Hi all, I've been investigating the use of Pagel's lambda and run into a problem comparing trees that have a low speciation rate and no extinction with trees that have a high speciation rate and lots of extinction. I'm comparing phylogenetic signal in two characters. In the first, I simulate a character under Brownian evolution over a 200 taxon tree and then prune it down to 100 taxa. The second character was evolved over the pruned-down tree. I then estimated Pagel's lambda for both characters on the pruned-down tree. Doing this 500 times, I found that the error rate in the estimate of lambda was not the same for the two methods of generating the data. The difference was small but statistically significant. To my understanding though, there shouldn't be a difference. The rate of evolution is the same in both cases and the tree topology and branch lengths are identical. If the characters are evolving in a Brownian way, shouldn't there be, on average, no difference between the characters with respect to phylogenetic signal? Could this be an artifact of using fastBM to simulate the characters? Is there another function that I should be using to simulate a character evolving under Brownian motion?

Here's the script I'm using: library(phytools) library(TreeSim) library(adephylo) age<-2 lambda <- 2.0 mu <- 0.5 frac <-0.6 sample_size<-100 smalltrees<-list() Bigtrees<-sim.bd.taxa.age(200,500,lambda,mu,frac,age) ## using sim.bd.taxa.age to get an ultrametric tree with known age subtrees<-list() sig<-vector() sig_sub<-vector() for (i in 1:500){ var<-fastBM(Bigtrees[[i]],nsim=1) tips_to_drop<-sample(1:200, 200-sample_size, replace=FALSE) subtrees[[i]]<-drop.tip(Bigtrees[[i]], tips_to_drop) var1<-fastBM(subtrees[[i]],nsim=1) sig_sub[i]<-phylosig(subtrees[[i]],var,method="lambda")$lambda sig[i]<-phylosig(subtrees[[i]],var1,method="lambda")$lambda} lamb<-(2-sig) ## reflected the data around lambda=1 so that standard deviation has a symmetric distribution lamb<-c(lamb,sig) lamb_sub<-(2-sig_sub) lamb_sub<-c(lamb_sub,sig_sub) var.test(lamb,lamb_sub) Best regards, - Will Gelnaw [[alternative HTML version deleted]] _______________________________________________ R-sig-phylo mailing list - R-sig-phylo@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-phylo Searchable archive at http://www.mail-archive.com/r-sig-phylo@r-project.org/