PATTERNS IN PHYLOGENETIC TREE BALANCE WITH VARIABLE AND EVOLVING SPECIATION RATES.

Aspects of phylogenetic tree shape, and in particular tree balance, provide clues to the workings of the macroevolutionary process. I use a simulation approach to explore patterns in tree balance for several models of the evolutionary process under which speciation rates vary through the history of diversifying clades. I demonstrate that when speciation rates depend on an evolving trait of individuals, and are therefore "heritable" along evolutionary lineages, the resulting phylogenies become imbalanced. However, imbalance also results from some (but not all) models of "nonheritable" speciation rate variation. The degree of imbalance increases with the magnitude of speciation rate variation, and then for gradual evolution (but not punctuated equilibria) reaches an asymptote short of the theoretical maximum. Very high levels of rate variation are required to produce imbalance matching that found in real data (estimated phylogenies from the systematic literature). I discuss implications of the simulation results for our understanding of macroevolution.