Clade

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

A clade^{[note 1]} is a group consisting of an organism and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life".^[1] The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological classification. In cladistics (which takes its name from the term), clades are the only acceptable units.

The term was coined by English biologist Julian Huxley.

Definitions

Clade and ancestor

A clade is termed monophyletic, meaning it contains one ancestor which can be an organism, population, or species and all its descendants.^{[note 2][2][3]} The term clade refers to the grouping of the ancestor and its living and/or deceased descendants together. The ancestor can be a theoretical or actual species.

Clade definition

Three methods of defining clades are featured in phylogenetic nomenclature: node-, stem-, and apomorphy-based:

• In node-based definition, clade name A refers to the least inclusive clade containing taxa (or specimens) X, Y, etc., and their common ancestor. The ancestor is the branch point, or node.
• In stem-based definition, A refers to the most inclusive clade containing X, Y, etc., and their common ancestor, down to where Z branches off below A. Taxa are included between the node of A and down to (but not including) the branching point to Z; that is, the stem of A.
• In apomorphy-based definition, A refers to the clade identified by an apomorphy found in X, Y, etc., and their common ancestor. This definition is basically similar to the Linnaean system.

Clades as constructs

In cladistics, the clade is a hypothetical construct based on experimental data. Clades are found using multiple (sometimes hundreds) of traits from a number of species (or specimens) and analysing them statistically to find the most likely phylogenetic tree for the group. Although similar in some ways to a biological classification of species, the method is statistical and more open to scrutiny than traditional methods. Although taxonomists use clades as a tool in classification where feasible, the taxonomic "tree of life" is not the same as the cladistic. The traditional genus, family, etc. names are not necessarily clades; though they will often be.

Clade names

In Linnaean systematics, the various groups are ordered into a series of taxonomic ranks (the familiar order, family etc). These ranks will by convention dictate the ending to names for some groups. Clades do not by their nature fit this scheme, and no such restriction exists as to their names. There is however a convention for naming more or less inclusive groups, which are given prefixes like crown- or pan-, see Crown group.

Taxonomy and systematics

Early phylogenetic tree by Haeckel, 1866

The idea of "clade" did not exist in pre-Darwinian Linnaean taxonomy, which was based only on morphological similarities between organisms, though many of the better known animal groups in Linnaeus' original Systema Naturae (notably among the vertebrate groups) represent clades. With the publication of Darwin's theory of evolution in 1859, taxonomy gained a theoretical basis, and the idea that systematic units represent branches on the evolutionary tree of life was born. In the century and a half since then, taxonomists have worked to make the taxonomic system reflect evolution. However, as the Tree of Life branches rather unevenly, the hierarchy of the Linnaean system does not always lend itself well to represent clades. When it comes to naming, cladistics and Linnaean taxonomy are not always compatible. Particularly higher level taxons in Linnaean taxonomy often represent evolutionary grades rather than clades, i.e. clades where one or two sub-branches have been excluded. Typical examples include bony fishes, who include the ancestor of tetrapods, and reptiles, ancestral to both birds and mammals.^{[note 3]}

In phylogenetic nomenclature, clades can be nested at any level, and do not have to be slotted into a rank in an overall hierarchy. In contrast, the Linnaean units of "order", "class" etc. must be used when naming a new taxon. As there are only seven formal levels to the Linnaean system (whereof species is the lowest), there is a finite amount of sub- and super-units that can be applied. As taxonomic trees (cladograms) become more detailed, some researchers intimately familiar with the topography of the trees they are working with have opted to dispense with the ranks all together, using clade names without Linnaean ranks. The preference of one system over the other is mainly one of application: Cladistics give details, but require intimate knowledge; the Linnaean system gives a well ordered overview, at the expense of details of the phylogenetic tree.

In a few instances, the Linnaean system has actually impeded our understanding of the phylogeny and broad evolutionary patterns. The best known example is the interpretation of the strange fossils of the Burgess Shale and the subsequent idea of a "Cambrian Explosion" ^[4] With the application of cladistics, and the rejection of any significance of the concept of Phyla, the confusion of the late 20th century over the Burgess animals has been resolved. It appears there never was an "explosion" of major bauplans with subsequent extinctions.^[5] The seemingly weird critters themselves have been found to be representatives of a group, the Lobopodia, that includes arthropods, water bears and velvet worms.^[6]

In most instances the two systems are not at odds, however. The cladistic statement, that the clade Lobopodia contains (among others) the Arthropoda, Tardigrada and Onychophora, is factually identical to the Linnaean evolutionary statement that the group Lobopodia is ancestral to the phyla Arthropoda, Tardigrada and Onychophora. The difference is one of semantics rather than phylogeny.

See also

• Cladistics
• Phylogeny
• Paraphyly
• Polyphyly
• Phylogenetic nomenclature
• Binomial nomenclature
• Crown group

Notes

1. ↑ (from Ancient Greek κλάδος, klados, "branch")
2. ↑ A semantic case has been made that the name should be "holophyletic," but this term has not yet acquired widespread use. For more information, see Holophyletic
3. ↑ The term "reptile" is here to be understood as traditionally defined, e.g. Romer & Parson (1985): The Vertebrate Body. (6th ed.) Saunders, Philadelphia. There are other (cladistic) definitions of "reptile" that exclude the first amniotes and the synapsid line, see Sauropsida.

References

External links

• 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