Answer:
There are four ways I know of that the frequency of alleles in a gene pool can change over time.
1. natural selection - If one organism has an allele that is beneficial to the survival of the population it will become selected for and will be mated with. Over time the amount of offspring with this gene/allele will increase while other genes that were not selected for because may decrease.
2. genetic drift - By chance one organism may not mate during the mating season or they die. For whatever reason the organism doesn't pass on their gene and so, especially if this gene is rare, it may decrease the frequency of this allele in the gene pool or make it extinct all together. Smaller populations are more susceptible to genetic drift as it causes their allele frequencies to fluctuate more dramatically.
3. population bottleneck - Due to some chance occurrence such as an environmental change a large majority of the population dies. The alleles from those that survive may not necessarily represent the alleles from the larger population and some alleles could have been wiped from the gene pool resulting in a change in the allele frequency.
4. - Founder effect - A small portion of a larger population is removed and repopulates itself in a different environment. The alleles of the small population don't necessarily represent the alleles of the larger population and so may not have alleles that were present in the larger population. This also changes the allele frequency