A clade is a taxonomic group comprising a single common ancestor and all the descendants of that ancestor.^[1] Any such group is considered to be a monophyletic group, and can be represented by both a phylogenetic analysis, as in a tree diagram, and by a cladogram (see cladistics), or simply as a taxonomic reference. "Clade" is derived from the ancient Greek , klados, "branch."

The grouping of reptiles and birds is generally believed to be monophyletic.

In cladistics, 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.

Relation to paraphyletic and polyphyletic groups

Paraphyletic Group - if Reptilia is defined to not include Aves.

Polyphyletic Group - if warm blooded animals are defined as a group.

Phylogenetic alternatives

To avoid the pitfalls of traditional Linnaean taxonomy in phylogenetic nomenclature, three new methods of phylogenetic naming 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.

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

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