Figure 1. Evolution under Fixed Goals (FG) and Modularly Varying Goals (MVG).

Examples of data from a series of studies [18],[19] that suggest that modularity spontaneously evolves when goals change over time in a modular fashion (modularly varying goals or MVG). (A) Logic circuits made of NAND gates evolved under a constant goal (fixed goal, abbreviated FG) that does not vary over time, G ₁ = (x XOR y) AND (w XOR z). The circuit is composed of 10 NAND gates. Evolution under a constant goal typically yields compact non-modular circuits. (B) Circuits evolved under MVG evolution, varying every 20 generations between goal G ₁ and goal G ₂ = (x XOR y) OR (w XOR z). Note that these two goals share the same sub-goals, namely two XOR functions. Connections that are rewired when the goal switches are marked in red. Evolution under MVG typically yields modular circuits that are less compact, composed in this case of 11 gates. The circuits are composed of three modules: two XOR modules and a third module that implements an AND/OR function, depending on the goal.