Clade

Last updated

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 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. Clade-grade II.svg
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 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 biological phylogenetics, a clade (from Ancient Greek κλάδος (kládos) 'branch'), also known as a monophyletic group or natural group, [1] is a grouping of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. [2] In the taxonomical literature, sometimes the Latin form cladus (plural cladi) is used rather than the English form.[ citation needed ] Clades are the fundamental unit of cladistics, a modern approach to taxonomy adopted by most biological fields.

Contents

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.

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. [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'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 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. In 1876 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.

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[ when? ] 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] Ages of clades cannot be directly observed. They are inferred, either from stratigraphy of fossils, or from molecular clock estimates. [16]

Viruses

Phylogenetic tree of the SIV and HIV viruses showing clades (subtypes) of the virus. HIV-SIV-phylogenetic-tree straight.svg
Phylogenetic tree of the SIV and HIV viruses showing clades (subtypes) of the virus.

Viruses, and particularly RNA viruses form clades. [17] These are useful in tracking the spread of viral infections. HIV, for example, has clades called subtypes, which vary in geographical prevalence. [18] HIV subtype (clade) B, for example is predominant in Europe, the Americas and Japan, whereas subtype A is more common in east Africa. [19]

See also

Notes

  1. ↑ 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 .

Related Research Articles

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.

<span class="mw-page-title-main">Monophyly</span> Property of a group of including all taxa descendant from a common ancestral species

In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria:

  1. the grouping contains its own most recent common ancestor, i.e. excludes non-descendants of that common ancestor
  2. the grouping contains all the descendants of that common ancestor, without exception

In biology, phylogenetics is the study of the evolutionary history of life using genetics, which is known as phylogenetic inference. It establishes the relationship between organisms with the empirical data and observed heritable traits of DNA sequences, protein amino acid sequences, and morphology. The results are a phylogenetic tree—a diagram setting the hypothetical relationships between organisms and their evolutionary history.

In biology, phenetics, also known as taximetrics, is an attempt to classify organisms based on overall similarity, usually with respect to morphology or other observable traits, regardless of their phylogeny or evolutionary relation. It is related closely 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.

<span class="mw-page-title-main">Paraphyly</span> Type of taxonomic group

Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic with respect to the excluded subgroups. In contrast, a monophyletic grouping includes a common ancestor and all of its descendants.

<span class="mw-page-title-main">Systematics</span> Branch of biology

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.

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.

<span class="mw-page-title-main">Taxon</span> 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, especially in the context of rank-based ("Linnaean") nomenclature. 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.

<span class="mw-page-title-main">Polyphyly</span> Property of a group not united by common ancestry

A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. 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. It is contrasted with monophyly and paraphyly.

<span class="mw-page-title-main">Phylogenesis</span>

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

<span class="mw-page-title-main">Sauropsida</span> Taxonomic clade

Sauropsida is a clade of amniotes, broadly equivalent to the class Reptilia, though typically used in a broader sense to also include extinct stem-group relatives of modern reptiles and birds. The most popular definition states that Sauropsida is the sibling taxon to Synapsida, the other 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 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.

<span class="mw-page-title-main">Apomorphy and synapomorphy</span> 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.

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

<span class="mw-page-title-main">Evolutionary grade</span> 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.

<span class="mw-page-title-main">Plesiomorphy and symplesiomorphy</span> Ancestral character or trait state shared by two or more taxa

In phylogenetics, a plesiomorphy and symplesiomorphy are synonyms for an ancestral character shared by all members of a clade, which does not distinguish the clade from other clades.

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 method, by 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 regulated currently by the International Code of Phylogenetic Nomenclature (PhyloCode).

<span class="mw-page-title-main">Taxonomic rank</span> Level in a taxonomic hierarchy

In biology, taxonomic rank is the relative or absolute level of a group of organisms in a hierarchy that reflects evolutionary relationships. Thus, the most inclusive clades have the highest ranks, whereas the least inclusive ones have the lowest ranks. Ranks can be either relative and be denoted by an indented taxonomy in which the level of indentation reflects the rank, or absolute, in which various terms, such as species, genus, family, order, class, phylum, kingdom, and domain designate rank. This page emphasizes absolute ranks and the rank-based codes require them. However, absolute ranks are not required in all nomenclatural systems for taxonomists; for instance, the PhyloCode, the code of phylogenetic nomenclature, does not require absolute ranks.

