[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
----
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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