[R-sig-phylo] How to detect phylogenetic signal (lambda) in one unscaled trait?

Joe Felsenstein joe at gs.washington.edu
Tue Mar 22 23:56:24 CET 2011

Alberto Gallano wrote:

> I think I was not clear with what I said about the log transformation, and I
> see now what you mean about using log-log when using regression. Though It
> does seem to me that logging two variables in a ratio context:
> log(Y) / log(X)
> or
> log(Y / X)
> would influence phylogenetic signal levels greatly, since, while the order
> of trait means is maintained, the magnitude of differences between trait
> means is altered (means with higher values are dragged toward the centre of
> the distribution). I assume, then, that this is nothing to worry about.

The above two are *not* equivalent.  If a trait is doing a Brownian
Motion on the log scale, the latter,   log(Y/X) is OK, as it is equivalent
to to  log(Y) - log(X), a straightforward linear combination.   

But   log(Y)/log(X)  could get you into real trouble, particularly
if   X  got near 1, in which case the ratio could blow up.

Joe Felsenstein         joe at gs.washington.edu
 Department of Genome Sciences and Department of Biology,
 University of Washington, Box 355065, Seattle, WA 98195-5065 USA

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