Colonization involves demographic bottlenecks that can reduce genetic diversity. Such genetic depletion may result in reduced fitness due to increased levels of inbreeding. This has raised the question of how invasive species can successfully expand outside their native range despite demographic bottlenecks - the Genetic Paradox of Invasions (GPI). Most discussion on the GPI focused on invasions, in which genetic depletion was prevented by specific mechanisms such as genetic admixture. Fewer attempts were made to identify invaders that spread successfully despite genetic depletion and to explain how these species could overcome the negative effects of inbreeding. In my thesis, I addressed these open aspects of the GPI with a literature review and synthesis, in which I developed a new approach to explain the GPI. Subsequently, I investigated this new approach empirically with an intercontinental field study, population genetic analyses, a breeding- and a stress manipulation experiment.