Although there are no options listed, it's almost universally correct that the thing most likely to change the gene pool of a population of chipmunks is something in their immediate environment, like a change in plant-life, predators, and/or food sources.
NON DISJUNCTION: when 2 homologous chromosomes move to the same pole during meiosis. One daughter cell will be missing one chromosome while the other will have an extra chromosome.