How adaptation appears and is later refined by natural selection has been the object of intense theoretical work. However, the testing of these theories is limited by our ability to estimate the strength of natural selection in nature. Using a long-term cline series, we estimate the selection coefficients acting on different alleles at the same locus to analyze the allele replacement observed in the insecticide resistance gene Ester in the mosquito Culex pipiens in the Montpellier area, southern France. Our method allows us to accurately account for the resistance allele replacement observed in this area since 1986. A first resistance allele appeared early, which was replaced by a second resistance allele providing the same advantage but at a lower cost, itself being replaced by a third resistance allele with both higher advantage and cost. It shows that amelioration of the adaptation (here resistance to insecticide) through allele replacement was successively achieved by selection of first a generalist allele (i.e., with a low fitness variance across environments) and later a specialist allele (i.e., with a large fitness variance across environments). More generally, we discuss how precise estimates of the strength of selection obtained from field data help us understand the process of amelioration of adaptation.