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{{Short description|Group of a common ancestor and all descendants}}

A '''clade''' is defined as a taxonomic group of [[organism]]s consisting of a single common ancestor and all the descendants of that ancestor. Any such group is considered to be a [[monophyletic]] group of organisms, and can be represented by both a [[phylogenetic]] analysis, as in a tree diagram, or by a [[cladogram]] (see [[cladistics]]), or simply as a [[taxonomic]] reference.

{{Other uses}}

{{Use dmy dates|date=July 2020}}

[[File:Clade-grade II.svg|thumb|400px| [[Cladogram]] (a branching tree diagram) illustrating the relationships of organisms within groups of taxa known as clades. The vertical line ''stem'' at the base represents the [[last common ancestor]]. The blue and orange 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 [[Paraphyly|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''' ({{etymology|grc|''κλάδος'' (kládos)|branch}}), also known as a '''[[Monophyly|monophyletic]] group''' or '''natural group''',<ref>{{cite book |last1=Martin |first1=Elizabeth |title=A Dictionary of Biology |last2=Hin |first2=Robert |publisher=Oxford University Press |year=2008}}</ref> is a group of [[organism]]s that is composed of a [[common ancestor]] and all of its descendants.<ref>{{cite book |title=Assembling the Tree of Life |url=https://archive.org/details/assemblingtreeli00crac_335 |url-access=limited |editor-first=Joel |editor-last=Cracraft |editor2-first=Michael J. |editor2-last=Donoghue |chapter=Introduction |page=[https://archive.org/details/assemblingtreeli00crac_335/page/n18 1] |publisher=Oxford University Press |year=2004 |isbn=978-0-19-972960-9}}</ref> Clades are the fundamental unit of [[cladistics]], a modern approach to taxonomy adopted by most biological fields.

If a clade proves robust in different cladistic analyses using different sets of data, it may be adopted into [[taxonomy]] and become a [[taxon]]. Not all taxa, however, are considered to be clades. [[Reptile]]s, for example, are a [[paraphyly|paraphyletic]] group because they do not include [[aves]] ([[birds]]), which are thought to also have evolved from the common ancestor of the reptiles.

The common ancestor may be an individual, a [[population]], or a [[species]] ([[extinct]] or [[Extant taxon|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.

In [[cladistics]], a clade that is located within another more inclusive clade is said to be "nested" within that clade. Nested clade analysis is beneficial in many ways. For instance, it enables us to detect range expansions in isolated geographic areas.

Over the last few decades, the cladistic approach has revolutionized biological classification and revealed surprising evolutionary relationships among organisms.<ref name="Palmer-2009">{{Cite book |title=Evolution: The Story of Life |last=Palmer |first=Douglas |publisher=University of California Press |year=2009 |___location=Berkeley |pages=13}}</ref> Increasingly, taxonomists try to avoid naming [[Taxon|taxa]] that are not clades; that is, taxa that are not [[Monophyly|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.<ref>{{Cite journal |last=Pace |first=Norman R. |date=2006-05-18 |title=Time for a change |journal=Nature |volume=441 |issue=7091 |pages=289 |bibcode=2006Natur.441..289P |doi=10.1038/441289a |issn=1476-4687 |pmid=16710401|s2cid=4431143 |doi-access=free }}</ref>

==Phylogenetic nomenclature==

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|Cladistics § In disciplines other than biology]].

Phylogenetic nomenclature is formulated in terms of evolution and common descent rather than the type specimens, categorical ranks, and morphological characters. The later is used most commonly in cladistic analysis. Taxon names are strictly connected to phylogenetic tree topology and evolutionary history. In taxonomy, each name is attached to a clade taxonomic group containing a common ancestor and all its descendants. Phylogenetic nomenclature discards categorical ranks. The problem with ranks are evident when one considers biodiversity lineages and clades. Questions like "how many lineages are there?" or "how many clades are there?" become pointless, since there are no answers. These are relative concepts, illustrating the fractal nature of the tree of life and the need to let a phylogenetic hypothesis be the focus, rather than the categories, when biodiversity is quantified. Phylogenetic nomenclature helps to put focus on phylogenetic trees by offering an explicit link between names and parts of species history, that is, clades.

==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]].<ref name="Dupuis-1984">{{cite journal | last=Dupuis | first=Claude | year=1984 | title=Willi Hennig's impact on taxonomic thought | journal=Annual Review of Ecology and Systematics | volume=15 | pages=1–24 | doi=10.1146/annurev.es.15.110184.000245| doi-access=free }}</ref><ref name="Huxley-1957">{{cite journal | last = Huxley | first = J. S. | year =1957 | title = The three types of evolutionary process | journal=Nature | volume=180 | issue = 4584 | pages=454–455 | doi=10.1038/180454a0| bibcode=1957Natur.180..454H | s2cid = 4174182 }}</ref>

==Traditional binomial nomenclature==

Many commonly named groups – [[rodent]]s and [[insect]]s, 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 [[mammal]]s that split off after the end of the period when the clade [[Dinosaur]]ia stopped being the dominant terrestrial [[vertebrate]]s 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.

