Study of adaptive evolutionary changes in populations of invasive species can be advanced through the joint application of quantitative and population genetic methods. Using purple loosestrife as a model system, we investigated the relative roles of natural selection, genetic drift, and gene flow in the invasive process by contrasting phenotypic and neutral genetic differentiation among native European and invasive North American populations (QST - FST analysis). Our results indicate that invasive and native populations harbor comparable levels of AFLP variation, a pattern consistent with multiple independent introductions from a diverse European gene pool. However, it was observed that the genetic variation reduced during subsequent invasion, perhaps by founder effects and genetic drift. Comparison of genetically-based quantitative trait differentiation (QST) to its expectation under neutrality (FST) revealed no evidence of disruptive selection (QST > FST) or stabilizing selection (QST < FST). One exception was found for only one trait (the number of stems) showing significant sign of stabilizing selection across all populations. It suggests that the difficulties in distinguishing the effects of non-adaptive population processes and natural selection. Multiple introductions of purple loosestrife may have created a genetic mixture from diverse source populations and increased population genetic diversity, but its link to the adaptive differentiation of invasive North American populations needs further research.