The Phylogeographic History and Contemporary Evolution of the Invasive Species Avena barbata Pott ex Link in California
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Understanding the factors that precipitate a successful colonization of a new geographic region is a major goal in both ecology and evolution. In my doctoral thesis, I examined evolutionary patterns (Ch. 2), ecological patterns (Ch. 3), and eco-evolutionary processes (Ch. 4 & 5) associated with the invasion of a highly-selfing, annual grass, Avena barbata Pott ex Link into California from the Mediterranean Basin. Because colonizing populations may experience severe genetic bottlenecks, multiple introductions of different genetic variants may be an important source of variation facilitating adaptation to the novel habitat. In the second chapter of my thesis, I show that there have been at least three introductions of A. barbata into California, and that substantial spatial mixing has occurred between haplotypes within populations in California. There was also evidence of population structure due to a north-south cline. Although niche overlap between the endemic and novel habitat may facilitate biological invasions, niche shifts may also occur during invasion. In the third chapter, a species distribution model (SDM) was employed to characterize and compare the niches in the Iberian Peninsula (home range of A. barbata) to niches in California (invaded range). Niche separation was observed between the two geographic regions, suggesting that A. barbata occupies a different environment in California and may be evolving. Traits with a known genetic basis may prove useful in tracking contemporary evolutionary change and provide insights on adaptive versus neutral processes. In the fourth chapter, I compared the contemporary frequency of two binary, heritable characters: lemma color and leaf sheath pubescence to that reported in past studies from the 1970s. Due to natural selection, I found that light lemma color and leaf sheath pubescence had increased in frequency. Although intuition suggests that large plants should be at a selective advantage within populations, recent discussion in the literature suggests the opposite. The idea that small plants with lower total fecundity may be selectively favoured contradicts basic Fisherian principles. In the fifth chapter, using a simulation and field data from A. barbata recombinant inbred lines (RILs), I show that individuals with large body size are always favored.