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.