Adaptation occurs by gene replacement (or transient balanced polymorphism). Replacement may be caused by selection (local or global) and/or genetic drift among alleles. In addition, historical events may blur the respective effects of selection and drift during the course of replacement. We address the relative importance of these processes in the evolution of insecticide resistance genes in the mosquito Culex pipiens. The resistance allele, Ester2, has a broad geographic distribution compared to the other resistance alleles. To distinguish between the different processes explaining this distribution, we reviewed the literature and analysed updated data from the Montpellier area of southern France. Overall, our data indicate that Ester2 prevails over other Ester resistance alleles in moderately treated areas. Such conditions are common and favour the hypothesis of selection acting at a local level. This places an emphasis on the importance of ecological conditions during the evolution of resistance. Finally, we highlight that historical events have contributed to its spread in some areas.