A geographic selection mosaic in a generalized plant–pollinator–herbivore system

Abstract

The concept of Selection Mosaic is central to the Geographic Mosaic Theory of Coevolution. Most information on coevolving interactions, however, comes from specialized organisms. In contrast, an accurate understanding of the effect of geographically varying evolutionary dynamics on the evolution of generalist organisms is lacking, although these kinds of organisms are the most frequent in nature. In flowering plants, pollinators and herbivores are important selective agents. In this study we investigate whether a geographic selection mosaic for floral traits in a generalist plant,Erysimum mediohispanicum(Brassicaceae), can be mediated by the interplay of mutualistic and antagonistic interacting organisms. In eight populations we quantified the selection exerted by these organisms on several plant traits. We found significant spatial variation in pollinator assemblage. In different populations, the main pollinators belonged to different functional groups (beeflies, large bees, small bees, and beetles). Damage by ungulates also varied among populations. Consequently, we found that different populations were under different selective regimes, and the traits affected by selection depended on the local interaction intensity with pollinators and mammal herbivores. Some traits, such as flower number and stalk height, were selected similarly in most populations. Other traits, such as corolla diameter and tube length, were selected only in some populations. Finally, we found divergent selection for some traits, such as corolla tube width and corolla shape, which were selected in contrasting directions in different localities. This spatial variation in selective scenarios results in populations with strong selective regimes (hot spots) intermingled with populations with weak selective regimes (cold spots). Four important outcomes emerge from theE. mediohispanicumselection mosaic. (1) Interactions with generalist organisms may produce strong selection. (2) Spatial changes in main pollinators result in divergent selection across populations. (3) Geographic mosaics depend on a balance between mutualistic and antagonistic selection. (4) Selection mosaics operate at fairly small spatial scales. These findings will surely contribute to expanding the conceptual framework of the Geographic Mosaic Theory of Coevolution.