Why are rewardless flowers diverse? A simulation study

Background/Question/Methods

Many species of rewardless flowers exhibit polymorphism in flower color. Previous empirical studies suggest that learning by pollinators imposes negative frequency-dependent selection on flower color and maintains color polymorphism in rewardless flowers. However, the evolutionary mechanism underlying the color polymorphism of rewardless flowers has not been fully understood theoretically.

To explore the evolutionary mechanism, I developed an individual-based simulation model describing the evolution of flower color in a population of rewardless flowers with animal-driven pollen transfer. The model assumed flower color as a continuous trait evolving through the accumulation of small changes due to point mutation, and explicitly considers the color vision and learning processes of pollinators. In addition, I performed a simulation with parameter values from a real pollination system on a rewardless flower species with two polymorphic flower colors.

Results/Conclusions

Starting from a population consisting of only one flower color, a polymorphism of discrete colors evolved when pollinators could learn rewardless flowers but with inaccurate discrimination between similar flower colors. When pollinator’s color discrimination was accurate, flower color in a population evolved to show a continuous distribution across the whole gradation of possible flower colors. These results suggest that inaccurate color discrimination by pollinators is a key component for the evolution of discrete color polymorphism.

When pollinators perform inaccurate color discrimination, intermediate flower colors will suffer high risk of being recognized as rewardless flower by pollinators, because these pollinators have learnt flowers of similar colors as rewardless flower. This reduces fitness of plants with intermediate colors and thus causes disruptive selection, a general evolutionary force driving polymorphism.

In the simulation with parameter values from a real system with two polymorphic flower colors, a comparable flower color polymorphism with two or three colors was successfully evolved, supporting the plausibility of the evolutionary mechanism in my simulation model.

In conclusion, I argue that inaccurate color discrimination by pollinators causes a disruptive selection that drives the evolution of color polymorphism of rewardless flowers