• Aristal branches;
  • development time;
  • phenotypic plasticity;
  • pigmentation;
  • sternopleural bristles;
  • thorax length

Two contrasting views can characterize the attitude of many studies toward reaction norms (RNs). An “optimistic” view attempts to use a linear model to describe RN variation; and a “pessimistic” view emphasizes RNs complexity without using any model to describe them. Here, we have analyzed the shape of 40 RNs of five traits of Drosophila mediopunctata in response to 11 temperatures. Our results, along with several other studies, show that RNs are typically curves best explained by nonlinear models. Estimating the set of 40 RNs on the basis of three rather than 11 temperatures produces a scenario, typical of the pessimistic view, where the linear model is either nonsignificant or a poor explanatory model. Moreover, we show that RN nonlinearity can significantly affect the conclusions of studies using the linear model. We propose a middle ground view on RNs which recognizes their general nonlinearity. Such view could, on the one hand, explain part of the important phenomenon of genotype–environment interaction emphasized by the pessimistic view. Moreover, it may explain features and patterns which are being ignored by the optimistic view. We suggest the parabolic model as first step to reveal patterns which were ignored before, or not fully appreciated.