The role of pathogens in creating and maintaining host polymorphism

Terrestrial and marine ecosystems exhibit remarkable species diversity and richness. Among the hypotheses put forward to explain this diversity —its origins and the forces sustaining it— one prominent hypothesis posits that pathogens could be pivotal in generating and maintaining species diversity within ecosystems. To explore this hypothesis, we analyzed a host-pathogen system, examining the specific conditions that might facilitate pathogen-driven diversification within the host population, with a focus on both the emergence and persistence of diversity. By applying a fecundity-transmission trade-off in which a non-evolving pathogen influences its evolving host, we observed that pathogens only infrequently drive host diversification, and only around intermediate trade-off conditions. Furthermore, while these rare diversification events result in stable coexistence, evolved diversity is low, consisting of two coexisting host morphs. Because the sole evolution of hosts in response to pathogens offers limited opportunity for diversification, we propose additional mechanisms that could foster higher levels of diversity within the system.