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Mutation has a paradoxical role in evolution: it is the ultimate source of new genetic variation which enables adaption in natural populations, yet most mutations have an adverse effect on fitness. How mutation in the presence of selection contributes to standing genetic variation is a fundamental concept to evolutionary biology, encapsulated best by the mutation-selection balance class of theoretical models. The magnitude of effect that a mutation has on fitness will determine how strongly natural selection will act to remove it from the population, where mutations of large effect are purged quickly, and weak mutations can persist for hundreds of generations, evolving under stochastic drift. Despite their importance, the fitness effects of mutations remain poorly understood. Mutation- selection balance models fail to reconcile empirical estimates of mutation, standing genetic variation and strength of selection, which may …