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 simil ar to those introduced previously in which between-species divergence is estimated as a function of time since divergence but also takes int o account the possible statistical nonindependence of trait values mea sured 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 th e standard Brownian motion model of evolutionary change and to allow f or tests of neutral evolution versus evolution under stabilizing selec tion. Applications of the method to test specific hypotheses of phenot ypic evolution (including the adequacy of Brownian motion to describe real data), to compare rates of change of different types of character s or different groups of organisms, and to estimate branch lengths in units of expected variance of change as required by most comparative-m ethod data analysis techniques are discussed.