Various hypothetical design spaces for two parameters x1 and x2. A Single optimum peak in the design space. For two parameters, only one particular narrow setting leads to a high performance. B Ridge-like optimum. When the two parameters are concurrently changed, an extended optimum can be achieved, but not when only one is changed. C Flat optimum with a high plateau. Both the parameters can be changed within wide boundaries, still leading to an optimum. This indicates the robustness of performance with respect to parameter changes. D A ‘corridor’ (arrow) that links two flat optima. Performance might decrease a little, but still be comparatively high. Interlinked areas of high performance facilitate evolution in various ways, increasing evolvability.