Naming clades by explicit ancestry-based definitions. A definition fixes the rule connecting a name to ancestry; an inferred phylogenetic tree determines which organisms satisfy that rule.
A named organism, species or other specified reference used to anchor a clade definition. Internal specifiers are included and external specifiers excluded.
A definition selecting the largest clade containing specified internal reference taxa but excluding specified external reference taxa. With one of each, it includes organisms more closely related to the internal than to the external specifier.
A definition selecting the smallest clade containing specified internal reference taxa: their last common ancestor and every descendant. The specifiers need not be living.
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