 Here, he proposes a trade-off between specialized adaptation to local conditions in the absence of migration and generalized adaptation to global conditions in the presence of migration. In modern terms, this is called genotype-by-environment interaction, where the selective effect, \(s\), of a gene changes with change in the environment. A gene might be adaptive in one environmental context (i.e., \(s \gt 0)\) but maladaptive in another (i.e., \(s \lt 0)\). Migration between local environments mixes the adaptive and maladaptive responses to selection and reduces the average magnitude of gene frequency change. In this sense, genotype-by-environment interaction for fitness is viewed as an evolutionary constraint because it limits the rate of gene frequency change. The restraint can be removed simply by stopping gene flow and the mixing of genes across different local environments. Thus, the fixed selective effect illustrated in  Figure 1,  must be considered an average of variable selective effects across environments. Clearly, large local effects of opposite sign must be averaged when there is gene flow among habitats and the averaging tends to reduce the magnitude of a gene’s selective effect. Ford also suggests that the genetic mechanism involves “gene complex[es] balanced to fit their own local environment”. That is, he claimed interactions among genes, or epistasis, contributes to local adaptation. Thus, Ford invokes genotype-by-environment interactions for fitness as well as gene-gene interactions for fitness in his cases of rapid evolution, although both reduce the average selective effect of a gene. This argument also runs counter to his assertion that the adaptive advantage of a gene is typically 25 percent in natural populations. Both kinds of interaction change the depiction of the threshold separating natural selection from random genetic drift  (Fig. 1);  in fact they tend to raise it and thereby increase the domain of random genetic drift. Before turning to interaction effects, I will examine a representative discussion of ecological genetics of random genetic drift using data from a natural population.