Much effort has been devoted to understanding the function of extrafloral nectaries (EFNs) for ant-plant-herbivore interactions. However, the pattern of evolution of such structures throughout the history of plant lineages remains unexplored. In this study, we used the hypothesis of plant defense mediated by ants as a theoretical framework to test more specific hypotheses about the adaptive role of EFNs during plant evolution. Emphasis was given to different processes (neutral or adaptive) and factors (habitat change and tradeoffs with new trichomes) that may have affected the evolution of ant-plant associations. We measured 7 EFN quantitative traits in all 105 species included in a well-supported phylogeny of the tribe Bignonieae (Bignoniaceae), and collected field data on ant-EFN interactions in 32 species. We identified a positive association between ant visitation (a surrogate of ant guarding) and the abundance of EFNs in vegetative plant parts, and rejected the hypothesis of phylogenetic conservatism of EFNs, with most traits presenting K-values < 1. Modeling the evolution of EFN traits using maximum likelihood approaches further suggested adaptive evolution, with static-optimum models showing a better fit than purely drift models. Finally, the abundance of EFNs was associated to habitat shifts (with a decrease in the abundance of EFNs from forest to savannas), and a potential trade-off between the abundance of EFNs and estipitate glandular trichomes (trichomes with sticky secretion) was detected. These evolutionary associations appeared as evidence of divergent selection between species, and could also explain K-values < 1. Overall, our results suggest that the evolution of EFNs was likely associated with adaptive processes. Experimental studies with multiple lineages, including savanna and forest species, could improve our understanding of the functional role of nectaries on plants.