Pollinator-mediated floral selection in the introduced range of <i>Mimulus guttatus</i>

Introduced flowering plants often encounter novel pollinator assemblages in their non-native ranges, potentially reshaping plant-pollinator interactions and driving evolutionary change. We tested whether variation in pollinator assemblages mediates local adaptation of introduced Mimulus guttatus (Phrymaceae). Using a reciprocal transplant experiment between the southern and northern extremes of the British Isles, we combined pollinator censuses with measurements of floral traits and fruit production to assess pollinator assemblages, phenotypic differentiation and selection. Pollinator assemblages differed between sites, with more abundant bumblebees in the northern garden. Pollinator visitation rate was positively associated with fruit set, indicating that pollinators remain essential for sexual reproduction in the introduced range. Plants from northern populations received more visits in the northern than in the southern garden, and more visits than southern populations in the northern garden. This pattern paralleled a previously reported local and home advantage in fruit production. Corolla width was under positive and stabilizing selection mediated by bumblebees in the north, suggesting that pollinator-driven selection contributes to floral differentiation between regions. Our findings demonstrate that M. guttatus has undergone pollinator-mediated adaptive differentiation within two centuries in the British Isles, highlighting how strong biotic pressures can generate rapid ecological diversification during plant introductions.