Fig. 4. A morphological Baldwin effect and its relationship to energy efficiency and stability.

a Progression of mean (n = 100) iterations to achieve the 75th percentile fitness of the initial population for the lineages of the best 100 agents in the final population across 3 evolutionary runs. b Fraction of stable morphologies (see “Methods”) averaged over 3 evolutionary runs per environment. This fraction is higher in VT and MVT than FT, indicating that these more complex environments yield an added selection pressure for stability. c Mean cost of work (see “Methods”) for same lineages as in (a). d Learning curves for different generations of an illustrative agent evolved in FT indicate that later generations not only perform better but also learn faster. Thus overall evolution simultaneously discovers morphologies that are more energy efficient (c), stable (b), and simplify control, leading to faster learning (a). Error bars (a, c) and shaded region (b) denote 95% bootstrapped confidence interval.