Evolutionary impacts of assisted gene flow: Fitness consequences of hybridization along a geo-climatic gradient in an annual prairie legume

Abstract

Rapid climate change, alongside substantial habitat fragmentation, presents new challenges for biodiversity conservation in the tallgrass prairie. Therefore, capacity for adaptation in situ will be necessary for long-term population persistence. Populations with larger genetic variation likely have greater capacity for adaptation, but limited population sizes and restricted gene flow decrease this potential. Assisted gene flow (AGF) aims to increase adaptive capacity by introducing genetic material from populations that have undergone selection in warmer or drier environments. However, the addition of genetic diversity from geographically and genetically distant populations may disrupt local adaptation and ultimately decrease population performance. Here, I study how hybridization between populations over a geo-climatic gradient impacts their evolutionary capacity in response to warming temperatures. I used Chamaecrista fasciculata, a buzz-pollinated prairie annual to investigate the immediate and longer-term consequences of assisted gene flow in the Minnesota tallgrass prairie. In Chapter 1, I found that local gene flow increased population fitness compared to long distance or no gene flow scenarios in both ambient and warmed conditions and that the offspring of long-distance hybridization were limited by their capacity to germinate in Minnesota. In Chapter 2, I assessed how hybridization along this geo-climatic gradient impacted the capacity for ongoing adaptation and its dependence on environmental conditions. I found that the VA(W) of hybrids was more clearly expressed in warming conditions compared to the focal populations, thus providing evidence of capacity for adaptation in hybrid populations exposed to warmer temperatures. In Chapter 3, I evaluated the impact of hybridization on floral trait plasticity. Although I detected plasticity in almost all floral traits and found no significant difference in trait values between populations, the effect of those traits on population fitness differed significantly between populations. Together these chapters provide an empirical examination of the theoretical expectations of gene flow on a complex landscape and provide substantial evidence of the importance of maintaining genetically variable populations in a changing climate.