Variation originally comes from mutation, and the rate of mutation might often be slow compared to the rate of natural selection. Other factors that could counter this and keep alleles in a population include diploidy/the existence of heterozygotes, selection for different phenotypes at different times, selection for more than one trait at one time, and gene flow from other populations with different allelic frequencies.
Work Step by Step
This question asks, in essence, that you review the chapter for variation-maintaining mechanisms. These are found in the section on the Hardy-Weinberg equilibrium and elsewhere in the chapter.