Causes of Gene Pool Changes

Mutations

HIV virus in action.

Mutations is a change the occur in the DNA of an organism. If this defect is able to be passed through the generations that come, inheritable mutation, it can cause a change in the gene pool of a population. There are three types of mutation, some are neutral, some are beneficial, and most of them are harmful for the individual that has it.
Mutation increase the diversity of a gene pool.  The more genetic variation, caused by mutation, the greater the chances of selective advantage, which will change the environment at some degree.
A good example of mutation in humans is the presence of some individuals that are resistant to the HIV virus, due to a homozygous mutation, which will cause the lack of the functioning receptor of the virus in the cell, which will prevent the individual to be infected.

Gene Flow 

Gene flow describes the change on gene pools due to migration of individuals. It may increase the genetic variation in one population, however, it can decrease in another population -- the one that emigration is occurring.

Wolves are usually the best example of gene flow, since they migrate several times during their life spam. Thus, some wolves in certain region share some genetic traits with wolves from a different region.

Non-Random Mating

Random mating in a population means that there is no prediction of who is going to mate with who, thus leading to a greater genetic variation. However, in the many animal populations, some individual choose to mate with other individuals based on their physical and mental traits. This leads to a non-random mating because it prevents some individuals to breed. Conclusion of this is that some animals will not contribute with the gene pool of the next generation of their specie.
Another example of non-random mating is inbreeding. This is a process which animals that are closely related breed together.

In many population of lions, only the alpha-male is allowed to breed with all the females.

Genetic Drift

Genetic drift is the change in allele frequencies due to chance events in small breeding populations. When only few individuals reproduce in small population it may cause the extinction of certain alleles. A good example of Genetic Drift would be if we took a very small population of five roses ( 3 AA, 1 Aa, and 1 aa). if only the AA type reproduce, in a couple of generation, the allele A will be the only one present in the genetic code of individuals.
Genetic Drift can also occur when the population decreases due to climate changes, disease, or fragmentation of habitat. However, in stable and large population, Genetic Drift are unusual to happen.

The Founder Effect

The founder effect is the process of which few individual start a new and isolated population, limiting the genetic variation. This often occurs on islands. When a really small number of birds start a new population in a isolated island, their offspring will carry genetic information from the parents and other birds from the same species, but that live in different regions. However, as time passes by, the newest generation of the birds on the isolated island are more likely to be very different genetically compared to the birds that are found in other non-isolated regions.

The Bottleneck Effect

Bottleneck Effect is the change in Gene Pool that is a result from a rapid decrease in population size. It usually occurs in species that are close to extinction. this effect can be related  to the Genetic Drift.

Natural Selection and Human Activities

These are very common factors that influence the gene pools in many species. While natural selection "eliminate" weaker alleles, human activities are able to vanish alleles even if they are helpful for the specie.