Range-wide floral trait variation reflects shifts in pollinator assemblages, consistent with pollinator-mediated divergence despite generalized visitation

Floral trait evolution mediated by pollinators is important in the diversification of flowering plants, yet few studies have demonstrated the range-wide geographic variation in both floral traits and pollinators which represents a predicted precursor for pollinator-mediated speciation. This study explores whether geographic variation in pollinator interactions underlies the observed patterns of floral divergence both 1) among species of the Castilleja purpurea complex (C. purpurea, C. citrina and C. lindheimeri) and the congener C. sessiliflora, as well as 2) within C. sessiliflora, across its wide geographic range. We sampled floral visitors and floral traits (morphology and color) at 23 populations across a 1900 km-wide study area in 1–3 years, with reproductive fitness (fruit set) data for 18 of these populations. A wide diversity of pollinator functional groups visited the focal species, including bees, butterflies, hawkmoths and hummingbirds, and visitor assemblages varied among species and across geography. We identified relationships between floral traits and visitation by certain pollinator groups, which often aligned with predictions based on pollination syndromes. Despite visitor assemblages being largely generalized across most populations, we found that the observed changes in floral traits were associated with shifts in the relative frequencies of key pollinator functional groups. Hence this study demonstrates that variation in pollinator assemblages across the distributions of taxa may underlie divergence in floral traits and suggests that highly specialized relationships may not be required for early stages of pollinator-mediated floral divergence. Our extensive sampling of 23 populations over multiple years across a large geographic area highlights the value of range-wide studies for characterizing patterns of divergence mediated by ecological interactions.