Lineage Branching During Recovery from Simulated Mass Extinction

How do population lineages diversify to fill new ecological niches? What governs the dynamics of population recovery from near-extinction? These questions are particularly urgent in our current age of climate-driven mass extinction. We investigate these questions using a computational model of evolutionary dynamics in which simulated organisms reproduce by bacterial fission or assortative mating on a two-dimensional phenotype space. This model has been shown to undergo a directed-percolation-like nonequilibrium phase transition from survival to extinction as system parameters such as maximum mutation size or death rate are varied. Here, we use methods from coalescent theory to show that population lineages undergo a structural change near the extinction-survival transition, with a sharp divergence in the time to most recent common ancestor (TMRCA). We also simulate mass extinctions, both in the neighborhood of the phase transition and in the survival regime, by either increasing the death rate of organisms, or increasing the parameter that controls their competition. We then analyze lineages, TMRCA, and other measures of population structure during successful and unsuccessful recoveries from mass extinction events.