Organisms can respond to fluctuating environments by adaptations occurring at a similar timescale to that of environmental fluctuations. We investigated how the various timescales of environmental fluctuations affect competition between different adaptation mechanisms, phenotypic plasticity and rapid evolution. Evolution was expressed as natural selection that is the change in the frequency of two specialist genotypes with the fixed trait, each of which fits to either of the two environmental states. Phenotypic plasticity was expressed as a flexible generalist genotype that can change its trait to fit to the fluctuating environments. Then, the advantage of the specialist and generalist genotypes were compared as the timescale of environmental fluctuations changes. The results indicate that the mechanisms allowing plastic organisms dominant to evolvable organisms are different depending on the timescales of environmental fluctuations. Therefore, phenotypic plasticity is not always dominant in fluctuating environments, and whether plasticity or rapid evolution is dominant changes idiosyncratically depending on the timescale of environmental fluctuations.