Genetic diversity is required for natural selection to act, but natural selection can reduce or eliminate diversity. What process can restore genetic diversity to a population?

Natural selection reduces genetic diversity by favoring certain alleles over others. Mutation is the ultimate source of new genetic variation - it creates novel alleles that didn't exist before. Genetic drift reduces diversity (especially in small populations), and stabilizing selection specifically reduces variation by favoring intermediate phenotypes. Only mutation introduces new genetic diversity.