Aim: Mutualistic systems involving plants and brood pollinators show intriguing cases where a single plant is pollinated by several closely related sympatric species. This study aims at identifying the mechanisms involved in such an extreme case, where 10 sympatric weevil species are only associated with African oil palm inflorescences. We assessed the patterns and timing of speciation of this peculiar assemblage to explore the main drivers of its diversification. Location: Tropical Africa. Taxon: The African oil palm Elaeis guineensis Jacquin (Arecaceae, Cocoseae), weevil genera Elaeidobius Kuschel and Prosoestus Faust (Curculionidae, Derelomini). Methods: We first reassessed the status of the weevil species assemblage associated with the African oil palm based on a range of molecular species delimitation analyses. Second, we reconstructed a dated molecular phylogeny using Bayesian inference and maximum likelihood and carried out historical biogeographical analyses accounting for the main biogeographical barriers in tropical Africa. Results: Our analyses generally confirm the validity of the species associated with the African oil palm and indicate that they form a monophyletic group. Dating analyses indicate that they originated about 7-8 million years ago (Mya) and that most speciation events occurred during the Pliocene and Pleistocene. Historical biogeography analyses reveal that one geographical barrier played a bigger role during their diversification. Age estimates suggest that extant weevil species remained in sympatry along the range of their host during the last 0.2 Mya. Main conclusions: This case represents a remarkable example of a speciose mutualistic monophyletic insect assemblage. Our results indicate that past climatic oscillations in Western and Central Africa - where cycles of range contractions and expansions of the host drove the divergence of the weevils in allopatry - and the predominant role of a mountain range were instrumental in the emergence of the sympatric assemblage.