A clade (from Ancient Greek: κλάδος, klados, "branch") or monophylum (see monophyletic) is a life-form group consisting of a common ancestor and all its descendants—representing a single "branch" on the "tree of life".
The common ancestor may be an individual, a population or even 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.
Many commonly named groups are clades, for example, rodents, or insects; because in each case, their name comprises a common ancestor with all its descendant branches. Rodents, for example, are a branch of mammals that split off after the end of the dinosaurs 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 comprises even smaller clades. The clade "rodent" is in turn included in the mammal, vertebrate and animal clades.
Often, however, common words for kinds of living things name only part of a clade. "Lizards" is not a clade, and neither is "monkeys". Snakes are a clade, and lizards and snakes together make a bigger clade, but "lizard" excludes snakes based on anatomy. Snakes are still part of that clade even though they have evolved to look different. The term "lizard" puts geckos and Gila monsters in the same category but excludes snakes. Since Gila monsters are more closely related to snakes than they are to geckos, the term "lizard" does not name a clade. In the same way, "monkeys" is not a clade because it excludes apes. Apes are more closely related to Old World monkeys than they are to New World monkeys, so any clade big enough to include New World and Old World monkeys has to include apes, too. Finally, many definitive clades lack common terms for them. For example, there is no term for the clade that includes all monkeys and apes (humans included). Terms such as "vertebrate" and "mammal" resulted from modern taxonomy, since folk taxonomy did not have terms for these clades.
Clade and ancestor
A clade is by definition monophyletic, meaning it contains one ancestor (which can be an organism, a population, or a species) and all its descendants.[note 1] 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 being derived from "clade" by Ernst Mayr (1965). The results of phylogenetic/cladistic analyses are tree-shaped diagrams called cladograms; they, and all their branches, are phylogenetic hypotheses.
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, 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.
- 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 diagram to the right, the strepsirrhine clade, including the lemurs and lorises, is basal to the hominoids, the apes and humans. Some authors have used "basal" differently, using it to mean a clade that is "more primitive" or less species-rich than its sister clade; others consider this usage to be incorrect.
Nomenclature and taxonomy
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 publication of Darwin's theory of evolution in 1859, the idea was born that groups used in a system of classification should represent branches on the evolutionary tree of life. In the century and a half since then, 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; also, taxon names cannot be defined in a way that guarantees them to refer to clades. For these and other reasons, phylogenetic nomenclature has been developed; it is still controversial.
- Binomial nomenclature
- Biological classification
- Crown group
- Phylogenetic nomenclature
- Adaptive radiation
- 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.
- 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.
- Huxley, J. S. (1957). "The three types of evolutionary process". Nature 180: 454–455. doi:10.1038/180454a0.
- "International Code of Phylogenetic Nomenclature. Version 4c. Chapter I. Taxa.". 2010. Retrieved 22 September 2012.
- 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.
- 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.
- Krell, F.-T. & Cranston, P. (2004). "Which side of the tree is more basal?". Systematic Entomology 29 (3): 279–281. doi:10.1111/j.0307-6970.2004.00262.x.
- "Choosing the Book title 'Clade'". Penguin Group Australia. 2015. Retrieved 20 January 2015.
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- Evolving Thoughts: Clade
- DM Hillis, D Zwickl & R Gutell. "Tree of life". An unrooted cladogram depicting around 3000 species.
- Phylogenetic systematics, an introductory slide-show on evolutionary trees University of California, Berkeley