The pollination biology of the cactus Pachycereus pecten‐aboriginum was studied in a tropical location in western Mexico (ca. 18° N latitude) to compare with data from a northern population (ca. 28° N latitude). Throughout this range, the nectar‐feeding bat Leptonycteris curasoae is resident within the tropics but migratory in its northern range. The hypothesis was tested that if a predictable bat presence has been an important force in the evolution of pollination systems in columnar cacti, P. pecten‐aboriginum will have a specialized pollination system within the tropics and a generalized pollination system in northern populations. In both areas, pollination experiments showed that P. pecten‐aboriginum has a self‐incompatible, hermaphroditic breeding system. In the tropical area, flowers open at night and close early in the morning. Nectar is secreted only during the night, and flowers are exclusively pollinated by three species of nectar‐feeding bats, indicating a specialized pollination system. In contrast, anthesis and nectar secretion in northern populations occur during the night and day, allowing visitation and effective pollination by both nocturnal and diurnal pollinators. This study provides evidence of divergence mediated by pollinator predictability (resident vs. migrant), through shifts from short to long anthesis and nectar production periods from southern to northern populations.