This paper introduces a novel study on the sense of valency as a vital process for achieving adaptation in agents through evolution and developmental learning. Unlike previous studies, we hypothesise that behaviour-related information must be underspecified in the genes and that additional mechanisms such as valency modulate final behavioural responses. These processes endow the agent with the ability to adapt to dynamic environments. We have tested this hypothesis with an ad hoc designed model, also introduced in this paper. Experiments have been performed in static and dynamic environments to illustrate these effects. The results demonstrate the necessity of valency and of both learning and evolution as complementary processes for adaptation to the environment.