Parsimony in Systematics: Philosophical Issues

Systematists observe patterns of sameness and difference in the characteristics of taxa. On this basis, they attempt to reach conclusions about phylogenetic relationships. Parsimony and other principles of inference have been proposed to bridge the gap. Methods have proliferated, as have justifications and criticisms of them; the question of which method is most reasonable has become increasingly controversial. Methods differ in their specifications of what information must be available if phylogenetic inference is to be undertaken and in their view of the kind of hypothesis that can be constructed using phylogenetic inference. Parsimony stipulates that the investigator must be able to distinguish between the ancestral (plesiomorphic) and the derived (apomorphic) form of every characteristic used. t Given this information, the preferred geneological hypothesis is the one that requires the fewest homoplasies. A homoplasy occurs when two taxa independently evolve the same state of some character; the concept subsumes both parallelism and convergence. Parsimony may also be described as holding that synapomorphies--matches with respect to derived characteristics---count as evidence of a phylogenetic relationship, but that simplesiomorphies~matches with respect to ancestral characteristics---do not. Synapomorphy and simplesiomorphy do not imply homology; matchings may be due to inheritance from a common ancestor or to independent evolution.