The ecological and evolutionary study of plant invasion processes is of exceeding importance in today's changing environment. However, few studies have addressed the impact of natural selection on invasive plant species. While scientists have been able to detect selection in natural populations, most studies are not replicated in space or time leading to unreliable statistical estimates and tentative causal analyses. My objective was to further our knowledge of selection dynamics in the wild by working in the area of invasion biology through studies that combine both field and controlled settings. It has been suggested that plant invasion affords us the ability to better assess the speed and predictability of local adaptation by natural selection, and that there are at least two mechanisms by which species can become invasive: through rapid local adaptation and/or through augmented phenotypic plasticity. I conducted a three-year phenotypic selection analysis on invasive Fallopia japonica (Japanese knotweed) and a native relative, Persicaria virginiana (woodland knotweed) and have shown that natural selection is generally strong in these field populations and is highly variable, in both strength and direction, through time. To assess the level of local adaptation in F. japonica and compare it to P. virginiana, I conducted a reciprocal transplant experiment in the field using two contrasting light regimes. Neither species is particularly locally adapted to different light regimes and both show evidence of possessing traits that respond plastically to the different light conditions. I examined this phenotypic plasticity with a common garden experiment. Both species show plasticity for traits relating to increased fitness in the different light treatments (ambient, and 50% shade) that would aid either one in the establishment and invasion of a novel light habitat. However, they do this with different strategies. P. virginiana has a more robust response, having similar fitness in both treatments and so can be classified here as a Jack-of-all-trades. F. japonica had a more opportunistic response with increased fitness in the higher quality (light) treatment, making it a Master-of-some.