I present two models for the genesis of phenotypic variation in age and size at maturity in male guppies. Both are qualitatively consistent with the observation that age at maturity increases and size at maturity decreases as food availability (and growth rate) decline. The first model (“fixed development”) has a constant size which signals the initiation of the process of maturation and a constant time interval between the initiation and completion of maturation. There is thus no response in the pattern of development to differences in growth rate. Variation in age at maturity results from the different amounts of time required to reach the size at initiation. Variation in size results from different amounts of growth between the initiation and completion of maturation. Phenotypic variation is therefore an epiphenomenon of expressing a fixed pattern of development in different environments. In the second model (“flexible development”), fast growers initiate maturation at a larger size than slow growers; this results in a later age and larger size at maturation than their fast growing counterparts in the fixed development model. Slow growers mature at an earlier age and smaller size than their counterparts in the fixed development model. In the second model, the pattern of development changes with the environment. Two experiments with guppies demonstrate that the second model accurately describes their pattern of development. I conclude that phenotypic plasticity is in part the product of plastic development and may represent an adaptation. Testing whether or not the response is adaptive will require further investigations. A general message of this investigation is that characterizing the reaction norm alone is not sufficient to evaluate the adaptive significance of plasticity; one must also consider the processes that generate this plasticity.