Mutualists and antagonists drive among-population variation in selection and evolution of floral display in a perennial herb

Spatial variation in the direction of selection drives the evolution of adaptive differentiation. However, few experimental studies have examined the relative importance of different environmental factors for variation in selection and evolutionary trajectories in natural populations. Here, we combine 8 y of observational data and field experiments to assess the relative importance of mutualistic and antagonistic interactions for spatial variation in selection and short-term evolution of a genetically based floral display dimorphism in the short-lived perennial herb Primula farinosa. Natural populations of this species include two floral morphs: long-scaped plants that present their flowers well above the ground and short-scaped plants with flowers positioned close to the ground. The direction and magnitude of selection on scape morph varied among populations, and so did the frequency of the short morph (median 19%, range 0–100%; n = 69 populations). A field experiment replicated at four sites demonstrated that variation in the strength of interactions with grazers and pollinators were responsible for among-population differences in relative fitness of the two morphs. Selection exerted by grazers favored the short-scaped morph, whereas pollinator-mediated selection favored the long-scaped morph. Moreover, variation in selection among natural populations was associated with differences in morph frequency change, and the experimental removal of grazers at nine sites significantly reduced the frequency of the short-scaped morph over 8 y. The results demonstrate that spatial variation in intensity of grazing and pollination produces a selection mosaic, and that changes in biotic interactions can trigger rapid genetic changes in natural plant populations.Spatial variation in the intensity of biotic interactions is an integral part of the geographic mosaic model of coevolution (1, 2), and may result in divergent selection and the maintenance of genetic variation in traits influencing the strength and outcome of interactions (3, 4). However, few studies have presented quantitative estimates of spatiotemporal variation in selection on traits influencing the outcome of biotic interactions across more than a handful of populations. In plants, variation in the composition of the mutualist and antagonist assemblages may result in spatially varying selection on morphology, phenology, and life-history traits (e.g., 5–12). Of particular interest are traits such as floral display that may be subject to conflicting selection from mutualists and antagonists, and where the magnitude and direction of net selection should depend on the relative strength of these interactions (13–20).Experimental manipulation of environmental conditions is a powerful approach to identify agents of selection and to determine the evolutionary consequences of changes in the selection regime (21, 22). Experimental manipulation of pollen deposition (6, 23, 24) and interactions with herbivores (25–28) can be used to assess the roles of pollinators and herbivores for patterns of selection. Conflicting selection on floral traits by pollinators and herbivores have been inferred in many systems (15, 19, 20), but no study has simultaneously manipulated the intensity of both interactions to determine their relative importance for spatiotemporal variation in selection on plant traits. There is also a lack of studies experimentally examining the importance of biotic interactions for the evolutionary trajectories of natural plant populations (28, 29).Here, we combine long-term observational data and field experiments to examine causes and consequences of spatial and temporal variation in selection on floral display in the rosette-forming, short-lived, perennial herb Primula farinosa. This species offers an ideal system to examine the outcome of conflicting selection by mutualists and antagonists. It is dimorphic for scape length, with a long-scaped morph displaying the umbellate inflorescence well above the soil surface and a short-scaped morph with the inflorescence very close to the ground. The segregation of scape morphs in controlled crosses is consistent with scape morph being determined by a single biallelic locus with a dominant allele coding for short scape (SI Text, SI Segregation of Scape Morphs in Crosses and Table S1). This difference in floral display affects interactions with both pollinators and antagonists. In previous studies, we have shown that seed production in the long-scaped morph is less likely to be limited by pollen availability (14, 30, 31), whereas the short-scaped morph is less frequently attacked by seed predators (14, 18, 32, 33). The inflorescence of the long-scaped morph should also have a higher probability of being damaged by grazers compared with that of the short-scaped morph. These interactions influence plant fitness largely via fruit production, which is a key fitness component of the study species and straightforward to quantify. In P. farinosa populations in the study area, plant mortality is high, overall fitness is strongly influenced by successful seedling recruitment (34), and total seed production is significantly correlated with number of intact mature fruits produced (r = 0.838, n = 442).We documented variation in scape morph frequencies among 69 populations and asked the following questions (1): Does selection on scape length vary among populations and years? We quantified selection on scape morph in about 40 populations in each of 2 y, and in five populations across 5 y (2). What are the drivers of variation in selection on scape morph? We documented the relationship between grazing intensity and selection on scape morph, and with a field experiment, we tested the hypothesis that spatial variation in grazing pressure and pollination intensity cause among-population variation in selection on scape morph (3). Do among-population differences in selection result in different evolutionary trajectories? We used observational data to examine whether changes in scape morph frequencies were correlated with estimates of selection on scape morph, and an 8-y field experiment to test whether the exclusion of grazers resulted in a reduced frequency of the short-scaped morph.