<span class="mw-page-title-main">Outline of evolution</span> Overview of and topical guide to change in the heritable characteristics of organisms

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

<span class="mw-page-title-main">Character evolution</span>

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

  1. ↑ Martin, Elizabeth; Hin, Robert (2008). A Dictionary of Biology. Oxford University Press.
  2. ↑ Cracraft, Joel; Donoghue, Michael J., eds. (2004). "Introduction". Assembling the Tree of Life . Oxford University Press. p.  1. ISBN   978-0-19-972960-9.
  3. 1 2 Palmer, Douglas (2009). Evolution: The Story of Life. Berkeley: University of California Press. p. 13.
  4. ↑ Pace, Norman R. (18 May 2006). "Time for a change". Nature. 441 (7091): 289. Bibcode:2006Natur.441..289P. doi: 10.1038/441289a . ISSN   1476-4687. PMID   16710401. S2CID   4431143.
  5. ↑ Dupuis, Claude (1984). "Willi Hennig's impact on taxonomic thought". Annual Review of Ecology and Systematics. 15: 1–24. doi: 10.1146/annurev.es.15.110184.000245 .
  6. ↑ Huxley, J. S. (1957). "The three types of evolutionary process". Nature. 180 (4584): 454–455. Bibcode:1957Natur.180..454H. doi:10.1038/180454a0. S2CID   4174182.
  7. ↑ Huxley, T.H. (1876): Lectures on Evolution. New York Tribune. Extra. no 36. In Collected Essays IV: pp 46–138 original text w/ figures
  8. ↑ Brower, Andrew V. Z. (2013). "Willi Hennig at 100". Cladistics. 30 (2): 224–225. doi: 10.1111/cla.12057 .
  9. 1 2 "Evolution 101". page 10. Understanding Evolution website. University of California, Berkeley. Retrieved 26 February 2016.
  10. ↑ "International Code of Phylogenetic Nomenclature. Version 4c. Chapter I. Taxa". 2010. Archived from the original on 15 June 2010. Retrieved 22 September 2012.
  11. ↑ Envall, Mats (2008). "On the difference between mono-, holo-, and paraphyletic groups: a consistent distinction of process and pattern". Biological Journal of the Linnean Society. 94: 217. doi: 10.1111/j.1095-8312.2008.00984.x .
  12. ↑ Nixon, Kevin C.; Carpenter, James M. (1 September 2000). "On the Other "Phylogenetic Systematics"". Cladistics. 16 (3): 298–318. doi: 10.1111/j.1096-0031.2000.tb00285.x . PMID   34902935. S2CID   73530548.
  13. 1 2 Krell, F.-T. & Cranston, P. (2004). "Which side of the tree is more basal?". Systematic Entomology. 29 (3): 279–281. Bibcode:2004SysEn..29..279K. doi: 10.1111/j.0307-6970.2004.00262.x . S2CID   82371239.
  14. ↑ Smith, Stacey (19 September 2016). "For the love of trees: The ancestors are not among us". For the love of trees. Retrieved 23 March 2019.
  15. ↑ Harmon 2021.
  16. ↑ Brower, A. V. Z., Schuh, R. T. 2021. Biological Systematics: Principles and Applications (3rd edn.). Cornell University Press, Ithaca, NY.
  17. ↑ Yamaji R, Saad MD, Davis CT, Swayne DE, Wang D, Wong FY, McCauley JW, Peiris JS, Webby RJ, Fouchier RA, Kawaoka Y, Zhang W (May 2020). "Pandemic potential of highly pathogenic avian influenza clade 2.3.4.4 A(H5) viruses". Reviews in Medical Virology. 30 (3): e2099. doi:10.1002/rmv.2099. PMC   9285678 . PMID   32135031.
  18. ↑ Stebbing J, Moyle G (2003). "The clades of HIV: their origins and clinical significance". AIDS Reviews. 5 (4): 205–13. PMID   15011999.
  19. ↑ Sarabia I, Bosque A (November 2019). "HIV-1 Latency and Latency Reversal: Does Subtype Matter?". Viruses. 11 (12): 1104. doi: 10.3390/v11121104 . PMC   6950696 . PMID   31795223.

Bibliography

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.