Patterns of phenotypic selection for oil and nectar in Monttea aphylla (Plantaginaceae) in a geographic mosaic of interactions with pollinators

•A plant with flowers that simultaneously offer oil and nectar as pollinator rewards.•Phenotypic selection on rewards varied across a changing ecological backdrop.•Selection occurred in a few hotspots in opposite directions at geographic extremes.•Oil was favoured where specialist oil-collecting bees were scarce.•Geographic selection structure agreed with phenotypic differentiation.