Abstract
A method for quantifying levels of observed homoplasy in cladistic analyses is proposed. The homoplasy slope is a function that describes the relationship between number of taxa in a cladistic analysis and the observed number of extra steps per character. The homoplasy slope ratio (HSR) relates the homoplasy slope of a cladistic analysis (real data) to that of an analysis based on random binary data sets of the same size. Tests show that HSR is not correlated with numbers of characters and taxa, and this quality makes the index particularly useful for comparing the levels of observed homoplasy among cladistic analyses based on data sets of different sizes. All of the other homoplasy indices currently available-including the consistency index (CI), retention index (RI), and homoplasy excess ratio (HER)-are correlated with number of characters and/or number of taxa. (Homoplasy indices; homoplasy comparison; homoplasy slope ratio; consistency index; homoplasy excess ratio; retention index.) The discussion of homoplastic evolution commonly has focused on how it compli- cates the reconstruction of phylogenies. Systematists distinguish three kinds of nonhomologous similarity or homoplasy (Futuyma, 1986): parallelism, convergence, and reversal. These homoplasies are not a major problem for phylogenetic recon- struction if they are identified prior to the analysis and recoded or excluded from the data set. Additional homoplasies may be discovered during the cladistic analysis as incongruent character states that lengthen the cladogram beyond the sum of the range of the characters (Lmin) by any number of extra steps (EXS). The number of extra steps is thus a measure of character-state distri- butions that do not fit the hypothesized phylogeny. The literature about homoplastic evo-
Published Version
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