Clade
In biological phylogenetics, a clade (from Ancient Greek κλάδος (kládos) 'branch'), also known as a monophyletic group or natural group,[1] is a grouping of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree.[2] In the taxonomical literature, sometimes the Latin form cladus (plural cladi) is used rather than the English form. Clades are the fundamental unit of cladistics, a modern approach to taxonomy adopted by most biological fields.
For other uses, see Clade (disambiguation).
The common ancestor may be an individual, a population, or a species (extinct or extant). Clades are nested, one in another, as each branch in turn splits into smaller branches. These splits reflect evolutionary history as populations diverged and evolved independently. Clades are termed monophyletic (Greek: "one clan") groups.
Over the last few decades, the cladistic approach has revolutionized biological classification and revealed surprising evolutionary relationships among organisms.[3] Increasingly, taxonomists try to avoid naming taxa that are not clades; that is, taxa that are not monophyletic. Some of the relationships between organisms that the molecular biology arm of cladistics has revealed include that fungi are closer relatives to animals than they are to plants, archaea are now considered different from bacteria, and multicellular organisms may have evolved from archaea.[4]
The term "clade" is also used with a similar meaning in other fields besides biology, such as historical linguistics; see Cladistics § In disciplines other than biology.
Naming and etymology[edit]
The term "clade" was coined in 1957 by the biologist Julian Huxley to refer to the result of cladogenesis, the evolutionary splitting of a parent species into two distinct species, a concept Huxley borrowed from Bernhard Rensch.[5][6]
Many commonly named groups – rodents and insects, for example – are clades because, in each case, the group consists of a common ancestor with all its descendant branches. Rodents, for example, are a branch of mammals that split off after the end of the period when the clade Dinosauria stopped being the dominant terrestrial vertebrates 66 million years ago. The original population and all its descendants are a clade. The rodent clade corresponds to the order Rodentia, and insects to the class Insecta. These clades include smaller clades, such as chipmunk or ant, each of which consists of even smaller clades. The clade "rodent" is in turn included in the mammal, vertebrate and animal clades.