Spurring plant diversification: are floral nectar spurs a key innovation?

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Abstract
High levels of species diversity in taxonomic groups have often been explained by a key innovation. However, the difficulty in establishing a causal role between a proposed key innovation and increased species diversity, as well as in substantiating that diversity patterns are different from null models has led to major criticisms of key innovation hypotheses. Here we show that patterns of diversification within and among clades that have evolved floral nectar spurs strongly support the hypothesis that floral nectar spurs represent a key innovation. Both reproductive success and reproductive isolation can be influenced by simple changes in nectar spur morphology and the acquisition of nectar spurs in a wide array of plant groups is highly correlated with increased species diversity.