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Abstract
Natural selection is a major evolutionary factor driving population differentiation and local adaptation. When plants are exposed to different levels of abiotic factors, such as water limitation, their ability to survive and reproduce may be affected. Continuous exposure to such factors can lead to adaptations to resist the stressor and potentially lead to adaptive differentiation. Identification of selective agents, such as drought, could help to determine if populations are adaptively differentiated and on what traits selection is acting. Granite outcrops of the southeastern US are rare, geographically isolated habitats harboring many endemic plant species. Additionally, they are subjected to water limitation due to their patchy shallow soils, high evaporation rates and severe run-off from the rock surface. Helianthus porteri (Compositae, A. Gray, Pruski) is an endemic annual sunflower found on these granite outcrops. It germinates in late March and begins flowering in late August, thus persisting through the harshest environmental conditions. Here we investigated the potential for adaptive differentiation to drought among populations of H. porteri. We found drought to be an important selective agent, leading to differential plant performance among three natural populations of H. porteri and significantly affecting plant survival. While we found that these same populations are genetically differentiated when grown in a common environment, the trait patterns observed were not consistent with local adaptation for drought. Investigation of population genetics of H. porteri revealed that there is little genetic structure among populations (FST), indicating they are genetically similar across the species range. Additionally, we found high levels of genetic diversity (He) within populations. Taken together, our results indicate that while drought is an important selective agent among populations of H. porteri, the high levels of genetic diversity indicate that gene flow may be counteracting the effects of selection. Thus there does not appear to be adaptive differentiation to drought among populations of H. porteri. Future studies which directly assess local adaptation, such as reciprocal transplants, are needed to assess if populations are local adapted to factors other than drought, which may be influencing population differentiation.