Evolution is coupled with branching across many granularities of life

Abstract

Across many different scales of life, the rate of evolutionary change is often accelerated at the time when one lineage splits into two. The emergence of novel protein function can be facilitated by gene duplication (neofunctionalisation); rapid morphological change is often accompanied with speciation (punctuated equilibrium); and the establishment of cultural identity is frequently driven by sociopolitical division (schismogenesis). In each case, the change resists re-homogenisation; promoting assortment into distinct lineages that are susceptible to different selective pressures, leading to rapid divergence. The traditional gradualistic view of evolution struggles to detect this phenomenon. We have devised a probabilistic framework that constructs phylogenies, tests hypotheses, and improves divergence time estimation when evolutionary bursts are present. As well as assigning a clock rate of gradual evolution to each branch of a tree, this model also assigns a spike of abrupt change, and independently estimates the contributions arising from each process. We provide evidence of abrupt evolution at the time of branching for proteins (aminoacyl-tRNA synthetases), animal morphologies (cephalopods), and human languages (Indo-European). These three cases provide unique insights: for aminoacyl-tRNA synthetases, the trees are substantially different from those obtained under gradualist models; Cephalopod morphologies are found to evolve almost exclusively through abrupt shifts; and Indo-European dispersal is estimated to have started around 6000 BCE, corroborating the recently proposed hybrid explanation. This work demonstrates a robust means for detecting burst-like processes, and advances our understanding of the link between evolutionary change and branching events. Our open-source code is available under a GPL license.