HOMOPLASY SLOPE RATIO - A BETTER MEASUREMENT OF OBSERVED HOMOPLASY IN CLADISTIC ANALYSES

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.