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On the Three‐Taxon Approach to Parsimony Analysis
Author(s) -
Laet Jan,
Smets Erik
Publication year - 1998
Publication title -
cladistics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.323
H-Index - 92
eISSN - 1096-0031
pISSN - 0748-3007
DOI - 10.1111/j.1096-0031.1998.tb00344.x
Subject(s) - taxon , systematics , a priori and a posteriori , character (mathematics) , statement (logic) , tree (set theory) , biology , mathematics , taxonomy (biology) , zoology , epistemology , paleontology , combinatorics , geometry , philosophy
The following three basic defects for which three‐taxon analysis has been rejected as a method for biological systematics are reviewed: (1) character evolution is a priori assumed to be irreversible; (2) basic statements that are not logically independent are treated as if they are; (3) three‐taxon statements that are considered as independent support for a given tree may be mutually exclusive on that tree. It is argued that these criticisms only relate to the particular way the three‐taxon approach was originally implemented. Four‐taxon analysis, an alternative implementation that circumvents these problems, is derived. Four‐taxon analysis is identical to standard parsimony analysis except for an unnatural restriction on the maximum amount of homoplasy that may be concentrated in a single character state. This restriction follows directly from the basic tenet of the three‐taxon approach, that character state distributions should be decomposed into basic statements that are, in themselves, still informative with respect to relationships. A reconsideration of what constitutes an elementary relevant statement in systematics leads to a reformulation of standard parsimony as two‐taxon analysis and to a rejection of four‐taxon analysis as a method for biological systematics.