Phylogenetically conservative trait correlation: quantification and interpretation

Correlation across species between two quantitative traits, or between a trait and a habitat property, can suggest that a trait value is effective in sustaining populations in some contexts but not others. It is widely held that such correlations should be controlled for phylogeny, via phylogenetically independent contrasts PICs or phylogenetic generalised least squares PGLS. Two weaknesses of this idea are discussed. First, the phylogenetically conservative share of the correlation ought not to be excluded from consideration as potentially ecologically functional. Second, PGLS does not yield a complete or accurate breakdown of A-B covariation, because it corresponds to a generating model where B predicts variation in A but not the reverse. Multi-response mixed models using phylogenetic covariance matrices can quantify conservative trait correlation CTC, a share of covariation between traits A and B that is phylogenetically conservative. Because the evidence is from correlative data, it is not possible to split CTC into causation by phylogenetic history versus causation by continuing reciprocal selection between A and B. Moreover, it is quite likely biologically that the two influences have acted in concert, through phylogenetic niche conservatism. Synthesis: The CTC concept treats phylogenetic conservatism as a conjoint interpretation alongside ongoing influence of other traits. CTC can be quantified via multi-response phylogenetic mixed models.