Caveats
In the model, in order to demonstrate the role of floral abundance in determining the optimal pollination system, plants could evolve on any combination of pollinators without evolutionary restrictions. Plants, however, have constrained abilities to track the most effective pollination system (Fenster et al. 2004). Evolution occurs along the lines of least resistance contingent on the genetic material available, making some shifts in pollination systems more likely to occur in certain plant lineages (Stebbins 1970; Van der Niet & Johnson 2012). Those evolutionary limitations represent an important nuance in flower diversification. Plants should adapt to the most effective pollination strategy that represent a line of least resistance. Therefore, many plants are expected to not be optimally adapted to their pollination environment. For instance, bilateral symmetry is considered to facilitate floral specialization (Sargent 2004) and radially symmetrical flowers may have limited ability to evolve floral specialization. Additionally, the evolution of specialized pollination rewards, such as oil and fragrance, that allow pollination by specific pollinator functional groups (e.g. oil and fragrance collecting bees) might often be contingent on particular floral traits (preaptations) already being in place (Armbruster 2011). Although transitions between “normal” and specialized rewards and between different specialized rewards are frequent (Armbruster 2011), shifts in pollination systems that require transitions between rewards might be more evolutionary constrained than shifts between pollinators using the same reward.
Even in the absence of evolutionary constraints, because there is a net flux of genes from large to small populations, the smallest, and often peripheral, populations of a given plant species are expected to match the local selective pressures relatively poorly (García-Ramos & Kirkpatrick 1997; Kirkpatrick & Barton 1997; Kay & Sargent 2009). Considering the importance of floral abundance in determining the optimal pollination strategy, small peripheral populations should rarely match such optimal conditions. Furthermore, large fluctuations in population abundance might prevent adaptation to the most effective pollinator when such fluctuations happen fast enough that adaptive tracking is not possible.
For a given plant species, its flower abundance in a community is the result of the species abundance and the average number of flowers produced per individual. Although I focused on variation in species abundance in the model, variation in average number of flowers per individual plant should have an additional direct effect on fitness: all else being equal, an individual that produces more flowers will also produce more gametes (pollen and ovules). Hence, in the instances where adapting to more effective pollinators is costly, flower evolution might conflict with the direct advantage of producing more flowers.