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The American Naturalist


Interspecific comparisons of phenotypes are used extensively to test hypotheses about the evolutionary forces shaping phenotypic variation, but comparative data analysis is complicated by correlations due to the common ancestry of species. The method of evolutionary contrasts removes such correlations by estimating the amount of character change between pairs of closely related species that has occurred since their most recent common ancestors. The original method allows character change to be estimated only along pairs of branches on a phylogeny, but many hypotheses address change along single branches. In this article the method of evolutionary contrasts is extended to allow character change along a set of single branches on a phylogeny to be estimated, expected variances are presented, and it is shown that these extensions also result in a set of contrasts that are not correlated because of common ancestry. These extensions will allow hypotheses to be tested concerning character change associated with host or habitat shifts, changes in breeding system (e.g., monogamy vs. polygyny, monoecy vs. dioecy), changes in life history (e.g., semelparity vs. iteroparity), and changes in quantitative characters in many other situations in which one is interested in character change along single branches.