Shifts in colour morph frequencies along an urbanisation gradient in the ground beetle Pterostichus madidus

Cities impose strong ecological constraints on many species. It is often difficult to know whether phenotypic responses to urbanisation are due to (adaptive) evolutionary responses, as opposed to e.g. plastic responses. A solution is to focus on traits for which variation is only or mostly genetic in origin, since changes are then likely to reflect evolutionary processes. For this purpose, we studied the leg-colour polymorphism of a common carabid beetle, Pterostichus madidus, along an urbanisation gradient. We observed that black-legged individuals were more frequent than red-legged individuals in urban areas. Whether these changes result from natural selection or non-selective evolutionary forces is uncertain. However, if this phenotypic change is adaptive in nature, higher urban temperatures are likely to be the driver. Specifically, our results are consistent with previous data showing that black-legged individuals have a behavioural advantage in warmer (micro)climates, and contradict the thermal melanism hypothesis that predicts they would experience stronger negative effects of higher temperatures in cities.