Spatiotemporal Ecological Chaos Enables Gradual Evolutionary Diversification Without Niches or Tradeoffs

Ecological and evolutionary dynamics are intrinsically entwined. On short time scales, ecological interactions determine the fate of new mutants and changes in the community they induce, while on longer time scales evolution shapes the whole community. How eco-evolutionary dynamics gives rise to the extensive coexisting diversity of strains found in many bacterial species is a major puzzle. In this paper we study the evolution of large numbers of closely related strains with generalized Lotka Volterra interactions but no niche structure. The host-pathogen-like interactions drive the ecological dynamics into a spatiotemporally chaotic state characterized by continual local blooms and busts. Upon the slow serial introduction of new strains, the community is found to diversify indefinitely, accommodating arbitrarily large numbers of strains in the absence of any kind of stabilizing niche interactions. This diversifying phase is robust to changes in evolutionary parameters, and persists even in the presence of a distribution of general, nonspecific fitness differences between individual strains, which explicitly break the assumption of tradeoffs inherent in much previous work. However, gradual increase of the general fitnesses in the ecosystem slows down the diversification. Quantitative analysis of the range of behaviors is carried out by a combination of analytical methods and simulations. Building on a dynamical-mean field-theory understanding of the ecological dynamics, an approximate effective model captures the effects of evolution on the distributions of key properties, such as strain abundances. This work establishes a potential scenario and a theoretical framework for understanding how the interplay between evolution and ecology can give rise to extensive fine-scale diversity. Future avenues for investigation are suggested, including the effects of the build-up of relatedness between strains, how conditioning on the evolutionary history affects the ecological interactions and dynamics, and application to coevolution of the diversity of a bacterial and a phage species.