Herbicide resistant invasive weeds provide a unique system in which to study the evolution of adaptive traits. In most species, it can be difficult to determine which novel traits are adaptive as opposed to fixed due to drift and/or linkage. However, the adaptive trait, herbicide resistance, and the corresponding selective environments are extremely tractable. During the last decade, resistance to the herbicide glyphosate has evolved in weedy populations of Lolium sp (Poaceae) populations in agricultural systems of California, USA. Earlier work indicated that a non-synonymous mutation in the 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) gene is associated with the resistant phenotype within California. The research conducted for this dissertation assessed the processes facilitating the evolution of glyphosate resistance within California populations of Lolium sp and if selection for the adaptive trait is associated with sympatric population divergence.

The ability of an organism to adapt to new selection pressures and evolve new traits is dependent on the presence of adaptive genetic variation within populations. Research described in this dissertation assessed whether of genetic variability resulting from hybridization and/or gene duplication contributed to the evolution of glyphosate-resistance in Lolium populations. In Chapter 1, I used neutral nuclear and chloroplast DNA to assess the evolutionary history of Lolium sp., and determine if populations were of hybrid origin. The analysis indicated that California glyphosate-resistant and susceptible plants were not hybrids, but were most closely related to the L. multiflorum group. In Chapter 3, I assessed if duplication of the EPSPS locus contributed to L. multiflorum’s potential to evolve resistance to glyphosate. The analysis detected at least two EPSPS loci, one of which has glyphosate-resistant type alleles and one of which only has glyphosate-susceptible type alleles. These data support the hypothesis that new phenotypic functions can evolve from duplicated genes.

It has been hypothesized that strong divergent selection can disrupt gene flow between populations and cause sympatric populations to diverge genetically. Since glyphosate treatment is an extremely strong selection agent, I studied if populations under varying intensities of glyphosate treatment had diverged at neutral Simple Sequence Repeat (SSR) loci. As described in Chapter 2, patterns of genetic differentiation and population substructure were not associated with glyphosate-response phenotypes and glyphosate-treatment environments. However, gene flow between individuals differed between glyphosate-response phenotypes. Over time, this variation in gene flow may lead to genetic differentiation.