We investigate harvest–induced adaptive changes in age and size at maturation by modelling both plastic variation and evolutionary trajectories. Harvesting mature individuals displaces the reaction norm for age and size at maturation toward older ages and larger sizes and rotates it clockwise, whereas harvesting immature individuals has the reverse qualitative effect. If both immature and mature individuals are harvested, the net effect has approximately the same trend as when harvesting immature individuals only. This stems from the sensitivity of the evolutionary response, which depends on the maturity state of harvested individuals, but also on the type of harvest mortality (negatively or positively density dependent, density independent) and the value of three life–history parameters (natural mortality, growth rate and the trade–off between growth and reproduction). Evolutionary changes in the maturation reaction norm have strong repercussions for the mean size and the density of harvested individuals that, in most cases, result in the reduction of biomass—a response that population dynamical models would overlook. These results highlight the importance of accounting for evolutionary trends in the long–term management of exploited living resources and give qualitative insights into how to minimize the detrimental consequences of harvest–induced evolutionary changes in maturation reaction norms.