Evolving modular neural-networks through exaptation

Despite their success as optimization methods, evolutionary algorithms face many difficulties to design artifacts with complex structures. According to paleontologists, living organisms evolved by opportunistically co-opting characters adapted to a function to solve new problems, a phenomenon called exaptation. In this paper, we draw the hypotheses (1) that exaptation requires the presence of multiple selection pressures, (2) that Pareto-based multi-objective evolutionary algorithms (MOEA) can create such pressures and (3) that the modularity of the genotype is a key to enable exaptation. To explore these hypotheses, we designed an evolutionary process to find the structure and the parameters of neural networks to compute a Boolean function with a modular structure. We then analyzed the role of each component using a Shapley value analysis. Our results show that: (1) the proposed method is efficient to evolve neural networks to solve this task; (2) genotypic modules and multiple selections gradients needed to be aligned to converge faster than the control experiments. This prominent role of multiple selection pressures contradicts the basic assumption that underlies most published modular methods for the evolution of neural networks, in which only the modularity of the genotype is considered.