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A clade (; from grc, , ''klados'', "branch"), also known as a monophyletic group or natural group, is a group of
organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is a synonym for "Outline of life forms, life form". Organ ...
s that are monophyletic—that is, composed of a
common ancestor Common descent is a concept in evolutionary biology Evolutionary biology is the subfield of biology that studies the evolution, evolutionary processes (natural selection, common descent, speciation) that produced the Biodiversity, diversity ...
and all its lineal descendants - on a
phylogenetic tree
phylogenetic tree
. Rather than the English term, the equivalent
Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became the ...
term ''cladus'' (plural ''cladi'') is often used in taxonomical literature. The common ancestor may be an individual, a
population In biology, a population is a number of all the organisms of the same group or species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiv ...
, or a
species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individu ...
(
extinct Extinction is the termination of a kind of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of life. It is ...
or extant). Clades are nested, one in another, as each branch in turn splits into smaller branches. These splits reflect
evolutionary history The history of life on Earth traces the processes by which living and fossil organisms evolved, from the earliest emergence of life to the present. Earth formed about 4.5 billion years ago (abbreviated as ''Ga'', for ''Year#SI prefix multiplier ...
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 In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanism ...
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 are that
fungi A fungus (plural The plural (sometimes list of glossing abbreviations, abbreviated ), in many languages, is one of the values of the grammatical number, grammatical category of number. The plural of a noun typically denotes a quantity great ...
are closer relatives to animals than they are to plants,
archaea Archaea ( ; singular archaeon ) constitute a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria Bacteria (; common noun bacteria, s ...
are now considered different from
bacteria Bacteria (; common noun bacteria, singular bacterium) are a type of Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a Bacte ...
, 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 Historical linguistics, also termed diachronic linguistics, is the scientific study of language change over time. Principal concerns of historical linguistics include: # to describe and account for observed changes in particular languages # t ...
; see Cladistics § In disciplines other than biology.


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. 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.


History of nomenclature and taxonomy

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 – although as it happens, many of the better known animal groups in Linnaeus' original Systema Naturae (notably among the vertebrate groups) do represent clades. The phenomenon of convergent evolution is, however, responsible for many cases where there are misleading similarities in the morphology of groups that evolved from different lineages. 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. Thomas Henry Huxley, an early advocate of evolutionary theory, proposed a revised taxonomy based on a concept strongly resembling clades,Huxley, T.H. (1876): Lectures on Evolution. ''New York Tribune''. Extra. no 36. In Collected Essays IV: pp 46-13
original text w/ figures
/ref> although the term ''clade'' itself would not be coined until 1957 by his grandson, Julian Huxley. For example, the elder Huxley grouped birds with reptiles, based on fossil evidence. German biologist Emil Hans Willi Hennig (1913 – 1976) is considered to be the founder of cladistics. 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.”Evolution 101"
page 10
Understanding Evolution website. University of California, Berkeley. Retrieved 26 February 2016.
Taxonomists have increasingly worked to make the taxonomic system reflect evolution. When it comes to Nomenclature#Biology, naming, however, 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 there are not enough ranks 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, the full current classification of ''Anas platyrhynchos'' (the mallard duck) has 40 clades from ''Eukaryota'' down: see https://species.wikimedia.org/wiki/Anas_platyrhynchos and click on "Expand".


Definitions

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. A semantic case has been made that the name should be "holophyletic", but this term has not acquired widespread use. For more information, see ''holophyly''. 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#Phylogenetic definitions of clade names, Phylogenetic nomenclature§Phylogenetic definitions of clade names for detailed definitions).


Terminology

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. However, 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. This way one also avoids unintended and misconceived connotations about evolutionary advancement, complexity, diversity, ancestor status, and ancienity e.g. due to impact of sampling diversity and extinction. Basal clades should not be confused with stem groupings, as the latter is associated with paraphyletic or unresolved groupings.


In popular culture

''Clade'' is the title of a novel by James Bradley (Australian writer), James Bradley, who chose it both because of its biological meaning and also because of the larger implications of the word. An episode of ''Elementary (TV series), Elementary'' is titled "List of Elementary episodes#ep38, Dead Clade Walking" and deals with a case involving a rare fossil.


See also

* Adaptive radiation * Binomial nomenclature * Biological classification * Cladistics * Crown group * Monophyly * Paraphyly * Phylogenetic network * Phylogenetic nomenclature * Phylogenetics * Polyphyly


Notes


References


External links


Evolving Thoughts: "Clade"
* DM Hillis, D Zwickl & R Gutell.

. An unrooted cladogram depicting around 3000 species.
"Phylogenetic systematics, an introductory slide-show on evolutionary trees"
University of California, Berkeley {{Evolution Unranked clades, Biology terminology Philosophy of biology Phylogenetics 1950s neologisms