Eco-evolutionary interactions as a consequence of selection on a secondary sexual trait

Ecological and evolutionary population changes are often interlinked, complicating the understanding of how each is affected by environmental change. Using a male dimorphic mite as a model system, we studied concurrent changes in the expression of a conditional strategy and in the population in response to harvesting over 15 generations. We found evolutionary divergence in the expression of alternative male reproductive morphs—fighters and defenceless scramblers (sneakers)—caused by the selective harvesting of each male morph. Regardless of which morph was targeted, the direction of evolution of male morph expression in response to harvesting was always towards scramblers, which, in case of the harvesting of scramblers, we attributed to strong ecological feedback (reduced cannibalism opportunities for fighters) within the closed populations. Current evolutionary theory, however, predicts that the frequency of a morph always decreases when selected against: to understand phenotypic trait evolution fully, evolutionary theory would benefit from including ecological interactions, especially if traits have ecological consequences that in turn feedback to their evolutionary trajectory.