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contrastto nectar-flower species visited by a wide diversity of pollinators. The vast majority of the oil specieslack nectar as a pollinator reward; this suggests that the ancestors of plants with oil flowers had eithernectar-less flowers or faced strong selection against the production of nectar once oil secretion evolved.Monttea aphylla is one of the few species that simultaneously offers oil and nectar in the same flowerto pollinators. In particular, we studied phenotypic selection in 16 populations in the Monte desert ofArgentina that spanned the entire geographical range of the species. In each population, we determinedthe relationship between plant pollination success and floral rewards. Positive directional selection dif-ferentials were detected in three of the populations. One northern population favoured selection on oilproduction, two central or southern populations favoured selection on nectar production. Directionalselection gradients were consistent with the above results and correlational selection was significant forone northern population where pollination success was favoured by an increased selection on oil and adecreased selection on nectar. Geographic variation in phenotypic selection models showed latitudinalincrease in directional selection on nectar. Geographic variation in visitation frequencies of the specialistoil-collecting bee and relative abundance of other floral sources were related to an increase in directionalselection on nectar. Thus, evolution of rewards appears to respond to a geographic mosaic where eitheroil or nectar are favoured in different contexts of ecologically specialised oil collecting bees. Althoughphenotypic selection was detectable in few and was absent in most populations, the general pattern wasconsistent with current geographic differentiation in reward amounts.