Cycles.

Area of Science:

• Systematic Botany
• Phylogenetic Analysis
• Evolutionary Biology

Background:

• Three-taxon analysis (3ta) is a method used in phylogenetic reconstruction.
• Previous advocacy for 3ta by Nelson (1996) and Nelson & Platnick (1991) is based on presupposed conclusions.
• The role of plesiomorphies and character states in cladistic methods is debated.

Purpose of the Study:

• To evaluate the theoretical underpinnings of three-taxon analysis (3ta).
• To assess the compatibility of proposed 3ta methods with established cladistic principles.
• To examine the relationship between parsimony, evolutionary models, and cladogram interpretation.

Main Methods:

• Critical analysis of arguments supporting three-taxon analysis.
• Evaluation of the implications of treating all character states as terminal.
• Assessment of the treatment of plesiomorphies in cladistic methods.
• Examination of parsimony's correspondence with incompletely resolved cladograms.

Main Results:

• The proposal to treat all character states as terminal undermines three-taxon analysis.
• The exclusion of plesiomorphies from cladistic methods is shown to be problematic.
• Parsimony analysis does not align with certain interpretations of incompletely resolved cladograms.
• The theoretical basis for 3ta is found to be incoherent, sometimes favoring reversal over parallelism or disregarding reversed characters.

Conclusions:

• The arguments for three-taxon analysis are based on circular reasoning.
• Proposed modifications to cladistic methods for 3ta lack sound theoretical justification.
• Parsimony remains a viable method, with no superior alternative evolutionary model presented by proponents of 3ta.