Evolution 57 4 2003 pp 717 745 TESTING FOR PHYLOGENETIC SIGNAL IN COMPARATIVE DATA BEHAVIORAL TRAITS ARE MORE LABILE SIMON P BLOMBERG 1 THEODORE GARLAND JR 1 2 AND ANTHONY R IVES3 4 1 Department of Biology University of California Riverside California 92521 2 E mail tgarland citrus ucr edu 3 Department of Zoology University of Wisconsin Madison Wisconsin 53706 4 E mail arives facstaff wisc edu Abstract The primary rationale for the use of phylogenetically based statistical methods is that phylogenetic signal the tendency for related species to resemble each other is ubiquitous Whether this assertion is true for a given trait in a given lineage is an empirical question but general tools for detecting and quantifying phylogenetic signal are inadequately developed We present new methods for continuous valued characters that can be implemented with either phylogenetically independent contrasts or generalized least squares models First a simple randomization procedure allows one to test the null hypothesis of no pattern of similarity among relatives The test demonstrates correct Type I error rate at a nominal a 5 0 05 and good power 0 8 for simulated datasets with 20 or more species Second we derive a descriptive statistic K which allows valid comparisons of the amount of phylogenetic signal across traits and trees Third we provide two biologically motivated branch length transformations one based on the OrnsteinUhlenbeck OU model of stabilizing selection the other based on a new model in which character evolution can accelerate or decelerate ACDC in rate e g as may occur during or after an adaptive radiation Maximum likelihood estimation of the OU d and ACDC g parameters can serve as tests for phylogenetic signal because an estimate of d or g near zero implies that a phylogeny with little hierarchical structure a star offers a good fit to the data Transformations that improve the fit of a tree to comparative data will increase power to detect phylogenetic signal and may also be preferable for further comparative analyses such as of correlated character evolution Application of the methods to data from the literature revealed that for trees with 20 or more species 92 of traits exhibited significant phylogenetic signal randomization test including behavioral and ecological ones that are thought to be relatively evolutionarily malleable e g highly adaptive and or subject to relatively strong environmental nongenetic effects or high levels of measurement error Irrespective of sample size most traits but not body size on average showed less signal than expected given the topology branch lengths and a Brownian motion model of evolution i e K was less than one which may be attributed to adaptation and or measurement error in the broad sense including errors in estimates of phenotypes branch lengths and topology Analysis of variance of log K for all 121 traits from 35 trees indicated that behavioral traits exhibit lower signal than body size morphological life history or physiological traits In addition physiological traits corrected for body size showed less signal than did body size itself For trees with 20 or more species the estimated OU 25 of traits and or ACDC 40 transformation parameter differed significantly from both zero and unity indicating that a hierarchical tree with less or occasionally more structure than the original better fit the data and so could be preferred for comparative analyses Key words Adaptation behavior body size branch lengths comparative method constraint physiology Received March 29 2002 Accepted November 27 2002 A great triumph of biology has been the demonstration that organisms are descended from common ancestors and hence are related in a hierarchical fashion Mayr 1982 An observation of almost equal importance is that phylogenetically related organisms tend to resemble each other for most aspects of the phenotype e g hummingbirds look like hummingbirds elephants look like elephants We term this resemblance phylogenetic signal to avoid such terms as phylogenetic inertia or constraint which are inconsistently defined both conceptually and operationally e g see Antonovics and van Tienderen 1991 McKitrick 1993 Wagner and Schwenk 2000 Orzack and Sober 2001 Reeve and Sherman 2001 and whose original use was rather different from current use Blomberg and Garland 2002 As argued elsewhere Blomberg and Garland 2002 we believe that the evolutionary processes implied by phylogenetic inertia and constraint are difficult if not impossible to estimate from comparative data alone but see Hansen 1997 Orzack and Sober 2001 Schwenk and Wagner 2001 Therefore we use phylogenetic signal simply to describe a tendency pattern for evolutionarily related organisms to resemble each other with no implication as to the mechanism that might cause such resemblance process This use of phylogenetic signal is consistent with recent usage in systematic biology e g Hillis and Huelsenbeck 1992 and is also similar to the phylogenetic effect of Derrickson and Ricklefs 1988 and the phylogenetic conservatism of Ashton 2001 We do not use phylogenetic conservatism however because it seems to connote less change than might be expected from the phylogenetic structure of the taxa in question as argued below the existence of phylogenetic signal does not require the existence of such processes as that may imply We did not use the phylogenetic correlation of Gittleman et al 1996a because they used statistical approaches different from those presented here and because our methods do not involve correlation per se A crucial point that has not been sufficiently appreciated in the literature is that phylogenetic signal will occur to some extent under most simple stochastic models of character evolution along a tree with any amount of hierarchical structure Under such models as Brownian motion evolutionary changes are simply added to values present in the previous generation or at the previous node on a phylogenetic tree Thus members of lineages that have only recently diverged will necessarily on average tend to be similar as compared with more distantly related lineages This effect carries on to the 717 q 2003 The Society for the Study of Evolution All rights reserved 718 SIMON P BLOMBERG ET AL values for taxa at the tips of a phylogenetic tree e g extant species The presence of phylogenetic signal does not require a failure to evolve Wake 1991 along certain branches but only that the branch lengths of a
View Full Document
Unlocking...