ESTIMATING THE RATE OF PHENOTYPIC EVOLUTION FROM COMPARATIVE DATA

This study presents a method to estimate rates of evolutionary change in continuous characters from comparative data. The technique is similar to those introduced previously in which between-species divergence is estimated as a function of time since divergence but also takes into account the possible statistical nonindependence of trait values measured from phylogenetically related species in an approach similar to the independent contrasts methods used in interspecific data analysis. The use of Ornstein-Uhlenbeck processes is also proposed to extend the standard Brownian motion model of evolutionary change and to allow for tests of neutral evolution versus evolution under stabilizing selection. Applications of the method to test specific hypotheses of phenotypic evolution (including the adequacy of Brownian motion to describe real data), to compare rates of change of different types of characters or different groups of organisms, and to estimate branch lengths in units of expected variance of change as required by most comparative-method data analysis techniques are discussed.