Bridging the Micro-Macroevolutionary Divide with Performance Surface Analysis: Insights from Ruminants, Turtles, and Snakes

A central goal in evolutionary biology is reconciling how macroevolutionary patterns, assembled over geological timescales, are governed by microevolutionary processes that operate on populations over generational timescales. The phenotypic evolutionary response of a population to a selective pressure is a function of the direction and intensity of that pressure and the structure of heritable variation; evolutionary responses are channeled into directions with more variation. This field of research, linking micro- and macroevolution, has established that the structure of variation among species is typically proportional to variation structures within species. Empirical estimations of the directions of selection, however, are often omitted from these analyses. In this dissertation, I widen our understanding of the micro-macroevolutionary bridge by extending the methodological toolkit for phenotypic adaptive landscape analysis and testing the proportionality of variance at different evolutionary levels with respect to empirically estimated selection gradients. First, in Chapter 1 I introduce key themes and problems that are discussed throughout this dissertation. In Chapter 2 I use a comparative dataset of within-species cranial variation in ruminant artiodactyls to demonstrate that diversification proceeds along an allometric line of least resistance that is aligned with the browsing-grazing continuum. In Chapter 3 I discuss improvements that can be made to the newly introduced “functional adaptive landscape analysis” and reconcile alternative definitions of the “Pareto front”. In Chapter 4 I show that diversification of the jaws of Colubridae is eccentric and aligned with an allometric line of least resistance as well as a Pareto front that connects prey-handling and gape performance peaks. I extend the analysis of Colubridae into Alethinophidia to argue in Chapter 5 that differences in performance evolvability contributed to the asymmetric radiations of Colubriformes and ‘constrictors.’ In the concluding chapter (Chapter 6) I synthesize these results, provide directions for future research, and postulate that the alignment between microevolutionary evolvability and the Pareto front is selected for at multiple evolutionary levels. Together, these results introduce a robust research framework for micro-macroevolutionary analysis.