Evolutionary theory posits that adaptive potential is proportional to additive genetic variation in a population, but this prediction has not been tested empirically. I present results of an experimental study of laboratory populations of the red flour beetle Tribolium castaneum to test the degree and rate of adaptation to a novel resource as a function of genetic variation. I used four beetle strains differing in their acceptance of and performance on a novel resource (corn flour) to create populations with four levels of genetic variation, and measured their growth on the novel resource (corn flour) and on a mixture of traditional and novel resources (wheat and corn flour). To quantify adaptation, I used population growth rate and assays of fitness and development rate of individuals from each population. I quantified niche width using stable carbon isotope ratios. As predicted by theory, I find that niche expansion and degree of adaptation to corn, both increase as a function of genetic variation. However, the positive effect of genetic variation is sensitive to strain composition. Trade-offs between use of the alternative resources, as well as competitive outcomes between individuals of different strains may limit population niche expansion in spite of available genetic variation.