Clade

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Cladogram (family tree) of a biological group. The last common ancestor is the vertical line stem at the bottom. The blue and red subgroups are clades; each shows its common ancestor stem at the bottom of the subgroup branch. The green subgroup is not a clade; it is a paraphyletic group, because it excludes the blue branch which has descended from the same common ancestor. The green and blue subgroups together form a clade again. Clade-grade II.svg
Cladogram (family tree) of a biological group. The last common ancestor is the vertical line stem at the bottom. The blue and red subgroups are clades; each shows its common ancestor stem at the bottom of the subgroup branch. The green subgroup is not a clade; it is a paraphyletic group, because it excludes the blue branch which has descended from the same common ancestor. The green and blue subgroups together form a clade again.

A clade (from Ancient Greek κλάδος (kládos) 'branch'), also known as a monophyletic group or natural group, [1] is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. [2] Rather than the English term, the equivalent Latin term cladus (plural cladi) is often used in taxonomical literature.

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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. [3] 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. [4]

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.

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. [5] [6]

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

Early phylogenetic tree by Haeckel, 1866. Groups once thought to be more advanced, such as birds ("Aves"), are placed at the top. Haeckel arbol bn.png
Early phylogenetic tree by Haeckel, 1866. Groups once thought to be more advanced, such as birds ("Aves"), are placed at the top.

The idea of a clade did not exist in pre-Darwinian Linnaean taxonomy, which was based by necessity only on internal or external morphological similarities between organisms. Many of the better known animal groups in Linnaeus' 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 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. The publication of Darwin's 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, [7] 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. [7]

German biologist Emil Hans Willi Hennig (1913–1976) is considered to be the founder of cladistics. [8] 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. [3] [9]

Taxonomists have increasingly worked to make the taxonomic system reflect evolution. [9] When it comes to naming, this principle is not always compatible with the traditional rank-based nomenclature (in which only taxa associated with a 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 classification of Anas platyrhynchos (the mallard duck) with 40 clades from Eukaryota down by following this Wikispecies link and clicking on "Expand".

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"; its form with a suffix added should be e.g. "dracohortian".

Definition

Gavialidae, Crocodylidae and Alligatoridae are clade names that are here applied to a phylogenetic tree of crocodylians. Cladogram Crocodilia NL.PNG
Gavialidae, Crocodylidae and Alligatoridae are clade names that are here applied to a phylogenetic 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. [note 1] [10] [11] 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. [12]

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

Terminology

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. Primate cladogram.svg
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:

Age

The age of a clade can be described based on two different reference points, 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 clade. A clade's stem age is either the same as or older than its crown age. [15] Note that ages of clades cannot be directly observed. They are inferred, either from stratigraphy of fossils, or from molecular clock estimates [16]

Clade is the title of a novel by James Bradley, who chose it both because of its biological meaning and also because of the larger implications of the word. [17]

An episode of Elementary is titled "Dead Clade Walking" and deals with a case involving a rare fossil.

See also

Notes

  1. 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 .

Related Research Articles

Cladistics Method of biological systematics in evolutionary biology

Cladistics is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived characteristics (synapomorphies) that are not present in more distant groups and ancestors. However, from an empirical perspective, common ancestors are inferences based on a cladistic hypothesis of relationships of taxa whose character states can be observed. Theoretically, a last common ancestor and all its descendants constitute a (minimal) clade. Importantly, all descendants stay in their overarching ancestral clade. For example, if the terms worms or fishes were used within a strict cladistic framework, these terms would include humans. Many of these terms are normally used paraphyletically, outside of cladistics, e.g. as a 'grade', which are fruitless to precisely delineate, especially when including extinct species. Radiation results in the generation of new subclades by bifurcation, but in practice sexual hybridization may blur very closely related groupings.

Monophyly Property of a group of including all taxa descendant from a common ancestral species

In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor and all of its lineal descendants. Monophyletic groups are typically characterised by shared derived characteristics (synapomorphies), which distinguish organisms in the clade from other organisms. An equivalent term is holophyly.

Phylogenetics Study of evolutionary relationships between organisms

In biology, phylogenetics is the study of the evolutionary history and relationships among or within groups of organisms. These relationships are determined by phylogenetic inference methods that focus on observed heritable traits, such as DNA sequences, protein amino acid sequences, or morphology. The result of such an analysis is a phylogenetic tree—a diagram containing a hypothesis of relationships that reflects the evolutionary history of a group of organisms.

In biology, phenetics, also known as taximetrics, is an attempt to classify organisms based on overall similarity, usually in morphology or other observable traits, regardless of their phylogeny or evolutionary relation. It is closely related to numerical taxonomy which is concerned with the use of numerical methods for taxonomic classification. Many people contributed to the development of phenetics, but the most influential were Peter Sneath and Robert R. Sokal. Their books are still primary references for this sub-discipline, although now out of print.

Paraphyly Type of taxonomic group

In taxonomy, a group is paraphyletic if it consists of the group's last common ancestor and most of its descendants, excluding a few monophyletic subgroups. The group is said to be paraphyletic with respect to the excluded subgroups. In contrast, a monophyletic group includes a common ancestor and all of its descendants. The terms are commonly used in phylogenetics and in the tree model of historical linguistics. Paraphyletic groups are identified by a combination of synapomorphies and symplesiomorphies. If many subgroups are missing from the named group, it is said to be polyparaphyletic.

