Shift in climate, shift in selection: Experimental warming misaligns mean phenotype and adaptive peak for the goldenrod gall fly

Climate change is expected to alter selection regimes acting on diverse traits across many species in both direct and indirect ways. Higher temperatures may alter metabolisms, phenologies, resources, and ultimately trophic dynamics. Many studies have examined how eco-evolutionary dynamics are affected by changing climates, but few are able to quantify selection and attribute it specifically. We used the goldenrod ball-gall system, a tightly linked group of interacting species, to test the phenotypic, phenological, and fitness changes when exposed to experimental warming. The larvae of the fly Eurosta solidaginis induce galls on stems of Solidago altissima . Previous research has shown that larvae inducing small galls are more vulnerable to attack by the parasitoid wasp, Eurytoma gigantea , whereas those inducing large ones are more likely to be attacked by avian predators. Thus, natural enemies impose stabilizing selection on the fly for gall size. Under experimental warming, S. altissima showed no phenological change. Gall initiation was accelerated under warming, but gall growth plateaued earlier. This caused significantly smaller final gall diameters in the Warmed treatment. While this smaller size led to treatment differences in attack rates by some enemies, the optimal gall size for Eurosta was similar in both treatments. Warming thus caused mean gall size to deviate more strongly from the optimum, thereby inducing directional selection. The shape of the fitness function was similar between treatments; hence the strength of stabilizing selection was unchanged. These results show that warming could influence species’ phenotypes and their selective environment through altered species interactions.