Gene expression differences between genotypes are not homogeneous across environments. Across a temperature gradient expression differences are most pronounced at extreme temperatures. The similarity of gene expression between genotypes may be the outcome of higher efficacy of purifying selection in frequently encountered environments. However, the alternative explanation of decanalization at extreme, stressful environments cannot be ruled out. To distinguish between these two explanations we contrasted gene expression differences between two D. melanogaster strains cultivated at high or low larval density. We find more differences in gene expression for flies grown at low density, than for high density flies. Since low larval density is less stressful and the D. melanogaster strains were maintained for >80 years at high density conditions, we conclude that the similar gene expression profile of the two genotypes at high larval density is the outcome of purifying selection. Genetic variation causing expression differences at low larval density were probably not purged because this environment was rarely encountered. Our results suggest that expression differences between genotypes may frequently not be the outcome of adaptive changes, but may also reflect lack of purifying selection.