Clade

(a branching tree diagram) illustrating the relationships of organisms within groups of taxa known as clades. The vertical line (stem) at the base (bottom) represents the last common ancestor. The blue and red subgroups are clades, each defined by a common ancestor stem at the base of its respective subgroup (branch). The green subgroup alone, however, is not a clade; it is a paraphyletic group relative to the blue subgroup because it excludes the blue branch, which shares the same common ancestor. Together, the green and blue subgroups form a clade. In biology, a clade ( ) ( ), also known as a monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach to taxonomy adopted by most biological fields. 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.…

(a branching tree diagram) illustrating the relationships of organisms within groups of taxa known as clades. The vertical line (stem) at the base (bottom) represents the last common ancestor. The blue and red subgroups are clades, each defined by a common ancestor stem at the base of its respective subgroup (branch). The green subgroup alone, however, is not a clade; it is a paraphyletic group relative to the blue subgroup because it excludes the blue branch, which shares the same common ancestor. Together, the green and blue subgroups form a clade. In biology, a clade ( ) ( ), also known as a monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach to taxonomy adopted by most biological fields. 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. 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. 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== 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. ==Definition== tree of crocodylians. A clade is by definition monophyletic, meaning that it contains one ancestor which can be an organism, a population, or a species and all its descendants. The ancestor can be known or unknown; any and all members of a clade can be extant or extinct. ==Clades and phylogenetic trees== The science that tries to reconstruct phylogenetic trees and thus discover clades is called phylogenetics or cladistics, the latter term coined by Ernst Mayr (1965), derived from "clade". The results of phylogenetic/cladistic analyses are tree-shaped diagrams called cladograms; they, and all their branches, are phylogenetic hypotheses. Three methods of defining clades are featured in phylogenetic nomenclature: node-, stem-, and apomorphy-based (see Phylogenetic nomenclature for detailed definitions). ==Viruses== of the SIV and HIV viruses showing clades (subtypes) of the virus. Viruses, and particularly RNA viruses, form clades. These are useful in tracking the spread of viral infections. HIV, for example, has clades called subtypes, which vary in geographical prevalence. HIV subtype (clade) B, for example is predominant in Europe, the Americas and Japan, whereas subtype A is more common in east Africa. ==Notes== ==References== ==External links== Evolving Thoughts: "Clade" DM Hillis, D Zwickl & R Gutell. "Tree of life". An unrooted cladogram depicting around 3000 species. "Phylogenetic systematics, an introductory slide-show on evolutionary trees" – University of California, Berkeley

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