Gernot Huber asked --
1. Why do you have to force a linear regression through the origin
when using phylogenetically independent contrasts (PICs)?
Because their expectations are zero. A contrast could be either
X1 - X2 or X2 - X1, for one thing. The expectations of X1 and X2
are identical
This is also covered in the following paper (and others):
Garland, T., Jr., P. H. Harvey, and A. R. Ives. 1992. Procedures for the
analysis of comparative data using phylogenetically independent contrasts.
Systematic Biology 41:18-32.
Available here:
