R help - Jun 2012 - Obtaining r-squared values from phylogenetic autoregression in ape

Hello,

I am trying to carry out a phylogenetic autoregression to test whether my
data show a phylogenetic signal, but I keep calculating bizzare R-squared
values.

My script is:
> library(ape)
> x <-
"((((((((R:1,GK:1)d:1,(MW:1,G:1)n:1)a:1,SPW:1)a:1,WB:1)b:1,(((SPBF:1,PBF:1)n:1,(HBF:1,SWF:1)c:1)i:1,(PE:1,((P:1,C:1)w:1,(MF:1,HF:1)l:1)b:1)c:1)x:1)a:1,((((((CHB:1,AD:1)a:1,BA:1,SSB:1)i:1,(HB:1,SB:1)b:1)x:1,SC:1)x:1,((BRH:1,PH:1)e:1,(WLH:1,GH:1)b:1)d:1)x:1,DBF:1)a:1)a:1,((WW:1,(OT:1,SW:1)v:1)b:1,B:1)z:1)a:1,((DS:1,GS:1)j:1,(SSS:1,LS:1,(ES:1,SS:1)a:1)b:1)e:1)a;"
> treeX <- read.tree(text=x)
> treeX <- compute.brlen(treeX, method = "Grafen")
> data <-  c(4.854185,  6.008532,  6.221286,  4.369945, 10.044475,
5.801292,  5.128374,  5.540995,  4.566704, 10.188250,  7.121077,
4.469329,  4.815972,  7.798617,  5.892205,  4.853027,  5.080509,
7.982360,  7.518022,  5.675702, 11.989929,  6.760587,  7.433313,
7.906303,  7.235088,  7.131338,  5.582816, 6.769775, 11.886225,  5.589256,
5.626147,  4.714369,  6.040151,  9.098583,  5.194043,  8.830687,
6.231105)
> species <- c( "AD",   "B",    "BA",  
"BRH" , "C",    "CHB",  "DBF",
"DS",   "ES",  "G",    "GH",  
"GK",   "GS",  "HB",   "HBF", 
"HF",
"LS",   "MF",  "MW",   "OT",  
"P",    "PBF",  "PE",   "PH", 
"R",
"SB","SC",   "SPBF", "SPW", 
"SS",   "SSB", "SSS",  "SW",  
"SWF",  "WB",
"WLH",  "WW")
> names(data) <- species
> cor.mat <- vcv.phylo(treeX, cor=TRUE)
> regr <- compar.cheverud(data, cor.mat)
> regr$rhohat
[1] 5.541462
> 1 - var(regr$residuals)/var(data)
          [,1]
[1,] -1.333095


I don't understand why the autoregression coefficient falls outside the
interval -1 to 1, or why the calculation for obtaining an R-squared
produces a value that doesn't make sense.

Have I made a mistake in the application of this method, or have I
misunderstood when it is appropriate to use phylogenetic autoregression?

Thanks for your help.

Louise

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