Cladogram (a branching tree diagram) illustrating the relationships of organisms within groups of taxa known as clades. The vertical line (stem) at the base (bottom) represents thelast common ancestor. The blue and red subgroups are clades, each defined by a common ancestor stem at the base of its respective subgroup (branch). The green subgroup alone, however, isnot a clade; it is aparaphyletic 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.
Inbiology, aclade (//kleɪd//) (from Ancient Greek κλάδος (kládos)'branch'), also known as amonophyletic group ornatural group,[1] is a group oforganisms that is composed of acommon ancestor and all of its descendants.[2] Clades are the fundamental unit ofcladistics, a modern approach totaxonomy adopted by most biological fields.
The common ancestor may be an individual, apopulation, or aspecies (extinct orextant). Clades are nested, one in another, as each branch in turn splits into smaller branches. These splits reflectevolutionary history as populations diverged and evolved independently. Clades are termedmonophyletic (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 namingtaxa 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 thatfungi are closer relatives to animals than they are to plants,archaea are now considered different frombacteria, and multicellular organisms may have evolved from archaea.[4]
The termclade was coined in 1957 by the biologistJulian Huxley to refer to the result ofcladogenesis, the evolutionary splitting of a parent species into two distinct species, a concept Huxley borrowed fromBernhard Rensch.[5][6]
Many commonly named groups –rodents andinsects, 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 ofmammals that split off after the end of the period when the cladeDinosauria stopped being the dominant terrestrialvertebrates 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 aschipmunk orant, respectively, each of which consists of even smaller clades. The clade "rodent" is in turn included in the mammal,vertebrate and animal clades.[citation needed]
Early phylogenetic tree byHaeckel, 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-DarwinianLinnaean taxonomy, which was based by necessity only on internal or externalmorphological similarities between organisms. Many of the better known animal groups in Linnaeus's originalSystema Naturae (mostlyvertebrate groups) do represent clades. The phenomenon ofconvergent evolution is responsible for many cases of misleading similarities in themorphology of groups that evolved from different lineages.[citation needed]
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 evolutionarytree of life. The publication of Darwin'stheory of evolution in 1859 gave this view increasing weight. In 1876Thomas Henry Huxley, an early advocate of evolutionary theory, proposed a revised taxonomy based on a concept strongly resembling clades,[7] although the termclade itself would not be coined until 1957 by his grandson,Julian Huxley.
German biologistEmil Hans Willi Hennig (1913–1976) is considered to be the founder ofcladistics.[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 tonaming, this principle is not always compatible with the traditionalrank-based nomenclature (in which only taxa associated with arank 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.[citation needed]
The name of a clade is conventionally a plural, where the singular refers to each member individually. A unique exception is the reptile cladeDracohors, which was made byhaplology from Latin "draco" and "cohors", i.e. "thedragoncohort"; its form with a suffix added should be e.g. "dracohortian".[citation needed]
Gavialidae, Crocodylidae and Alligatoridae are clade names that are here applied to aphylogenetic tree of crocodylians.
A clade is by definitionmonophyletic, 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.
The science that tries to reconstruct phylogenetic trees and thus discover clades is calledphylogenetics orcladistics, the latter term coined byErnst Mayr (1965), derived from "clade". The results of phylogenetic/cladistic analyses are tree-shaped diagrams calledcladograms; they, and all their branches, are phylogenetic hypotheses.[12]
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 benested within that clade. In the diagram, thehominoid clade, i.e. the apes and humans, is nested within the primate clade.[citation needed]
Two clades aresisters if they have an immediate common ancestor. In the diagram, lemurs and lorises are sister clades, while humans and tarsiers are not.[citation needed]
A cladeA isbasal to a cladeB ifA branches off the lineage leading toB before the first branch leading only to members ofB. In the adjacent diagram, thestrepsirrhine/prosimian clade, is basal to thehominoids/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.[13] 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][13][14] Basal clades should not be confused with stem groupings, as the latter is associated with paraphyletic or unresolved groupings.
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 itssister 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 fromstratigraphy offossils, or frommolecular clock estimates.[16]
Phylogenetic tree of the SIV and HIV viruses showing clades (subtypes) of the virus.
Viruses, and particularlyRNA viruses form clades.[17] These are useful intracking 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]
^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, seeholophyly.
