Clade
<p>A clade is by definition <a href="http://en.wikipedia.org/wiki/Monophyletic" title="Monophyletic" class="mw-redirect">monophyletic</a>, meaning it contains one ancestor (which can be an organism, a <a href="http://en.wikipedia.org/wiki/Population" title="Population">population</a>, or a <a href="http://en.wikipedia.org/wiki/Species" title="Species">species</a>) and all its descendants.<sup id="cite_ref-3" class="reference"><a href="http://en.wikipedia.org/wiki/Clade#cite_note-3">[note 1]</a></sup><sup id="cite_ref-4" class="reference"><a href="http://en.wikipedia.org/wiki/Clade#cite_note-4">[3]</a></sup><sup id="cite_ref-5" class="reference"><a href="http://en.wikipedia.org/wiki/Clade#cite_note-5">[4]</a></sup> The ancestor can be known or unknown; any and all members of a clade can be extant or extinct.
The science that tries to reconstruct phylogenetic trees and thus discover clades is called <a href="http://en.wikipedia.org/wiki/Phylogenetics" title="Phylogenetics">phylogenetics</a> or <a href="http://en.wikipedia.org/wiki/Cladistics" title="Cladistics">cladistics</a>, the latter term being derived from "clade" by <a href="http://en.wikipedia.org/wiki/Ernst_Mayr" title="Ernst Mayr">Ernst Mayr</a> (1965). The results of phylogenetic/cladistic analyses are tree-shaped diagrams called <i><a href="http://en.wikipedia.org/wiki/Cladogram" title="Cladogram">cladograms</a></i>; they, and all their branches, are phylogenetic hypotheses.<sup id="cite_ref-6" class="reference"><a href="http://en.wikipedia.org/wiki/Clade#cite_note-6">[5]</a></sup>
Three methods of defining clades are featured in phylogenetic nomenclature: node-, stem-, and apomorphy-based:</p>
<ul>
<li>In node-based definition, clade name A refers to the least inclusive clade containing taxa (or specimens) X, Y, etc. A is the clade consisting of the last common ancestor of these taxa together with all the descendents of that ancestor. The ancestor is the branch point, or node.</li>
<li>In stem-based definition, A refers to the most inclusive clade containing X and Y, etc., but not U or V etc. A consists of all organisms more closely related to both X and Y than to either U or V.</li>
<li>In apomorphy-based definition, A refers to the clade containing X with an apomorphy α (a trait) together with the earliest ancestor from which X inherited α and all other descendants of that ancestor.</li>
</ul>
<p>The relationship between clades can be described in several ways:</p>
<ul>
<li>A clade located within a clade is said to be <i>nested</i> within that clade. In the diagram, the <a href="http://en.wikipedia.org/wiki/Ape" title="Ape">hominoid</a> clade, the apes and humans, is nested within the primate clade.</li>
<li>Two clades are <i><a href="http://en.wikipedia.org/wiki/Sister_group" title="Sister group">sisters</a></i> if they have an immediate common ancestor. In the diagram, lemurs and lorises are sister clades.</li>
<li>A clade <i>A</i> is <i><a href="http://en.wikipedia.org/wiki/Basal_%28phylogenetics%29" title="Basal (phylogenetics)">basal</a></i> to a clade <i>B</i> if <i>A</i> branches off the lineage leading to <i>B</i> before <i>B</i> 's first member. In the diagram to the right, the <a href="http://en.wikipedia.org/wiki/Strepsirrhini" title="Strepsirrhini">strepsirrhine</a> clade, including the lemurs and lorises, is basal to the <a href="http://en.wikipedia.org/wiki/Hominoidea" title="Hominoidea" class="mw-redirect">hominoids</a>, the apes and humans. Some authors have used "basal" differently to mean a clade that is "more primitive" or less species-rich than its sister clade; others consider this usage to be incorrect.</li>
</ul>
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