Sexual reproduction produces more genetic variation due to the recombination and through opportunities for error during meiosis. Mutation creates some variability as well but this is not passed on from somatic cells. Natural selection picks better adapted phenotypes and the genotypes which lead to them, reducing the frequency of others (some of which may be completely eliminated). With recombination gone (due to asexual reproduction only occurring) and natural selection reducing variation, mutation will not be able to keep up and genetic variation will decline.
Work Step by Step
To get this answer review the basic ideas behind natural selection and adaptation, and then skim the chapters on genetics, making a list of things which cause genetic variability to appear. Consider which factors operate when asexual reproduction is the rule, and how they balance-- will those promoting diversity win or those reducing it?