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The environment can be considered as a set of prerequisites. An animal performs at its best if it possesses all prerequisites for that environment. If some are missing, the performance is reduced. Each environment has its own set of prerequisites. However, some environments are so much alike that animals can use the same set of prerequisites. The more different two environments are, the more important it becomes to have the specific prerequisites to manage in either of the environments. Sometimes, the prerequisites are even adverse: if you have the one, you can’t have the other. For example, if you have a thick fur to tolerate cold, you can’t stand the heat. These adverse prerequisites are called trade-offs. An animal can be capable of handling one prerequisite, for example digesting low quality feed, but that requires a type of digestion physiology which makes it impossible to grow very fast when the feed quality is good. You can be good at one thing, but it automatically means you are not so good at something else. Often meeting a prerequisite in one environment only causes a small negative effect on performance in the other environment. The opposite can be much more of a problem: not being resistant to an infection is quite problematic in an environment with that infection.

The size of genotype by environment interaction

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Figure 4. Two examples of GxE in the sheep breeds Dorper and Red Masai. In the top graph the non-parallel reaction norms are an indication of the higher sensitivity of the Dorper to semi-arid conditions than the Red Masai, expressed in mortality rate. But the mortality rate of the Dorper remains higher in both environments. In the bottom graph the GxE is stronger and indicating that the Dorper is heaviest in hot and humid conditions, whereas the Red Masai is the heaviest in the semi-arid conditions. 

Thus:

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