Cladogram (family tree) of a biological group, showing the last common ancestor
of the composite tree, which is the vertical line 'trunk' (stem) at the
bottom, with all descendant branches shown above. The blue and red
subgroups (at left and right) are clades, or monophyletic (complete) groups; 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, which is an incomplete
clade here because it excludes the blue branch even though it has also
descended from the common ancestor stem at the bottom of the green
branch. The green subgroup together with the blue one forms a clade
again.
A clade (from Ancient Greek: κλάδος, klados, "branch"), also known as monophyletic group, is a group of organisms that consists of a common ancestor and all its lineal descendants, and represents a single "branch" on the "tree of life". Rather than the English term, the equivalent Latin term cladus (plural cladi) is often used in taxonomical literature.
The common ancestor may be an individual, a population, a species (extinct or extant), and so on right up to a kingdom and further. 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. 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 are 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.
Etymology
The term "clade" was coined in 1957 by the biologist Julian Huxley to refer to the result of cladogenesis, 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
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 – 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. 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 clades. For example, he 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.
Taxonomists have increasingly worked to make the taxonomic system reflect evolution. When it comes to naming, however, this principle is not always compatible with the traditional rank-based nomenclature. In the latter, only taxa associated with a rank can be named, yet 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.
Definitions
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. 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 here 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.
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 hominoid clade, i.e. the apes and humans, is nested within the primate clade.
- Two clades are 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 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 strepsirrhine/prosimian clade, is basal to the 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, who chose it both because of its biological meaning and also because of the larger implications of the word.
An episode of Elementary was titled "Dead Clade Walking" and dealt with a case involving a rare fossil.
