Dear Charlotte, No, you do not want to do this. As shown in Blomberg et al. (2003), the tests for statistical significance do not work well when you have fewer than about 20 species. The estimate of the K statistic (indicating the amoung of phylogentic signal) is fine for small sample sizes, but the test of its statistical significance is not. You can compare your value of K against the many other values reported in our paper to see if it is "typical" for that type of trait.
Note that K can range from close to zero to somewhat grater than one for real data. See the values in Blomberg et al. (2003). Typical values are in the range of about 0.4 to 0.8, and those values are tyupically significantly greater than zero when you have decent sample size (greater than 20 or so). A value a bit below one does NOT mean no phylogenetic signal. In general, it is better to use statistical models that simultaneously estiumate the strength of phylogenetic signal and the other parameters (e.g., partial regression slopes). However, unfortunately, this also does not work well with small samples sizes. So, I suggest you do both conventional statistical analyses and phylogenetically independent contrasts (which is equivalent to PGLS models without any sort of branch length transform). Hopefully, results will be similar. If not, then you probably cannot conclude much from your data. Sincerely, Ted Theodore Garland, Jr. Professor Department of Biology University of California, Riverside Riverside, CA 92521 Office Phone: (951) 827-3524 Wet Lab Phone: (951) 827-5724 Dry Lab Phone: (951) 827-4026 Home Phone: (951) 328-0820 Facsimile: (951) 827-4286 = Dept. office (not confidential) Email: tgarl...@ucr.edu http://www.biology.ucr.edu/people/faculty/Garland.html Experimental Evolution: Concepts, Methods, and Applications of Selection Experiments Edited by Theodore Garland, Jr. and Michael R. Rose http://www.ucpress.edu/book.php?isbn=9780520261808 (PDFs of chapters are available from me or from the individual authors) ________________________________________ From: r-sig-phylo-boun...@r-project.org [r-sig-phylo-boun...@r-project.org] on behalf of charlotte De Busschere [charlotte.debussch...@ugent.be] Sent: Thursday, June 30, 2011 8:45 AM To: r-sig-phylo@r-project.org Subject: [R-sig-phylo] small sample size and detecting phylogenetic signal with Blombergs K statistic Dear all, I'm currently describing phenotypic divergence patterns of several traits within a radiated genus (5 species). Several comparative studies suggest to take into account the phylogenetic relationships among the species by means of PIC or PLGS etc. for which I agree But before applying these methods I guess I should first detect whether there is Phylogenetic signal (PS)in the data and if not then I suppose I can use classic statistical methods (correct me if my reasoning is wrong) Hence I calculated Blombergs K statistics with Picante and almost all traits have K values<1 i.e. no PS (but no p values < 0.05) hence my question concerns about the accuracy of this statistic in a small data set (N=5) Blomberg et. al. 2003 (Evolution) mentioned that it is practically impossible to get a statistical significant result if N<7 Hence, can I put confidence in this statistic? Sincerely greetings, Charlotte Drs. Charlotte De Busschere Ghent University Biology Department Terrestrial Ecology Unit Ledeganckstraat 35 B-9000 Gent Belgium tel:++32 (0)92645039 http://www.ecology.ugent.be/terec/personal.php?pers=cd _______________________________________________ R-sig-phylo mailing list R-sig-phylo@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-phylo _______________________________________________ R-sig-phylo mailing list R-sig-phylo@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-phylo
