When Fitness Is Cheap: Pareto-Based Evolutionary Optimisation for Lightweight Neural Architectures
Evolutionary algorithms are well suited to neural architecture search and other combinatorial design problems, but their scalability is often limited by the high cost of fitness evaluation. This paper studies evolutionary multi-objective optimisation in a regime where fitness evaluations are effectively free, enabled by a training-free proxy for neural network expressivity. We propose SWAP-Lite, a Pareto-guided evolutionary algorithm that maintains an explicit archive of non-dominated solutions over representational capacity and deployment cost, yielding an anytime optimiser that exposes budget-feasible solutions throughout the search. Using a MobileNet-style architecture space as a case study, we instantiate the fitness function with a sample-wise activation pattern proxy and perform large-scale evolutionary searches with up to 105 architecture evaluations. Experiments on CIFAR-10 and ImageNet show that SWAP-Lite discovers compact architectures that are competitive with state-of-the-art training-based and zero-shot baselines, while reducing search cost by one to four orders of magnitude. Analysis of the evolutionary dynamics demonstrates that explicit bi-objective optimisation produces higher-quality constrained Pareto fronts and superior anytime hypervolume compared with random search, greedy local search, and single-objective evolutionary baselines.