Neuronal circuits: an evolutionary perspective.

To understand neural circuits completely, it is necessary to know not only how they work, but also why they work that way. Answers to the latter question have been almost teleological in their assumption of optimal design. However, close examination of certain systems has revealed features that apparently lack adaptive value. Their existence can be understood only if the evolution of these circuits is considered and, in particular, how nonadaptive determinants have guided that evoluton.