Floral nectar spurs and diversification

Key innovations are thought to be especially important adaptations that confer the ability to utilize resources in a novel manner and may therefore allow taxa to diversify. Here I review the data indicating that the evolution of floral nectar spurs represents a key innovation in Aquilegia and many additional groups. Aquilegia has apparently radiated recently and this radiation is closely associated with the evolution of nectar spurs. Numerous studies support the hypothesis that nectar spur morphology is important in plant reproduction and may provide a prezygotic reproductive isolating mechanism via differential pollinator visitation. The evolution of nectar spurs is highly correlated with increased species diversity across multiple independent lineages and thus provides strong support for the key innovation hypothesis. Previous studies have suggested that the evolution of nectar spurs may have been due to a simple genetic change in Aquilegia. I review these data and point out several cautions to this conclusion. Finally, I suggest that future research to determine the genetic basis for the development of nectar spurs will lead to especially interesting insights to the evolutionary origin of morphological novelties.