| Abstract | Phylogenetic systematics is a discipline with three major objectives: (1) reconstructing the phylogenetic (evolutionary) relationships among organisms, (2) studying character evolution in reference to such reconstructions, and (3) producing classifications that exactly reflect such relationships. Success in meeting objectives (1) and (2) depends upon the ability of the investigator to circumscribe monophyletic (natural) taxa. In turn, the ability to circumscribe monophyletic taxa depends on the ability of the investigator to determine which of two (or more) homologues is apomorphic (derived) because only apomorphic homologues justify monophyletic taxa. This determination may be made using outgroup comparisons and/or ontogeny. The criteria "common equals primitive" and "ancient equals primitive" are not recommended for use. There are several ways of meeting objective (3), including ranking, listing, indenting, or the use of numerical prefixes. Whatever system is used, phylogenetic classifications are superior to both phenetic and evolutionary taxonomic classifications for recovering information about genealogical descent and character evolution. Vicariance biogeography is a method of searching for congruent patterns of distribution among taxa and for comparing these distributions with earth history. The basic method, as currently practiced, is discussed. A recent analysis of Middle American fishes is used as a practical illustration.
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