Taxon-dependent diversification of life-history traits and the perception of phylogenetic constraints

Phylogenetic constraints on evolution have been invoked to explain variation of traits among taxa. We argue here that such variation merely records the results of evolutionary diversification and sheds no light on differences among taxa in the evolutionary responsiveness of phenotypes to their environments. We propose, alternatively, that differences in the distribution of variation within taxa indicate taxon-dependent differences in evolution. We use a simple model of evolutionary divergence to illustrate the diversification of trait values within and among taxa through time. Phylogenetic constraints, or differences in the evolutionary interaction between phenotype and environment, are recognized with this model by the orientation and shape of bivariate or multivariate distributions of traits, in which each character serves as an internal standard for evolutionary change in all other traits. To illustrate this concept, we use jack-knifed principal component analyses to demonstrate similarities and differences in the diversification of mammalian life histories.