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As the number of molecular studies con? tinues to grow, so does the problem of how to analyze multiple data sets. The impor? tance of this problem is indicated by the flurry of recent papers (reviewed by de Queiroz et al., 1995) addressing philosoph? ical and practical considerations facing systematists fortunate enough to have sev? eral pertinent data sets on hand. Perhaps the most rigorous practical treatment was that of Bull et al.(1993), who argued against combining data when there is de? monstrable heterogeneity among the dif? ferent data sets in the processes governing character evolution. These authors showed that combining simulated data sets gener? ated from the same topology but with dif? ferent rates of evolution leads to less ac? curate estimation of phylogeny than does analysis of the more slowly evolving data set alone. Although the validity of these re? sults is not in question for the models test …