The role of resource defensibility in facilitating sexually selected weapon evolution: an experimental evolution test

Animal weapons have evolved multiple times, primarily for battling for access to mates. Despite intra-sexual selection being common, exaggerated weapons have evolved relatively rarely. So why do exaggerated weapons not evolve more commonly? It has been hypothesized that three conditions are necessary for evolution of exaggerated weapons: high variance in reproductive success, patchy, high-value resources, and spatial environments conducive to one-on-one competition. Here, we test this hypothesis by performing experimental evolution in Drosophila melanogaster, utilizing heterogeneous environments where conditions facilitating territorial defense and opportunities for competitive interactions vary. We examine changes in sexually dimorphic morphology and male aggression that are predicted to occur, based on this model. We also examine whether condition dependence for sexual dimorphism has evolved after 35 and 75 generations of experimental evolution. Aggression did increase, albeit modestly, in environments that facilitate resource defense. Morphological changes are modest although with some trait-specific changes to allometry, generally in the opposite direction of our predictions. Condition dependence trends in the opposite direction from those predicted by our hypothesis as well. We discuss our results in the context of the necessary conditions for the evolution of exaggerated weapons, and if, and when condition dependence may evolve.