is said to be paraphyletic if it consists of all the descendants of the last common ancestor of the group's members minus a small number of monophyletic groups of descendants, typically just one or two such groups. For example, the group of reptiles, as traditionally defined, is paraphyletic: it contains the last common ancestor of the reptiles-including the extant reptiles as well as the extinct mammal-like reptiles-along with all descendants of that ancestor except for mammals and birds.
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. Ereshefsky has argued that that paraphyletic taxa are the result of anagenesis.
A taxon that is not paraphyletic or monophyletic is 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).
Many of the older classifications contain paraphyletic groups, including the traditional 2–6 kingdom systems and the classic division of the vertebrates. Examples of well-known paraphyletic groups include:
The following table shows some paraphyletic groups.
| Paraphyletic Taxon | Excluded Clades | Corresponding Monophyletic Taxon |
|---|---|---|
| Dicotyledons | Monocotyledoneae | Angiospermae |
| Even-toed ungulates | Cetacea | Cetartiodactyla |
| Reptiles | Mammalia, Aves (Birds) | Amniota |
| Lizards | Serpentes (Snakes) | Squamata |
| Prokaryotes | Eukaryota | Cellular Organisms |
| Bony fish | Tetrapoda | Euteleostomi |
| Pelycosaurs | Therapsida | Synapsida |
| Monkeys | Hominoidea | Simiiformes |
| Plagiaulacidans | Cimolodonta, Arginbaataridae | Multituberculata |
| Invertebrates | Vertebrata | Animalia |
| Gymnosperms | Angiospermae | Spermatophyta |
| Fish | Tetrapoda | Vertebrata |
| Prosimians | Simiiformes | Primates |
| Crustaceans | Hexapoda | Tetraconata |
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) has taken place in an environment 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 descendent group. The prokaryote group is another example; it is paraphyletic because it excludes many of its descendent organisms (the eukaryotes), but it is very useful because it has a clearly defined and significant distinction (no cell nucleus) from its excluded descendants.
Also, paraphyletic groups are involved in evolutionary transitions, the development of the first tetrapods from their ancestors for example. Any name given to these ancestors to distinguish them from tetrapods-"fish", for example-necessarily picks out a paraphyletic group, since the descendent tetrapods are not included.
The term "evolutionary grade" is sometimes used for paraphyletic groups.
Vivipary, the production of offspring without the involvement 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, at least one of the lineages that led to these species from their last common ancestor contains nonviviparous animals, the pelycosaurs ancestral to humans for example; vivipary appeared subsequently in the human lineage.
Independently-developed traits like these cannot be used to distinguish paraphyletic groups since paraphyly requires the excluded groups to be monophyletic. Pelycosaurs were descended from the last common ancestor of skinks and humans, so vivipary could be paraphyletic only if the pelycosaurs were part of an excluded monophyletic group. Since this group is monophyletic, it contains all descendents of the pelycosaurs; since it is excluded, it contains no viviparous animals. This doesn't work, since humans are among these descendents. Vivipary in a group that includes humans and skinks cannot be paraphyletic.
Though the last common ancestor of skinks and humans was not viviparous, this is not essential to the argument. The last common ancestor of porpoises and sharks, like these extant animals, probably had a dorsal fin on its back, but the Mesozoic ancestors of porpoises (a mammal group) did not have the fin, so it does not define a paraphyly. The presence of a trait in a last common ancestor is not sufficient for paraphyly.
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.
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