Ever since I worked as a weed biologist in Swaziland and the Lower Amazon Basin of Brazil, I have been fascinated by the ecology and evolution of invasive plants. Where do they come from, why are they invasive and can they be studied to investigate contemporary evolution? My early studies concerned two quite different problems: 1) crop mimicry in barnyard grasses (24) and 2) the relative importance of clonal vs. sexual reproduction in water hyacinth (10, 11). Later, in collaboration with former Ph.D. students Chris Eckert and Rob Colautti (now both on the faculty at Queens University) we investigated the evolution of local adaptation in the wetland invader Lythrum salicaria. By sampling seed collected from representative populations across the invasive eastern North American range and using common garden and quantitative genetic analyses we demonstrated: 1) abundant heritable variation for most life history traits; 2) reduced variance and increased skew of quantitative traits with northward migration; 3) strong clines in flowering time and plant size associated with latitude and length of growing season; and 4) evidence for genetic constraints at range limits between flowering time and size (265). Further, reciprocal transplant experiments and measurements of natural selection provided evidence indicating a fitness advantage to plants that flower earlier at a smaller size in northern populations (271). There are still many fascinating questions concerning the invasion genetics of purple loosestrife and we are currently investigating evidence for population differentiation for floral display traits. I have also conducted a collaborative study with colleagues in China from Beijing Normal University on a world wide survey of genetic diversity in water hyacinth (268). More recently, I have been collaborating with Nicolay Cunha, Haoran Xue and Stephen Wright on comparative population genomic studies of clonal diversity Eichhornia crassipes and E. azurea in the Pantanal wetlands of Brazil.