The introduction and spread of non-native species has become a global ecological and environmental problem. We will be able to develop a deeper understanding on the background of invasiveness from the studies of evolutionary changes that invasive species undergo from their introduction to the establishment and aggressive spread. The ultimate goal of this dissertation is to contribute a deeper knowledge on the ability of invasive species to respond to the novel environment and prevail via evolutionary changes, using purple loosestrife (Lythrum salicaria L. Lythraceae) as a model system.;The dissertation is composed of several journal papers. Chapter 1 is a general introduction and Chapter 2 is a review paper summarizing recent studies on the leading hypotheses on the mechanisms for invasiveness: EICA (Evolution of Increased Competitive Ability), the evolution of phenotypic plasticity, local adaptation, allelopathy, hybridization/polyploidization. Chapter 3 tests the evolution of phenotypic plasticity hypothesis with an empirical study on the phenotypic plasticity in native vs. invasive populations subject to experimentally manipulated water and nutrient environments. This study has been published in Ecology and expanded into a larger international collaborative project that aims to detect the genetic difference in demography and phenotypic plasticity between native and invasive populations. Essentially this is a reciprocal transplant study with multiple common gardens involving collaborators from New Jersey and Germany. The results from the Iowa common garden have been presented in Chapter 4, employing structural equation modeling approach (path analysis) to describe causal relationships between fitness and traits that contribute to fitness (fitness-related traits) considering the ontogeny of purple loosestrife. Chapter 5 is a technical note on the development of a genomic DNA extraction protocol for plants that have a large amount of secondary metabolites including polysaccharides and polyphenols. Chapter 6 is a population genetic study on the same populations used in Chapter 3, to examine the effect of major evolutionary forces on invasive populations - whether the formation of invasive populations is purely due to stochastic events such as genetic drift and migration, or instead by disruptive and/or stabilizing selection leading to locally adapted populations in the North America.