The changes in the allele frequencies within a population results in the change in the gene pool and the population. The forces that are responsible for change in the allele frequencies include the natural selection, genetic drift, mutation and the gene flow.
Natural selection is the process in which organisms that are better adapted to the environment are chosen and they produce more offspring. Genetic drift is the variation in the relative frequency of different genotypes in a small population leading to the disappearance of particular genes. Mutations are the sudden, heritable changes in the genetic composition of an organism. Gene flow refers to the transfer of the genetic variation from one population to the other.