Emergence time difference can create negative frequency dependence and maintain a colour polymorphism in Colias butterflies

Many species of Colias butterflies show a female-limited colour polymorphism where the dominant Alba allele leads to white wings, faster development and higher fecundity (hypothesized to arise from not allocating resources to pteridine pigment production), while orange females are more attractive to males. Attractiveness can benefit females because males provide nutrient-rich spermatophores. Despite decades of research and a long-standing understanding of the associated life-history trade-off, earlier modelling work has been unable to show how this trade-off could generate frequency-dependent selection. We construct a model that shows the importance of phenology for this polymorphism's stability. When generations are non-overlapping or partially overlapping, and there is an eclosion order where males precede Alba females that precede orange females, females use partially distinct temporal niches. Theory on temporal niches, however, is known not to automatically guarantee coexistence; translated to our case, Alba monopolization of males early in the season might simply deplete mate availability for late-eclosing orange females. However, with sufficiently high male ability to replenish spermatophores, we find negative frequency-dependence where competition intensifies mainly within a morph if it becomes common. In summary, given realistic assumptions, the trade-off between development time and attractiveness can maintain a stable polymorphism in Colias butterflies. This article is part of the theme issue 'Exploring negative frequency dependent selection across levels: from genetics to ecology and back again'.