In biological systematics, a clade (from ancient Greek κλάδος, klados, "branch") signifies a single "branch" on the "Tree of life", a group composed of a single ancestor and all its descendants.[1] The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological classification. In cladistics (which takes its name from the term), clades are the only acceptable units.
The term was coined by English biologist Julian Huxley.
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A clade is termed monophyletic, meaning it contains one ancestor, all its descendants, and only its descendants.[note 1][2] The ancestor can be an organism, population, or species.[3] The common ancestor of any group of reasonable size and most of that ancestor's descendants have typically been long extinct.[note 2] It is not necessary for a clade to contain any living representatives. In most cases, the actual ancestor is a hypothetical rather than an actual species, as the likelihood finding a fossil of the ancestral species is slim. Should an actual ancestor be found, the computer program used in cladistic analysis will not be able to recognise it as such, and will display it as the sister group to its descendant clade.[note 3]
Three methods of defining clades are featured in phylogenetic nomenclature: node-, stem-, and apomorphy-based:
In cladistics, the clade is a hypothetical construct based on experimental data. Clades are found using multiple (sometimes hundreds) of traits from a number of species (or specimens) and analysing them statistically to find the most likely phylogenetic tree for the group. Although similar in some ways to a biological classification of species, the method is statistical and more open to scrutiny than traditional methods. Although taxonomists use clades as a tool in classification where feasible, the taxonomic "Tree of life" is not the same as the cladistic. The traditional genus, family, etc. names are not necessarily clades; though they will often be.
The idea of "clade" did not exist in pre-Darwinian Linnaean taxonomy, which was based only on morphological similarities between organisms, though many of the better known animal groups in Linnaeus' original Systema Naturae (notably among the vertebrate groups) represent clades. With the publication of Darwin's theory of evolution in 1859, taxonomy gained a theoretical basis, and the idea that systematic units represent branches on the evolutionary Tree of Life was born. In the century and a half since then, taxonomists have worked to make the taxonomic system reflect evolution. However, as the Tree of Life branches rather unevenly, the hierarchy of the Linnaean system does not always lend itself well to represent clades. When it comes to naming, cladistics and Linnaean taxonomy are not always compatible. Particularly higher level taxons in Linnaean taxonomy often represent evolutionary grades rather than clades, i.e. clades where one or two sub-branches have been excluded. Typical examples include bony fishes, who include the ancestor of tetrapods, and reptiles, ancestral to both birds and mammals.[note 4]
In phylogenetic nomenclature, clades can be nested at any level, and do not have to be slotted into a rank in an overall hierarchy. In contrast, the Linnaean units of "order," "class" etc. must be used when naming a new taxon. As there are only seven formal levels to the Linnaean system (whereof species is the lowest), there is a finite amount of sub- and super-units that can be applied. As taxonomic trees (cladograms) become more detailed, some researchers intimately familiar with the topography of the trees they are working with have opted to dispense with the ranks all together, using clade names without Linnaean ranks. The preference of one system over the other is mainly one of application: Cladistics give details, but require intimate knowledge, the Linnaean system gives a well ordered overview, at the expense of details of the phylogenetic tree.
In a few instances, the Linnaean system has actually impeded our understanding of the phylogeny and broad evolutionary patterns. The best known example is the interpretation of the strange fossils of the Burgess Shale and the subsequent idea of a "Cambrian Explosion" [4] With the application of cladistics, and the rejection of any significance of the concept of Phyla, the confusion of the late 20th century over the Burgess animals has been resolved. It appears there never was an "explosion" of major bauplans with subsequent extinctions.[5] The seemingly weird critters themselves have been found to be representatives of a group, the Lobopodia, that includes arthropods, water bears and velvet worms.[6]
In most instances the two systems are not at odds, however. The cladistic statement, that the clade Lobopodia contains (among others) the Arthropoda, Tardigrada and Onychophora, is factually identical to the Linnaean evolutionary statement that the group Lobopodia is ancestral to the phyla Arthropoda, Tardigrada and Onychophora. The difference is one of semantics rather than phylogeny.
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A clade (Greek = branch) is a group of organisms including a common ancestor and all the next generations of that ancestor. Such a clade is monophyletic.[1] A part of a family tree, for example Progoneata is a clade of millipides including Symphyla, Pauropoda, and Diplopoda
The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological classification. It brings classification in line with phylogeny. In cladistics (which takes its name from the term), clades are the only acceptable units. The term was coined by English biologist Julian Huxley.
Some versions of cladistics have been the subject of controversy.[2][3]
   | DM Hillis, D Zwickl & R Gutell: ~3000 species Tree of Life - Download Graphic Images from the Hillis/Bull Lab |
| | Phylogenetic systematics, an introductory slide-show on evolutionary trees - Phylogenetic systematics, a.k.a. evolutionary trees |
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