Abstract
Significance Long-term morphological stasis is a major feature of the paleontological record, but the explanation for this pattern has been controversial. Here, we use the species-rich plant clade Malpighiaceae to determine whether long-term floral stasis is maintained by selection or developmental and genetic constraint. Our results, which use an explicit phylogenetic framework and comparative methods, strongly support selection. We hypothesize that this floral morphology has been maintained over tens of millions of years via their specialized pollinator interaction with oil-collecting bees. To our knowledge, this study in which stasis has been connected to such a plant-pollinator mutualism, is unique, and opens the door to future research on how this association may have enhanced diversification in this plant lineage.