Why are all colour combinations not equally represented as flower-colour polymorphisms?

Flower-colour polymorphism within the British flora appears more common in species whose flowers typically contain pink, purple or blue anthocyanin pigments rather than other coloured pigments. In this study we test the hypothesis that this variation in anthocyanin pigmentation may be maintained by selection related to environmental heterogeneity and stress tolerance. Observations were made of stem pigmentation, shoot dry mass and seed production in five polymorphic species under different droughted and well watered conditions. The results show that over both treatments the morphs did not differ in their fitness. However, a significant morph times treatment interaction revealed that the pigmented plants performed relatively better in the droughted conditions, while the unpigmented plants performed relatively better in the well watered treatment. The results support the idea that anthocyanin based flower-colour polymorphisms may be better considered as polymorphism for the presence or absence of anthocyanin pigmentation throughout the entire plant. This variation may be maintained by selection related to environmental heterogeneity and stress tolerance.