cladistics

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cladistics(cladism, phylogenetic systematics) A special taxonomic system, founded in 1966 by W. Hennig, that is applied to the study of evolutionary relationships. It proposes that common origin can be demonstrated by the shared possession of derived characters, characters in any group being either primitive or derived. In the cladograms used to portray these relationships, cladogenesis always creates two equal sister groups: the branching is dichotomous. Thus each pair of sister groups constitutes a monophyletic group with a common stem taxon, unique to the group, and a parent taxon always gives rise to two daughter taxa which must be given different names from each other and from the parent, so the parent species ceases to exist. Monophyletic groups are deduced by identifying synapomorphic character states.

A Dictionary of Ecology