Final answer:
Genetic drift is the factor that does not increase genetic variation within a population; it can actually reduce variation by random allele frequency changes.
Step-by-step explanation:
The factor that will not increase genetic variation within a population is c) Genetic drift. Genetic drift is a random process that can reduce genetic variation by causing random changes in allele frequencies due to chance events. In contrast, options a) Mutation and d) Migration introduce new alleles to a population, thereby increasing genetic variation. b) Overdominance preserves variation by giving a survival advantage to heterozygotes, and e) Genetic rescue involves introducing new genetic material to counteract the effects of inbreeding and reduce the likelihood of extinction.