Rapid adaptation of an invasive species to climate at a local and continental scale revealed by citizen science

Rapid evolution and evolutionary rescue are key mechanisms modulating the success of invasive species, allowing to rapidly adapt to new conditions and to recover from a bottleneck after the introduction. Despite being a well-known mechanism overall, examples of rapid evolution from invasive species and on broad geographic ranges are poorly documented. The box tree moth (Cydalima perspectalis) is a polymorphic invasive species that recently colonized Europe, but the ecological factors driving the morph frequencies in natural populations are still largely unknown. Here we took advantage of citizen science, that allows for an unprecedented amount of observations across a large spatial scale, and pheromone trapping, for a more targeted sampling, to detect differences in morph ratios for our model species across Europe and Asia. We then fitted linear regressions and generalized mixed models to detect what variables in the invasive environment drive their frequencies. We found that the melanic form of this moth is maintained at intermediate frequencies in the entire range, but that both at the continental scale and at the local scale, its frequency is higher in warmer and drier conditions. This finding supports the hypothesis of a rapid evolution of colour polymorphism in this invasive species, that established the observed cline in roughly 27 generations. Finally, we provide a physiological rationale of the observed cline, using an experimental approach, showing that melanic individuals have a better resistance to desiccation, compatible with their higher frequency in warmer and drier climates.