In phylogenetics, binomial names are associated with the relationships of each described species. But this creates a problem because it makes assumptions about relationships about the description of the species identification. It suggests that species or genera are a unique category, and this contradicts the idea of recognizing only clades and lineages.

== History of nomenclature and taxonomy ==

==Phylogenetic alternatives==

[[File:Haeckel arbol bn.png|thumb|right|upright|Early phylogenetic tree by [[Ernst Haeckel|Haeckel]], 1866. Groups once thought to be more advanced, such as birds ("Aves"), are placed at the top.]]

To avoid the pitfalls of traditional Linnaean naming in phylogenetic nomenclature, three new methods of phylogenetic naming have been proposed: node-, stem-, and apomorphy-based. In node-based naming, taxon name A might refer to the least inclusive clade containing X and Y. In stem-based naming, A would refer to the most inclusive clade containing X and Y but not Z. In apomorphy (derived feature)-based naming, A would refer to the clade identified by a feature synapomorphic (sharing a derivation) with a feature in specimen (taxon) X. Differences between a traditional approach and these phylogenetic alternatives become obvious when the phylogenetic hypothesis changes. Comparison between the traditional Linnaean approach to nomenclature and a phylogenetic alternative (node-based naming). Suppose that all we want to do is to give a name ("A") to a clade containing X and Y. In the Linnaean system this means that we have to introduce names for sister taxa, assigning all taxa to the categories species, genus, and family, and designate type species. No explicit reference to phylogeny is made. The phylogenetic alternative provides an explicit reference to evolutionary history, and nothing but the clade containing X and Y needs to be named. When the hypothesis of relationship changes, the phylogenetic alternative is cleaner and more explicit about what it refers to.

The idea of a clade did not exist in pre-[[Charles Darwin|Darwinian]] [[Linnaean taxonomy]], which was based by necessity only on internal or external [[morphology (biology)|morphological]] similarities between organisms. Many of the better known animal groups in Linnaeus's original ''[[Systema Naturae]]'' (mostly [[vertebrate]] groups) do represent clades. The phenomenon of [[convergent evolution]] is responsible for many cases of misleading similarities in the [[Morphology (biology)|morphology]] of groups that evolved from different lineages.

{{phylo}}

{{Evolution}}

With the increasing realization in the first half of the 19th century that species had changed and split through the ages, classification increasingly came to be seen as branches on the evolutionary [[tree of life (biology)|tree of life]]. The publication of Darwin's [[Evolution|theory of evolution]] in 1859 gave this view increasing weight. In 1876 [[Thomas Henry Huxley]], an early advocate of evolutionary theory, proposed a revised taxonomy based on a concept strongly resembling clades,<ref name="Huxley-1876">Huxley, T.H. (1876): Lectures on Evolution. ''New York Tribune''. Extra. no 36. In Collected Essays IV: pp 46–138 [http://aleph0.clarku.edu/huxley/CE4/LecEvol.html original text w/ figures]</ref> although the term ''clade'' itself would not be coined until 1957 by his grandson, [[Julian Huxley]].

===External links===

* [http://scienceblogs.com/evolvingthoughts/2007/01/clade_1.php Evolving Thoughts: Clade]

* [http://www.zo.utexas.edu/faculty/antisense/DownloadfilesToL.html DM Hillis, D Zwickl & R Gutell: ~3000 species Tree of Life] A cladogram?

German biologist [[Emil Hans Willi Hennig]] (1913–1976) is considered to be the founder of [[cladistics]].<ref>{{Cite journal|author=Brower, Andrew V. Z.|year=2013|title=Willi Hennig at 100|journal=Cladistics|volume=30|issue=2|pages=224–225|doi=10.1111/cla.12057|doi-access=free}}</ref>

[[Category:Phylogenetics]]

He proposed a classification system that represented repeated branchings of the family tree, as opposed to the previous systems, which put organisms on a "ladder", with supposedly more "advanced" organisms at the top.<ref name="Palmer-2009" /><ref name="Evolution 101">"Evolution 101". [http://evolution.berkeley.edu/evolibrary/article/evo_10 page 10]. Understanding Evolution website. University of California, Berkeley. Retrieved 26 February 2016.</ref>

Taxonomists have increasingly worked to make the taxonomic system reflect evolution.<ref name="Evolution 101"/> When it comes to [[Nomenclature#Biology|naming]], this principle is not always compatible with the traditional [[Linnaean taxonomy|rank-based nomenclature]] (in which only taxa associated with a [[Taxonomic rank|rank]] can be named) because not enough ranks exist to name a long series of nested clades. For these and other reasons, [[phylogenetic nomenclature]] has been developed; it is still controversial.

