Biological invasions represent major threats to biodiversity as well as large-scale evolutionary experiments. Invasive populations have provided some of the best known examples of contemporary evolution [3-6], challenging the classical view that invasive species are genetically depauperate because of founder effects. Yet the origin of trait genetic variance in invasive populations largely remains a mystery, precluding a clear understanding of how evolution proceeds. In particular, despite the emerging molecular evidence that multiple introductions commonly occur in the same place, their contribution to the evolutionary potential of invasives remains unclear. Here, by using a long-term field survey, mtDNA sequences, and a large-scale quantitative genetic experiment on freshwater snails, we document how a spectacular adaptive potential for key ecological traits can be accumulated in invasive populations. We provide the first direct evidence that multiple introductions are primarily responsible for such an accumulation and that sexual reproduction amplifies this effect by generating novel trait combinations. Thus bioinvasions, destructive as they may be, are not synonyms of genetic uniformity and can be hotspots of evolutionary novelty.