Abstract
Aim Trade‐offs allow individuals to optimize their fitness by tailoring the investment into different traits to variable environmental conditions, such as along geographical gradients. Trade‐offs thus can help in adjusting to changing thermal and insolation profiles, especially in small ectotherms, whose body temperature typically follows environmental temperatures closely. Two traits usually involved in latitudinal adaptation are body size and melanism. Since both traits are costly, individuals need to optimize investment into each trait. Here, we studied how environmental factors influence this trade‐off in the short and long‐term. Location Europe. Methods We raised flies from 15 latitudinal populations at three constant temperatures in a laboratory common garden to differentiate plastic and evolutionary responses to temperature. We further analysed how the different insolation components of the populations’ habitats influenced the evolution of the trade‐off. Results Male Sepsis thoracica (Diptera: Sepsidae) feature a sigmoid relationship between melanism and body size, defining two strikingly different male morphs: obsidian (small and black) and amber (large and orange). This relationship was altered by the developmental temperature, documenting its plasticity. The relationship further evolved across populations in response to the environmental characteristics of their habitat, notably temperature, insolation and UV radiation, suggesting that plasticity also has an underlying genetic basis. Nevertheless, melanism, but not body size, merely slightly increased with latitude. Main conclusions As the plastic and evolutionary responses of the relationship to temperature differed, plasticity does not necessarily follow the direction of evolution of this trade‐off, but rather adds to it. Our study evinces the role of several environmental factors in shaping the evolution of a plastic melanism—body size relationship defining a rare male polymorphism in temperate sepsid flies.