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A clade (from ancient Greek , klados, "branch") is a term used in modern biological systematics, the scientific classification of living and fossil organisms, to describe a monophyletic group, defined as a group consisting of a single common ancestor and all its descendants.

The study of clades is called cladistics.

The common ancestor of any group of reasonable size and most of that ancestor's descendants will usually be long extinct. It is not necessary for a clade to contain any living representatives.

The context

Ever since Darwin showed that all organisms share common ancestry, taxonomy has consistently attempted to represent and reflect the evolutionary history of organisms. The DNA and RNA analysis used in modern molecular biology has greatly helped in illuminating this history by providing large amounts of new phylogenetic information which was previously unavailable to taxonomists. These techniques of study are known as molecular phylogenetics, and they have given rise to the modern disciplines of cladistics and phylogenetic systematics.

This new information and new insights have made very clear what the limitations of the old Linnaean system of taxonomy were and are. As a result, many taxonomists are gradually revising in radical ways the taxonomy of the groups that they study. For an example of a taxonomy (in this case a taxonomy of the gastropods) which has been partially revised in order to incorporate insights from molecular work, see Taxonomy of the Gastropoda .

Updating taxonomy

The term "clade" did not exist in the older Linnaean taxonomy, which was by necessity based only on morphological similarities between organisms. The concept embodied by the word "clade" does not fit well into the rigid hierarchy that the Linnaean system of taxonomy uses; indeed, cladistics and Linnaean taxonomy are not really compatible.

Linnaean taxonomy demands that all organisms be placed neatly into a rigid, ranked, hierarchy of taxa, such that one individual kind of organism must belong in one of each of the categories: species, genus, family, order, class, phylum and kingdom. Because of this necessity to "file things away neatly", the Linnaean system is often very convenient indeed in organizing such things as large museum reference collections; however it does not represent well the process of change that actually happens over evolutionary time.

Because clades can be nested at any level, they do not have to be neatly slotted into a rank in an overall hierarchy. In contrast, the Linnaean taxa of "order," "class" etc. must all be used when naming a new taxon. They cannot be avoided, and each one implies a certain (admittedly very poorly defined) level of diversity which is supposed to be equivalent throughout the system.

Species arise by gradual modification, not sudden complete changes or jumps (although, see also "punctuated equilibrium"), thus there is no sound biological basis to make a distinction between a species and its "descendant" species. This is another important area where cladistics is more valuable to biologists than Linnaean taxonomy; intermediate taxa can be named according to their relationship to named taxa using the stem group terminology.

For example, the famous fossil organism Archaeopteryx has a lot of bird-like characteristics, but is not a true bird in the modern sense. It is, in effect, a 'great-aunt' of the group that contains all modern birds and their shared ancestors. The Linnaean system would define the taxon 'Aves' to include all modern birds, and base this taxon on a number of shared characteristics. Since modern birds are not descended from Archaeopteryx, and Archaeopteryx has characters which no living birds possess, it could not be included within the existing taxon 'Aves'. The scientists who described the organism had to make a decision: did they erect a new taxon, which would carry no inherent relationship to Aves; or did they modify the characteristics of the existing group to include the unusual fossil? In the case of Archaeopteryx, the describers opted for the latter option. However, this approach cannot be followed ad infinitum. Each time a fossil is found to be basal to the birds, this approach would require relaxing still further the criteria for inclusion in 'Aves'. Ultimately, as more transitional forms are found, the definition of 'Aves' would become so broad as to include all dinosaurs; as basal dinosaurs are described, the definition of 'Aves' would have to be extended still further to incorporate all reptiles... and so on - the logical conclusion, in the presence of a complete fossil record, would be that the definition of 'Aves' would be relaxed so far that it would include all life. In cladistic terms, however, Archaeopteryx can be considered a stem group to the bird clade - it branched off from the bird lineage before the first member of that lineage resembled a true bird.

Defining clade names

Since taxonomy intends to reflect evolutionary relationships, in order to be valid in evolutionary terms a taxon must be monophyletic—that is, it must be a clade. The definition of clades differ somewhat between Linnaean and cladistic nomenclature. Linnaean units are defined by a small number of key traits. Three methods of defining clades have been proposed in cladistics: node-, stem-, and apomorphy-based:

  • In node-based naming, taxon name A refers 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. This definition is basically similar to the Linnaean system.


Differences between a Linnaean or an apomorphy-based clade and a node-based one become obvious when the phylogenetic hypothesis changes.

Here is an example comparing the traditional Linnaean approach to a node-based naming definition:
  • Suppose that all we want to do is to name a clade ("A"), containing X and Y. In the Linnaean system we would assign all taxa to the relevant categories species, genus, and family, and then designate type species. No explicit reference to the actual phylogeny is made when these categories are used. The clade A would then be accompanied by a short definition of the "defining trait" (apomorphy). The node-based alternative is started with 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.


See also



Notes



References

  1. 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 simply be referred to as "holophyletic," but this term has not yet acquired widespread use. For more information, see Holophyletic
  2. This may not be true in very small and recent clades, where the "last common ancestor" and all of the ancestor's descendants are still living.


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