Systematics Study of the diversification and relationships among living things through time

Biological systematics is the study of the diversification of living forms, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees. Phylogenies have two components: branching order and branch length. Phylogenetic trees of species and higher taxa are used to study the evolution of traits and the distribution of organisms (biogeography). Systematics, in other words, is used to understand the evolutionary history of life on Earth.

Taxonomy (biology) Science of naming, defining and classifying organisms

In biology, taxonomy is the scientific study of naming, defining (circumscribing) and classifying groups of biological organisms based on shared characteristics. Organisms are grouped into taxa and these groups are given a taxonomic rank; groups of a given rank can be aggregated to form a more inclusive group of higher rank, thus creating a taxonomic hierarchy. The principal ranks in modern use are domain, kingdom, phylum, class, order, family, genus, and species. The Swedish botanist Carl Linnaeus is regarded as the founder of the current system of taxonomy, as he developed a ranked system known as Linnaean taxonomy for categorizing organisms and binomial nomenclature for naming organisms.

Cladogram Diagram used to show relations among groups of organisms with common origins

A cladogram is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to descendants, nor does it show how much they have changed, so many differing evolutionary trees can be consistent with the same cladogram. A cladogram uses lines that branch off in different directions ending at a clade, a group of organisms with a last common ancestor. There are many shapes of cladograms but they all have lines that branch off from other lines. The lines can be traced back to where they branch off. These branching off points represent a hypothetical ancestor which can be inferred to exhibit the traits shared among the terminal taxa above it. This hypothetical ancestor might then provide clues about the order of evolution of various features, adaptation, and other evolutionary narratives about ancestors. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational phylogenetics are now very commonly used in the generation of cladograms, either on their own or in combination with morphology.

Taxon Grouping of biological populations

In biology, a taxon is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and given a particular ranking, especially if and when it is accepted or becomes established. It is very common, however, for taxonomists to remain at odds over what belongs to a taxon and the criteria used for inclusion. If a taxon is given a formal scientific name, its use is then governed by one of the nomenclature codes specifying which scientific name is correct for a particular grouping.

Polyphyly Set of organisms that do not share an immediate common ancestor

A polyphyletic group or assemblage is a set of organisms, or other evolving elements, that have been grouped together based on characteristics that do not imply that they share a common ancestor that is not also the common ancestor of many other taxa. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of convergent evolution. The arrangement of the members of a polyphyletic group is called a polyphyly.

Phylogenesis

Phylogenesis is the biological process by which a taxon appears. The science that studies these processes is called phylogenetics.

Sauropsida Taxonomic clade

Sauropsida is a clade of amniotes, broadly equivalent to the class Reptilia. Sauropsida is the sister taxon to Synapsida, the clade of amniotes which includes mammals as its only modern representatives. Although early synapsids have historically been referred to as "mammal-like reptiles", all synapsids are more closely related to mammals than to any modern reptile. Sauropsids, on the other hand, include all amniotes more closely related to modern reptiles than to mammals. This includes Aves (birds), which are now recognized as a subgroup of archosaurian reptiles despite originally being named as a separate class in Linnaean taxonomy.

Evolutionary taxonomy, evolutionary systematics or Darwinian classification is a branch of biological classification that seeks to classify organisms using a combination of phylogenetic relationship, progenitor-descendant relationship, and degree of evolutionary change. This type of taxonomy may consider whole taxa rather than single species, so that groups of species can be inferred as giving rise to new groups. The concept found its most well-known form in the modern evolutionary synthesis of the early 1940s.

Apomorphy and synapomorphy Two concepts on heritable traits

In phylogenetics, an apomorphy is a novel character or character state that has evolved from its ancestral form. A synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have evolved in their most recent common ancestor. In cladistics, synapomorphy implies homology.

Crown group Monophyletic closure of a set of living species

In phylogenetics, the crown group or crown assemblage is a collection of species, composed of the living representatives of the collection, the most recent common ancestor of the collection, and all descendants of the most recent common ancestor. It is thus a way of defining a clade, a group consisting of a species and all its extant or extinct descendants. For example, Neornithes (birds) can be defined as a crown group, which includes the most recent common ancestor of all modern birds, and all of its extant or extinct descendants.

In phylogenetics, a sister group or sister taxon comprises the closest relative(s) of another given unit in an evolutionary tree.

Evolutionary grade Non-monophyletic grouping of organisms united by morphological or physiological characteristics

A grade is a taxon united by a level of morphological or physiological complexity. The term was coined by British biologist Julian Huxley, to contrast with clade, a strictly phylogenetic unit.

Phylogenetic nomenclature is a method of nomenclature for taxa in biology that uses phylogenetic definitions for taxon names as explained below. This contrasts with the traditional approach, in which taxon names are defined by a type, which can be a specimen or a taxon of lower rank, and a description in words. Phylogenetic nomenclature is currently regulated by the International Code of Phylogenetic Nomenclature (PhyloCode).

Outline of evolution Hierarchical outline list of articles related to evolution

The following outline is provided as an overview of and topical guide to evolution:

Character evolution

Character evolution is the process by which a character or trait evolves along the branches of an evolutionary tree. Character evolution usually refers to single changes within a lineage that make this lineage unique from others. These changes are called character state changes and they are often used in the study of evolution to provide a record of common ancestry. Character state changes can be phenotypic changes, nucleotide substitutions, or amino acid substitutions. These small changes in a species can be identifying features of when exactly a new lineage diverged from an old one.

References

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