Example the process of Speciation

All population that reproduce sexually contain some degree of genetic variation, likewise, all environments will have different selection pressures. As a population reproduces, the individuals best suited to that environment, in terms of climate, avoiding predators, catching prey etc, are more likely to reproduce and therefore they will be more likely to pass on the alleles that make them adapted to that specific environment. This means that the concentration of those alleles in the gene pool will increase, while the unfavourable alleles will decrease.
In cases where the population splits up and migrates to different environments, the selection pressure on the population will change, and therefore, individuals with a different set of alleles will be favoured compared to the first environment. As the population reproduces, the same process described above takes place, except different phenotypes are selected for, therefore the alleles that are more abundant in the gene pool will be different. Eventually, as this continues, the gene pools in the 2 populations become distinctly different, as does the resultant phenotype, this is known as speciation.