Clade

Cladogram (family tree) of a biological group (ancestor at the bottom). The red and blue boxes at right and left represent clades (i.e., complete branches). The green box in the middle is not a clade, but rather represents an evolutionary grade, an incomplete group, because the blue clade at left is descended from it, but is excluded.

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.^[1][2]

Definitions

Gavialidae, Crocodylidae and Alligatoridae are clade names that are here applied to a phylogenetic tree of crocodylians.

Clade and ancestor

A clade is by definition population, or a species) and all its descendants.^{[note 1][3][4]} 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.^[5]

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

Terminology

Cladogram of the primates, showing the nested nature of the primate tree

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.^[6]

Nomenclature and taxonomy

Early phylogenetic tree by Haeckel, 1866

The idea of a clade did not exist in pre-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.

See also

• Binomial nomenclature
• Biological classification
• Cladistics
• Crown group
• Monophyly
• Paraphyly
• Phylogenetic nomenclature
• Phylogenetics
• Polyphyly
• Adaptive radiation

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

References

• 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

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