Eco-Evolutionary Dynamics of Pathogen Epidemic Timing in a Seasonal Environment.

Seasonality structures the dynamics of infectious diseases, yet how pathogens evolve their seasonal timing remains unclear. Here, we develop an evolutionary epidemiology model to examine how pathogens adapt to seasonally varying environments, assuming that each strain favours a specific season for transmission. Analytical and numerical results reveal two contrasting regimes: phenological drift, where the preferred season continually advances through priority effects, and stable seasonal adaptation, where evolution converges to a fixed season. Under strong transmissibility, multiple seasonal morphs can coexist, forming polymorphic communities of epidemic timing. These outcomes result from the balance between a seasonal priority effect, rewarding early occupation of the niche and a seasonal stabilising effect, adjusting traits to the external optimum. Our findings show that epidemic seasonality itself can evolve as a life-history trait, providing a conceptual bridge between pathogen dynamics and the broader evolution of phenology in periodic environments.