Combining ecological and genetic approaches, we have examined the process of biological invasion in three different plant systems. In multiflora rose (Rosa multiflora) we used genetic markers to quantify the relative contributions of sexual reproduction via seed versus vegetative reproduction via clonal spread to local patterns of invasion (Jesse et al. 2010). In purple loosestrife (Lythrum salicaria) we compared phenotypic plasticity of native European versus invasive North American populations in response to variation in water and nutrient levels (Chun et al. 2007). This research included development of a new, two-stage multivariate approach to test two important attributes of multivariate vectors of phenotypic change: the magnitude and direction of mean trait differences between environments. We also investigated the relative roles of natural selection, genetic drift and gene flow in the invasive process by contrasting phenotypical and neutral genetic differentiation among native and invasive purple loosestrife populations (Qst - Fst analysis; Chun et al. 2009). In California populations of wild radish (Raphanus sativus X R. raphanistrum), we tested the prediction that if two previously isolated taxa mutually assimilate through hybridization and subsequent biparental introgression, and if their introgressed descendants have the same or higher fitness than their parents, then gene flow should result in the local extinction of parental taxa via replacement by hybrid derivatives (Hedge et al. 2006). Using morphological and genetic marker evidence, we show that these dramatic events may occur rapidly, even in a few generations.