14 - Rate Perspective on Early Bursts of Evolution

Summary

Two models of diversification are commonly recognized in evolutionary radiations: Brownian diffusion (BD) and Ornstein-Uhlenbeck diffusion (OU). A third narrowly-defined ‘early burst’ model (ER*) is a form of Brownian diffusion with a step rate decreasing through time, leading to exponential slowing of net rates of change. Darwin’s finches, Geospizini, are analyzed as a case study for comparison of BD, OU, and ER*. The step rate of evolutionary change in tarsus length for Geospizini can be estimated from temporal scaling of node differences analyzed in the context of a phylogeny. This yields an estimated step rate of h0 = 10−0.643 = 0.228 standard deviations per generation, which is close to the median step rate of h0 = 0.153 for field studies found here in Chapter 8. Forward modeling of Brownian diffusion, Ornstein-Uhlenbeck diffusion, and the narrow early burst model through 2.5 million generations of geospizine diversification indicates that BD, OU, and EB* are all are early burst models, with net disparity increasing most rapidly at the beginning of each radiation. Early bursts of evolution are common, as paleontologists and ecologists have recognized for many years.