In this paper we show how evolving robots can develop behaviors displaying a modular organization characterized by semi-discrete and semi-dissociable sub-behavioral units playing different functions. In our experiments, the development of differentiated behaviors is not realized through the sub-division of the control system into modules and/or through the utilization of differentiated training processes. Instead, it simply originates as a consequence of the adaptive advantage provided by the possibility to display and use functionally specialized behaviors. These are selected by evolution not only with respect to their capability to perform a given sub-function but also with respect to the capability to support smooth and effective transition with other behaviors. This is achieved by having different co- adapted behaviors and by evaluating the variation affecting the behaviors on the basis of the impact they have on the overall performance of the robots. Moreover this process enables the development of the ability to carry on preparatory actions that are necessary for the effective execution of the following behaviors. We refer to this type of modularity as functional modularity, since unlike structural modularity, it is not based on behavioral modules that are separated by clear boundaries and/or that are programmed or trained independently.