Complex structure of a circuit evolved in silicon. (A) The final evolved circuit for tone discrimination, a 10 × 10 array of cells. All connections that link an input and an output are marked. (Arrow from a cell) connection driven by origin cell's function; (square at arrowhead) connection selected as input for recipient cell function. (B) Minimal necessary components of final evolved circuit for tone discrimination. Missing cells can be fixed at constant value without affecting performance; gray cells cannot be fixed at constant value without affecting performance, although the cell has no path connecting to marked minimal necessary functional cells (Thompson 1997).