Polyploidy, resulting from whole genome duplication (WGD), is ubiquitous in nature and reportedly associated with extreme environments and biological invasions. Polyploidy comes with steep costs, raising questions about polyploid establishment, but acquire many other trait changes. Are the surprisingly high number of polyploid species a consequence of deterministic trait changes? And, under what circumstances is polyploid establishment and coexistence with their lower ploidy ancestors possible? In this project, I want to go beyond the trait-centered attempts to determine the drivers of polyploid establishment by considering their full cost-benefit balance and a fundamental ecological context. Putative establishment, persistence and coexistence with the competing ancestral ploidy is determined by how the new polyploid performs within the environmental conditions, usually including competition with its ancestor. Using the uniquely powerful experimental system of Spirodela polyrhiza (greater duckweed), I will investigate shifts of phenotype, cost-benefit balance and variability therein in replicated WGDs and in optimal and different stressful environments. Furthermore, I will study invasion of each ploidy in a population of the other in experimental microcosms, drawing on insights of modern coexistence theory. This will have polyploids and their diploid ancestors compete to quantify stabilizing and equalizing mechanisms in optimal, stressful and fluctuating environments.