Clade

The grouping of reptiles and birds (Aves) is generally believed to be monophyletic. Because 'reptiles' does not include birds, reptiles alone do not form a clade.

A clade^{[note 1]} is a monophyletic group - that is, a single common ancestor and all its descendants.^{[note 2][1]} The common ancestor of any reasonably sized group, and many of its descendants, will be not be living.^{[note 3]} It is not necessary for a clade to contain living representatives.

Cladistics and Linnean taxonomy have an uneasy relationship. Because Linnean taxonomy requires that nature is split into nameable pigeon-holes (species, genera, and higher taxa), it breaks down over evolutionary time, because species arise by gradual modification, not step changes. There is no biological basis to make a distinction between one species and a 'descendant' species. This is where cladistics trumps Linnean taxonomy; intermediate taxa can be named according to their relationship to named taxa using the stem group terminology^[2]; the disadvantage, however, is that sibling or parent-descendant pairs belong to different clades despite being, for all practical purposes, identical organisms.^{[citation needed]}

Naming clades

Valid taxa are monophyletic groups - that is, they are clades. However, traditional Linnaean taxonomy does not cope well in a cladistic framework: for instance, it cannot adequately classify stem groups.

Three methods of naming clades have been proposed: node-, stem-, and apomorphy-based. In node-based naming, taxon name A might refer to the least inclusive clade containing X and Y. In stem-based naming, A would refer to the most inclusive clade containing X and Y but not Z. In apomorphy (derived feature)-based naming, A would refer to the clade identified by a feature synapomorphic (sharing a derivation) with a feature in specimen (taxon) X. Differences between a traditional approach and these phylogenetic alternatives become obvious when the phylogenetic hypothesis changes.

Comparison between the traditional Linnaean approach to nomenclature and a phylogenetic alternative (node-based naming). Suppose that all we want to do is to give a name ("A") to a clade containing X and Y. In the Linnaean system this means that we have to introduce names for sister taxa, assigning all taxa to the categories species, genus, and family, and designate type species. No explicit reference to phylogeny is made. The phylogenetic alternative provides an explicit reference to evolutionary history, and nothing but the clade containing X and Y needs to be named. When the hypothesis of relationship changes, the phylogenetic alternative is cleaner and more explicit about what it refers to.

Nested clade analysis

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A clade that is located within another more inclusive clade is said to be "nested" within that clade. Nested clade analysis is beneficial in many ways. For instance, it enables the detection of range expansions in isolated geographic areas.

See also

• Phylogeny
• Paraphyly
• Polyphyly
• Phylogenetic nomenclature
• Binomial nomenclature

Notes

1. ^ "Clade" is derived from the ancient Greek κλάδος, klados, "branch."
2. ^ The term 'monophyletic group' is used in this article in the conventional sense of 'an ancestor and all its descendants'. A case has been made that semantically, such groups should be referred to as 'holophyletic', but this term has not yet acquired widespread use. For more information, see Holophyletic
3. ^ This may not be true in very small clades, where the 'last common ancestor' and all its descendants are all alive.

References

External links

• Evolving Thoughts: Clade
• A poster-sized cladogram showing the Tree of Life (Evolution)
• DM Hillis, D Zwickl & R Gutell: ~3000 species Tree of Life A cladogram?
• Phylogenetic systematics, an introductory slide-show on evolutionary trees University of California, Berkeley