natural selection and genetic drift are the two most important causes of evolutionary change. how do they differ? natural selection and genetic drift are the two most important causes of evolutionary change. how do they differ? natural selection focuses on the survival of individuals, while genetic drift refers to which individuals actually reproduce. only genetic drift has been observed in populations of finite size. natural selection involves a population moving toward a goal, while genetic drift is not directed. only natural selection can change the frequencies of alleles in a population. genetic drift is nonadaptive; it changes allele frequency without regard to fitness.