Does insular adaptation to subtropical conditions promote loss of plasticity over time?

Phenotypic plasticity (i.e. the ability to express different phenotypes under changing environmental conditions) is thought to play a key role in habitat adaptation, but little is known about how trait plasticity evolves following dispersal into novel island habitats. We hypothesize that shifts from seasonal Mediterranean climates to more stable (subtropical) island conditions would promote a net reduction in trait plasticity over time. To test this hypothesis, we set two common gardens with contrasting environmental (low resource vs. mesic) conditions, where we grew seedlings of wild olive (Olea europaea var. sylvestris) populations that represented two Canary Island lineages with different colonization times (old vs. young) and their Mediterranean ancestral lineage (N = 275 individuals). Plasticity was assessed for 12 morphological, photosynthetic and chemical traits by (i) subjecting half of the seedlings to simulated herbivore browsing (50% of aerial biomass removal) and (ii) comparing phenotypic values between both common garden settings. Simulated herbivore browsing induced few plastic responses, mostly restricted to photosynthetic traits, but these were similarly displayed by all lineages. Comparisons between common gardens revealed a contrasting response between the Mediterranean and both subtropical island lineages in leaf phenotypes. Furthermore, the older island lineage showed an overall lack of plasticity (i.e. environmental canalization) in morphological and chemical traits. These results suggest that, unlike photosynthetic traits that are fundamental for fast acclimation to environmental shifts, some developmental traits may lose plasticity over time as a result of phenotypic adjustment to subtropical insular conditions.