High-Diversity Coevolutionary Dynamics in a Model of Interacting Bacteria and Phage*

The diversity of microbes in nature extends far below the species level, with large numbers of strains coexisting in space and time. Furthermore, large population sizes ensure that there is a steady supply of new mutations entering any microbial ecosystem, and therefore evolution must play a continual role in shaping these communities. These processes motivate the question: how do ecology and evolution interact in microbial communities, and what are the characteristic patterns of diversity they produce? We explore this question in the context of a toy model of a single bacteria and phage species, each of which is comprised of many closely related strains. Interactions between each pair of bacteria and phage strains are parameterized by mutable phenotypes, and ecological dynamics exhibit boom-bust chaotic oscillations stabilized by rudimentary spatial structure. Over longer evolutionary timescales, we find that extensive diversity can be maintained in a dynamical evolutionary steady state, with biodiversity turning over at a constant rate and coexisting strains related by their shared evolutionary history. These findings suggest a possible high-diversity scenario for the type of "red queen" evolution conjectured to exist between hosts and pathogens.