EVOLUTIONARY MAINTENANCE OF GEOGRAPHIC VARIATION FOR FLOWER COLOR IN LEAVENWORTHIA STYLOSA

Abstract

Flower color polymorphism is a striking feature of some angiosperm species, especially when it is geographically structured. Geographic patterns of flower color can be maintained by either non-adaptive processes, like genetic drift, or by natural selection, acting directly or indirectly. Leavenworthia stylosa, an endemic to the cedar glades of middle Tennessee, occurs mostly in monomorphic populations of yellow or white flowered morphs. The overall objective of my study was to understand why most of the L. stylosa populations are monomorphic and what maintains the geographic pattern of flower color variation of the species. I studied the pollinator assemblages, flower color preferences, and their constancy in foraging and seed predation across the species range. To check the differences in abiotic factors in different sites, I studied the soil chemistry, water holding capacity and water loss in white and yellow sites. Additionally, I conducted a series of reciprocal transplant experiments at different life history stages and evaluated the performance of the early life history stages of the two morphs across a water-related stress gradient. In both white and yellow sites, pollinators favored the white morph over the yellow morph across the species range. Seed predation was lower on the white morph compared to the yellow morph. Bombylius showed exclusive visits to the white morph in white sites favoring the white morph and preventing the yellow morph from increasing in frequency. In yellow sites, the fitness-enhancing pollinators preferred yellow morph and fitness-reducing seed predators showed no preference. There was no difference between white and yellow sites in soil chemistry. But yellow sites had higher water holding capacity and higher water loss rate than white sites. During the early life history stages the white morph was more successful over yellow morph while during late life history stages yellow morph was more successful over white morph. The yellow morph showed limited evidence for local adaptation in flower number and fruit number survival through reproductive stage. Stress experiments showed conflicting results. Thus, it is likely that the maintenance of geographic variation for flower colors of L. stylosa is influenced by multiple selective agents including pollinators, seed predators and abiotic conditions.