Polyploidy, which occurs in roughly half of all flowering plants and an even higher percentage of grasses, is thought to be a major driver of adaptation. Higher numbers of copies of each gene in polyploid genomes can increase genetic diversity, which could drive shifts in habitat preference, adaptability, and fitness. To test the effects of increased ploidy, we compared genomic diversity, environmental niche, and fitness responses across climatic gradients between tetraploid and octoploid switchgrass. We found that the octoploids contained novel combinations of the ancestral tetraploid genetic diversity, which was linked to the expansion of switchgrass into unsuitable habitats for tetraploid populations. Our experiments revealed evidence of niche divergence, differential fitness, and a generalistspecialist trade-off between cytotypes.