Genetic diversity and adaptation in native and invasive populations of Geranium carolinianum
Shirk, Rebecca Yukiko
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The detrimental effects of invasive species in natural and economically-important systems have been well-recognized for decades. However, the evolutionary mechanisms underlying successful invasions are not well understood. Here, I address questions concerning the role of genetic diversity and adaptation in the invasive plant Geranium carolinianum, a mixed-mating winter annual native to North America and invasive in China. Using neutral genetic markers, I reconstruct the colonization history of invasive Chinese populations and assess the extent of founder effects. I identify multiple introduction events from diverse source populations near Nanjing, Jiangsu province, with sequential founder effects reducing genetic diversity during range expansion. Because founder effects and genetic diversity can be significantly affected by mating system, I assessed variation in mating system in native and invasive populations to test the hypothesis that colonizing populations have increased rates of self-fertilization. Populations vary from highly selfing to highly outcrossing, but overall, outcrossing rates are higher in Chinese populations, and this difference has a strong genetic basis. This suggests that high rates of outcrossing reduced the impact of founder effects during the colonization of China. Finally, to relate neutral genetic diversity with adaptive potential in ecologically relevant traits, I test for adaptive evolution across invasive and native populations. Despite reduction in both neutral and quantitative genetic diversity in invasive Chinese populations, there is standing genetic variation and a signature of adaptation for many phenotypic traits. Surprisingly, many traits under selection are not strongly associated with climatic gradients. Adaptive trajectories appear to be different in native and invasive populations, which may be due to differences in selection pressures, underlying genetic constraints, or both. Finally, we found little evidence for the evolution of increased invasiveness, and it is unlikely in the short term that this species will transition into a major invasive weed. Combined, these studies directly link factors affecting genetic diversity in colonizing populations to adaptive evolution in an invasive plant. These results advance our understanding of evolutionary factors affecting species invasions in general, as well as species introductions to China.