Mutation, genetic drift, gene flow and natural selection can increase variation in a population.
Mutation is the process by which new genetic variations arise within a population.
Genetic drift is a random fluctuation in the frequency of alleles in a population.
Gene flow is the movement of alleles into or out of a population as a result of migration.
Natural selection is the process by which certain traits become more or less common in a population over time due to the increased or decreased likelihood that individuals with those traits will survive and reproduce.