 Evolutionary processes, such as genetic drift and natural selection, can cause allele frequencies to differ between populations.For example, exposure to malaria-bearing mosquitoes over many generations may have caused the higher allele frequency among certain ethnic groups for the sickle hemoglobin (HbS) allele, a recessive mutation that causes sickle-cell disease but confers resistance against malaria.In this way, HbS genotype has become associated with the malarial environment.