Bridging the gap across micro-, macro- and mega- evolutionary timescales

In this thesis I explore evolution spanning many orders of magnitude, from micro- to macro- and mega- evolutionary timescales. In the first part of this thesis I investigate the connection between micro- and macro- evolution in the morphological traits of organisms. I aim to understand why phenotypic evolution is conservative over short timescales yet net change accumulates over longer timescales via macroevolutionary bursts. I find that a simple process-based model of microevolution playing out over rugged adaptive landscapes can account for the empirical pattern of divergence in animal body sizes over time and that macroevolutionary bursts can be explained by rare traversals of valleys in the adaptive landscape without invoking shifts in adaptive peaks themselves. Secondly, I explore the connection between the macroevolutionary rate of speciation and the rates of steps involved in the speciation cycle, encompassing the evolution of geographical, reproductive and ecological isolation between populations. I show via a mathematical model that this relationship is nonlinear so that macroevolutionary and microevolutionary rates may appear disconnected for certain steps of the speciation cycle. Thirdly, I investigate the macro and megaevolutionary dynamics of the evolution of bill size in relation to body size across the entire radiation of modern birds. I find that evolution is generally allometrically conservative over millions of years, but there have been multiple shifts in the slope and intercept of the allometric relationship between bill size and body size across different clades accumulating steadily over time. Finally, I turn my attention to a specific case study in macroevolution, hummingbird diversification and morphological evolution in relation to the evolution of foraging ecology. I test the hypothesis that diversification rate is associated with foraging ecology, specifically that a traplining ecology which is thought to entail ecological specialisation is an ‘evolutionary dead end’. I find no evidence for a relationship between foraging ecology and rates of diversification. Taken together, this thesis finds that the tempo and mode of evolution are heterogeneous over timescales. Nonetheless, our understanding of evolutionary patterns over different timescales can be bridged by understanding how underlying processes interact and scale over time.