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[[File:Clade-grade II.svg|thumb|right|334 px|Cladogram (family tree) of a biological group. The red and blue boxes represent ''clades'' (i.e., complete branches). The green box is not a clade, but rather represents an ''[[evolutionary grade]]'', an incomplete group, because the blue clade descends from it, but is excluded.]]
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A '''
==Definitions==
{{main|Phylogenetic nomenclature#Phylogenetic definitions|l1=Phylogenetic definitions}}
[[File:Cladogram Crocodilia NL.PNG|thumb|right|200 px|A [[cladogram]] of crocodiles, a visual representation of their relationship]]
===Clade and ancestor===
A clade is termed [[monophyletic]], meaning it contains one ancestor which can be an organism, [[population]], or [[species]] and all its descendants.<ref group=note>
A semantic case has been made that the name should be "holophyletic," but this term has not yet acquired widespread use. For more information, see ''[[holophyly]]''</ref><ref>{{cite web|title=The PhyloCode, Chapter 1|url=http://www.ohio.edu/phylocode/art1-3.html#chapter1|year=2009|publisher=International Society for Phylogenetic Nomenclature|accessdate=23 January 2010}}</ref><ref>{{cite journal
| doi = 10.1111/j.1095-8312.2008.00984.x
| title = On the difference between mono-, holo-, and paraphyletic groups: a consistent distinction of process and pattern
| year = 2008
| last = Envall
| first= Mat S.
| journal = Biological Journal of the Linnaean Society
| volume = 94
| page = 217 }}</ref> The term clade refers to the grouping of the ancestor and its living and/or deceased descendants together. The ancestor can be a theoretical or actual species.
===Clade definition===
Three methods of defining clades are featured in [[phylogenetic nomenclature]]: node-, stem-, and apomorphy-based:
* In node-based definition, clade name A refers to the ''least inclusive'' clade containing taxa (or specimens) X, Y, etc., and their common ancestor. The ancestor is the branch point, or ''node''.
* In stem-based definition, A refers to the ''most inclusive'' clade containing X, Y, etc., and their common ancestor, down to where Z branches off below A. Taxa are included between the node of A and down to (but not including) the branching point to Z; that is, the ''[[Crown group#Stem groups|stem]]'' of A.
* In [[apomorphy]]-based definition, A refers to the clade identified by an apomorphy (a trait) found in X, Y, etc., and their common ancestor.
In [[Linnaean taxonomy]], clades are defined by a set of traits (apomorphies) unique to the group. This system is basically similar to the apomorphy-based clades of phylogenetic nomenclature. The difference is one of weight: While phylogenetic nomenclature bases the group on an ancestor with a certain trait, Linnaean taxonomy uses the traits themselves to define the group.
===Clades as constructs===
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 [[analysis|analysing]] them [[statistics|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 (science)|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.
===Clade names===
In Linnaean systematics, the various groups are ordered into a series of [[taxonomic rank]]s (the familiar [[order (biology)|order]], [[family (biology)|family]] etc). These ranks will by convention dictate the ending to names for some groups. Clades do not by their nature fit this scheme, and no such restriction exists as to their names in [[cladistics]]. There is however a convention for naming more or less inclusive groups, which are given prefixes like ''crown-'' or ''pan-'', see [[Crown group]].
==Taxonomy and systematics==
[[File:Haeckel arbol bn.png|thumb|right|Early phylogenetic tree by [[Ernst Haeckel|Haeckel]], 1866]]
The idea of "clade" did not exist in pre-[[Charles Darwin|Darwinian]] [[Linnaean taxonomy]], which was based only on [[morphology (biology)|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 [[Evolution|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 [[Nomenclature#Biology|naming]], [[cladistics]] and [[Linnaean taxonomy]] are not always compatible. Particularly higher level taxons in Linnaean taxonomy often represent [[evolutionary grade]]s rather than clades, i.e. clades where one or two sub-branches have been excluded. Typical examples include [[Osteichthyes|bony fishes]], who include the ancestor of [[tetrapoda|tetrapods]], and [[reptiles]], ancestral to both [[bird]]s and [[mammal]]s.<ref group=note>The term "reptile" is here to be understood as traditionally defined, e.g. Romer & Parson (1985): ''The Vertebrate Body.'' (6th ed.) Saunders, Philadelphia. There are other (cladistic) definitions of "reptile" that exclude the first [[amniote]]s and the [[synapsid]] line, see [[Sauropsida]].</ref>
In [[phylogenetic nomenclature]], clades can be nested at any level, and do not have to be slotted into a [[ranking|rank]] in an overall hierarchy. In contrast, the Linnaean units of "[[order (biology)|order]]", "[[class (biology)|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 ([[cladogram]]s) 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]]"
<ref name=Budd2000>{{Cite journal
| last1 = Budd | first1 = G.E.
| last2 = Jensen | first2 = S.
| year = 2000
| title = A critical reappraisal of the fossil record of the bilaterian phyla
| journal = Biological Reviews
| volume = 75
| issue = 02
| pages = 253–295
| url = http://journals.cambridge.org/production/action/cjoGetFulltext?fulltextid=624
| doi = 10.1017/S000632310000548X
}}</ref>
With the application of cladistics, and the rejection of any significance of the concept of [[Phylum|Phyla]], the confusion of the late 20th century over the Burgess animals has been resolved. It appears there never was an "explosion" of major [[bauplan]]s with subsequent [[extinction]]s.<ref>{{cite journal | author = Erwin, D.H.| year = 2007| doi = 10.1111/j.1475-4983.2006.00614.x| title = Disparity: Morphological Pattern And Developmental Context| journal = Palaeontology| volume = 50| pages = 57 }}</ref> The seemingly weird critters themselves have been found to be representatives of a group, the [[Lobopodia]], that includes [[Arthropoda|arthropods]], [[Tardigrada|water bears]] and [[Onychophora|velvet worms]].<ref>{{Cite journal
| author1 = | first1 = R. J.
| author2 = | first2 = S. E.
| author3 = | first3 = R. J.
| last1 = Whittle
| author4 = | first4 = J.
| title = An Ordovician Lobopodian from the Soom Shale Lagerstätte, South Africa
| journal = Palaeontology
| volume = 52
| pages = 561–567
| year = 2009
| doi = 10.1111/j.1475-4983.2009.00860.x
| last2 = Gabbott
| last3 = Aldridge
| last4 = Theron
}}</ref>
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.
== See also ==
* [[Cladistics]]
* [[Phylogeny]]
* [[Paraphyly]]
* [[Polyphyly]]
* [[Phylogenetic nomenclature]]
* [[Binomial nomenclature]]
* [[Crown group]]
==Notes==
==References==<!-- OrgDiversEvol8:17. ZoolScripta36:607,37:337,38:101. -->
{{reflist
==External links==
* [http://scienceblogs.com/evolvingthoughts/2007/01/clade_1.php Evolving Thoughts: Clade]
* DM Hillis, D Zwickl & R Gutell. "[http://www.zo.utexas.edu/faculty/antisense/DownloadfilesToL.html Tree of life]". An unrooted cladogram depicting around 3000 species.
* [http://evolution.berkeley.edu/evolibrary/article/0_0_0/phylogenetics_01 Phylogenetic systematics, an introductory slide-show on evolutionary trees] [[University of California, Berkeley]]
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[[Category:Phylogenetics]]
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