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Shifts between bee and bird pollination
This is a story about change and lack of change. The subjects are penstemons. The flowers of certain closely related penstemons differ dramatically, whereas certain distantly related species are eerily similar. This is due to the repeated evolution of hummingbird pollination from bee pollination. Our chapter explores the biology surrounding these evolutionary shifts. Our chapter shall be an overview of studies done by ourselves and others on penstemon floral evolution as it pertains to shifts from bee to bird pollination. Detailed accounts of the methods and analyses have been or will be published elsewhere. The research program that we outline is intended to not merely document the extent to which plants conform to lists of syndrome characteristics. We take the opportunity here to focus on the evolutionary mechanisms that cause syndrome shifts, just as others have done for evolutionary shifts from outcrossing to selfing, from homostyly to heterostyly, from blooming early to blooming late, and from semelparity to iteroparity.