PHYLOGENIES, SPATIAL AUTOREGRESSION, AND THE COMPARATIVE METHOD - A COMPUTER-SIMULATION TEST

Brownian motion computer simulation was used to test the statistical p roperties of a spatial autoregressive method in estimating evolutionar y correlations between two traits using interspecific comparative data . When applied with a phylogeny of 42 species, the method exhibited re asonable Type I and II error rates. Estimation abilities were comparab le to those of independent contrasts and minimum evolution (parsimony) methods, and generally superior to a traditional nonphylogenetic appr oach (not taking phylogenies into account at all). However, the autore gressive method performed extremely poorly with a smaller phylogeny (1 5 species) and with nearly independent (''star'') phylogenies. In both of these situations, any phylogenetic autocorrelation present in the data was not detected by the method. Results show how diagnostic techn iques (e.g., Moran's I) can be useful in detecting and avoiding such s ituations, but that such techniques should not be used as definitive e vidence that phylogenetic correlation is not present in a set of compa rative data. The correction factor (a) proposed by Gittleman and Kot ( 1990) for use in weighting phylogenetic information had little effect in most analyses of 15 or 42 species with incorrect phylogenetic infor mation, and may require much larger sample sizes before significant im provement is shown. With the sample sizes tested in this study, howeve r, the autoregressive method implemented with this correction factor a nd correct phylogenetic information led to downwardly biased estimates of the absolute magnitude of the evolutionary correlation between two traits. Cautions and recommendations for implemention of the spatial autoregressive method are given; computer programs to conduct the anal yses are available on request.