Climate change–driven phenological shifts alter the temporal distributions of natural populations and communities, but we have little understanding of how these shifts affect natural populations. Using agent-based models, we show that the interaction of within-population synchrony (individual variation in timing) and timing of interspecific interactions shapes ecological and evolutionary dynamics of populations within a seasonal cycle. Low-synchrony populations had lower survival and biomass but relatively stronger individuals. These effects were surprisingly robust and did not require size-based competitive asymmetries. However, reducing population synchrony could either negatively or positively affect population demography depending on whether the phenology of the focal species was advanced or delayed relative to its competitor. Furthermore, selection for earlier hatching increased when the interspecific competitor arrived earlier and when population synchrony was high. These results emphasize the importance of variation in the phenology of individuals within populations to better understand species interactions and predict ecological and evolutionary outcomes of phenological shifts.