Mutant fixation in the presence of a natural enemy

Abstract

In the extensive literature about mutant invasion and fixation, populations are typically assumed to exist in isolation from their ecosystem. Yet, populations are part of ecological communities, and enemy-victim (e.g. predator-prey or pathogen-host) dynamics are particularly common. We use computational models to re-visit the established theory about mutant fixation in the presence of a natural enemy, which equally attacks both wild-type and mutant populations. We consider advantageous and disadvantageous mutants, whose fitness is unrelated to the infection. Using spatially structured agent-based as well as patch models of a population that is subject to infection, we investigate the fixation probability of a mutant that is introduced into the population at quasi-equilibrium. We find that infection significantly weakens selection. Thus, the presence of infection increases the fixation probability of disadvantageous mutants and decreases the fixation probability of advantageous mutants, with the magnitude of the effect rising with the infection rate. We show that this occurs because infection induces spatial structures, in which mutant and wild-type individuals are mostly spatially separated. Thus, instead of mutant and wild-type individuals competing with each other, it is mutant and wild-type "patches" that compete, resulting in smaller fitness differences and hence weakened selection. Because natural enemies such as infections are ubiquitous, this has broad applicability to natural systems. Our results imply that the burden of deleterious mutants in natural populations might be significantly higher than expected from mutant invasion theory developed in the absence of natural enemies.