As an example, see the full current{{when|date=May 2024}} classification of ''[[Anas platyrhynchos]]'' (the mallard duck) with 40 clades from ''[[Eukaryota]]'' down by following [[species:Anas  platyrhynchos|this Wikispecies link]] and clicking on "Expand".

{{evolution-stub}}

The name of a clade is conventionally a plural, where the singular refers to each member individually. A unique exception is the reptile clade [[Dracohors]], which was made by [[haplology]] from Latin "draco" and "cohors", i.e. "the [[dragon]] [[Cohort (taxonomy)|cohort]]"; its form with a suffix added should be e.g. "dracohortian".

[[ca:Clade]]

[[es:Clado]]

==Definition==

[[eo:Klado]]

{{Main|Phylogenetic nomenclature}}

[[fr:Clade]]

[[File:Cladogram Crocodilia NL.PNG|thumb|right|upright|Gavialidae, Crocodylidae and Alligatoridae are clade names that are here applied to a [[Phylogenetics|phylogenetic]] tree of crocodylians.]]

[[hu:Klád]]

[[it:Clade]]

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.<ref group=note>

[[ms:Klad]]

A semantic case has been made in 2008 that the name should be "holophyletic", but this term has not acquired widespread use. For more information, see ''[[holophyly]]''.</ref><ref>{{cite web|title=International Code of Phylogenetic Nomenclature. Version 4c. Chapter I. Taxa.|url=http://www.ohio.edu/phylocode/art1-3.html|year=2010|access-date=22 September 2012|archive-date=15 June 2010|archive-url=https://web.archive.org/web/20100615072940/http://www.ohio.edu/phylocode/art1-3.html|url-status=dead}}</ref><ref>{{cite journal

[[nl:Clade]]

| doi = 10.1111/j.1095-8312.2008.00984.x

[[pl:Klad]]

| title = On the difference between mono-, holo-, and paraphyletic groups: a consistent distinction of process and pattern

[[pt:Clado]]

| year = 2008

[[ru:Клада]]

| last = Envall

[[sv:Klad]]

| first= Mats

[[fi:Kliini]]

| journal = Biological Journal of the Linnean Society

[[uk:Клада]]

| volume = 94

| page = 217 | doi-access = free

}}</ref> The ancestor can be known or unknown; any and all members of a clade can be extant or extinct.

==Clades and phylogenetic trees==

{{Main|Phylogenetics|Cladistics}}

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 ''[[cladogram]]s''; they, and all their branches, are phylogenetic hypotheses.<ref>{{cite journal|last=Nixon|first=Kevin C.|author2=Carpenter, James M.|title=On the Other "Phylogenetic Systematics"|journal=Cladistics|date=1 September 2000|volume=16|issue=3|pages=298–318|doi=10.1111/j.1096-0031.2000.tb00285.x|pmid=34902935|s2cid=73530548|doi-access=free}}</ref>

Three methods of defining clades are featured in [[phylogenetic nomenclature]]: node-, stem-, and apomorphy-based (see [[Phylogenetic nomenclature#Phylogenetic definitions of clade names|Phylogenetic nomenclature§Phylogenetic definitions of clade names]] for detailed definitions).

== Terminology ==

[[File:Primate cladogram.svg|thumb|left|Cladogram of modern primate groups; all tarsiers are haplorhines, but not all haplorhines are tarsiers; all apes are catarrhines, but not all catarrhines are apes; etc.]]

The relationship between clades can be described in several ways:

* A clade located within a clade is said to be ''nested'' within that clade. In the diagram, the [[Ape|hominoid]] clade, i.e. the apes and humans, is nested within the primate clade.

* Two clades are ''[[Sister group|sisters]]'' if they have an immediate common ancestor. In the diagram, lemurs and lorises are sister clades, while humans and tarsiers are not.

* A clade ''A'' is ''[[Basal (phylogenetics)|basal]]'' to a clade ''B'' if ''A'' branches off the lineage leading to ''B'' before the first branch leading only to members of ''B''. In the adjacent diagram, the [[Strepsirrhini|strepsirrhine]]/[[prosimian]] clade, is basal to the [[Hominoidea|hominoids]]/[[ape]] clade. In this example, both Haplorrhine as prosimians should be considered as most basal groupings. It is better to say that the prosimians are the sister group to the rest of the primates.<ref name="Krell-2004">{{cite journal |last1=Krell |first1=F.-T. |last2=Cranston |first2=P. |year=2004 |title=Which side of the tree is more basal? |journal=Systematic Entomology |volume=29 |issue=3 |pages=279–281 |doi=10.1111/j.0307-6970.2004.00262.x |s2cid=82371239 |name-list-style=amp |doi-access=free |bibcode=2004SysEn..29..279K }}</ref> This way one also avoids unintended and misconceived connotations about evolutionary advancement, complexity, diversity and ancestor status, e.g. due to impact of sampling diversity and extinction.{{citation needed|reason=reads as editorial comment|date=March 2019}}<ref name="Krell-2004" /><ref>{{Cite web|url=http://for-the-love-of-trees.blogspot.com/2016/09/the-ancestors-are-not-among-us.html|title=For the love of trees: The ancestors are not among us|last=Smith|first=Stacey|date=2016-09-19|website=For the love of trees|access-date=2019-03-23}}</ref> Basal clades should not be confused with stem groupings, as the latter is associated with paraphyletic or unresolved groupings.

