Castellanos, María Clara. “The evolution of transitions between pollination modes in Penstemon.” (2003): 2197-2197.
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Adaptation to pollinators is considered a major force shaping floral form and function. One approach to studying the mechanisms behind floral diversification is to concentrate on pollinator shifts. A pollinator shift, or transition, is the change in a plant lineage, over evolutionary time, from being adapted to using one type of pollinator to adapting to another type. The purpose of this thesis was to investigate how mechanical aspects of pollination affect the evolution of transitions from insect-pollination to hummingbird-pollination, using the plant genus Penstemon as a model. Two discrete types of Penstemon flowers are distinguished, associated with either bee- or hummingbird-pollination. Convergent transitions to hummingbird-pollination have occurred at least 14 times independently within the group.
The superior pollination efficiency of hummingbirds over bees might account for the repeated transitions to hummingbird-pollination. I compared the two pollinators visiting bee-adapted Penstemon strictus and bird-adapted P. barbatus. On P. strictus, bees and birds had similar pollen transfer efficiencies, but hummingbirds were more efficient on P. barbatus than on P. strictus.
When specializing on hummingbird pollination, floral characters may change to improve the morphological fit between bird and flower, or to exclude the less efficient bees, or both. I experimentally modified floral traits in P. strictus to resemble bird-syndrome flowers, and found effects on pollen transfer, suggesting that restricting pollination by bees played an important role in the evolution of hummingbird-flowers. Additionally, I contrasted patterns of pollen presentation in the two types of flowers across penstemons. Previous models predicted less gradual pollen presentation by flowers with more efficient pollinators. After controlling for relatedness, I found less restricted presentation in hummingbird-pollinated penstemons, consistent with the finding that birds are more efficient than bees.
I propose that a change in nectar rewards that makes a Penstemon patch more attractive to hummingbirds can trigger a shift to more-efficient bird-pollination by simple floral changes. Such changes include characters that explicitly exclude bees. Shifts to hummingbird-pollination can be explained under such conditions, but unless the conditions are sustained, bee-pollination constitutes a stable strategy. Similar trade-offs probably contribute to the formation and maintenance of diversity in flower form across angiosperms.