The spatial scale of adaptation in common ragweed (Ambrosia artemisiifolia)

Abstract

Environmental variation at both large and small spatial scales can influence the ecological and evolutionary trajectory of plant populations. Decades of common garden and reciprocal transplant experiments have demonstrated that local adaptation is widespread. However, most experiments examining local adaptation compare populations at a single spatial scale. For my dissertation, I combined field experiments and population genetics to examine how environmental variation at three spatial scales—small (i.e., among populations), regional (i.e., urban vs. rural), and continental (i.e., across latitudes)—shapes local adaptation in common ragweed (Ambrosia artemisiifolia L.). I found evidence for adaptation to urban environments and latitudinal gradients in climate. More specifically, I found evidence that phenotypic divergence at small spatial scales (e.g. within a city) can be greater than divergence found at larger spatial scales. Across latitudes, I found that photoperiod influences the fitness of common ragweed populations, and the relevant spatial scale for adaptation varies across life history stages. In addition, for northern populations of ragweed, I found that climate change is already causing maladaptation. Lastly, my population genomic work with RNA-seq identified new traits of interest that may be involved in local adaptation, including stomatal opening and closing and seed dormancy. I discuss these results with regard to how they can help us understand local adaptation more broadly and in turn predict how plants may respond to climate change.