=== Age ===

The age of a clade can be described based on two different reference points, [[crown group|crown]] age and stem age. The crown age of a clade refers to the age of the most recent common ancestor of all of the species in the clade. The stem age of a clade refers to the time that the ancestral lineage of the clade diverged from its [[sister group|sister]] clade. A clade's stem age is either the same as or older than its crown age.{{sfn|Harmon|2021}} Ages of clades cannot be directly observed. They are inferred, either from [[stratigraphy]] of [[fossil]]s, or from [[molecular clock]] estimates.<ref>Brower, A. V. Z., Schuh, R. T. 2021. Biological Systematics: Principles and Applications (3rd edn.). Cornell University Press, Ithaca, NY.</ref>

==Viruses==

[[File:HIV-SIV-phylogenetic-tree straight.svg|right|thumb|[[Phylogenetic tree]] of the SIV and HIV viruses showing clades (subtypes) of the virus.]]

[[Viruses]], and particularly [[RNA viruses]] form clades.<ref name="Yamaji-2020">{{cite journal |vauthors=Yamaji R, Saad MD, Davis CT, Swayne DE, Wang D, Wong FY, McCauley JW, Peiris JS, Webby RJ, Fouchier RA, Kawaoka Y, Zhang W |title=Pandemic potential of highly pathogenic avian influenza clade 2.3.4.4 A(H5) viruses |journal=Reviews in Medical Virology |volume=30 |issue=3 |pages=e2099 |date=May 2020 |pmid=32135031 |pmc=9285678 |doi=10.1002/rmv.2099 |url=}}</ref> These are useful in [[epidemiology|tracking the spread of viral infections]]. [[HIV]], for example, has clades called subtypes, which vary in geographical prevalence.<ref name="Stebbing-2003">{{cite journal |vauthors=Stebbing J, Moyle G |title=The clades of HIV: their origins and clinical significance |journal=AIDS Reviews |volume=5 |issue=4 |pages=205–13 |date=2003 |pmid=15011999 |doi= |url=}}</ref> HIV subtype (clade) B, for example is predominant in Europe, the Americas and Japan, whereas subtype A is more common in east Africa.<ref name="Sarabia-2019">{{cite journal |vauthors=Sarabia I, Bosque A |title=HIV-1 Latency and Latency Reversal: Does Subtype Matter? |journal=Viruses |volume=11 |issue=12 |date=November 2019 |page=1104 |pmid=31795223 |pmc=6950696 |doi=10.3390/v11121104 |url= |doi-access=free }}</ref>

== See also ==

{{Div col|colwidth=20em}}

* [[Adaptive radiation]]

* [[Binomial nomenclature]]

* [[Biological classification]]

* [[Cladistics]]

* [[Crown group]]

* [[Evolutionary grade|Grade]]

* [[Monophyly]]

* [[Paraphyly]]

* [[Phylogenetic network]]

* [[Phylogenetic nomenclature]]

* [[Phylogenetics]]

* [[Polyphyly]]

{{div col end}}

==Notes==

<references group=note/>

==References==

{{Reflist}}

== Bibliography ==

{{refbegin}}

* {{cite web |last1=Harmon |first1=Luke J |title=11.2: Clade Age and Diversity |url=https://bio.libretexts.org/Bookshelves/Evolutionary_Developmental_Biology/Phylogenetic_Comparative_Methods_(Harmon)/11%3A_Fitting_Birth-Death_Models/11.02%3A_Clade_Age_and_Diversity |website=Biology [[LibreTexts]]: Evolutionary Developmental Biology |publisher=[[University of California Davis]] |access-date=7 July 2021 |date=3 January 2021}}

{{refend}}

==External links==

{{Wiktionary}}

* [http://scienceblogs.com/evolvingthoughts/2007/01/12/clade-1 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]]

{{Phylogenetics}}

{{Evolution}}

[[Category:Unranked clades| ]]

[[Category:Evolutionary biology terminology]]

[[Category:Philosophy of biology]]

[[Category:Phylogenetics]]

[[Category:1950s neologisms]]