Idea that adaptations tend to be constrained by tradeoffs.
The more effective a system, the more likely that a change to that system will affect it detrimentally. This is even if the change is otherwise beneficial. Adaptations, in other words, tend to supply a net benefit rather than necessarily nothing but benefit. There is no free lunch!
Adaptations thus are limited by what genetic variation is available to a population (i.e., existing variation), the nature of what adaptations already exist (historical constraints), and by the fact that adaptations themselves can be detrimental to organisms (evolutionary compromise).