Describing the genetic characteristics of populations is the goal of the field of biology called population genetics . Under certain conditions, allele and genotype frequencies will remain the same from generation to generation. Populations will remain in H-W equilibrium unless they are acted upon by evolutionary forces . When sampling error result in changes in allele and genotype frequencies, we call this effect genetic drift. Since genetic drift is a random process, the direction and magnitude of changes cannot be predicted for any given population.
Genetic drift causes alleles to disappear from populations thereby reducing genetic variation.
Genotypes that adapt individuals to their environment will survive and leave more offspring having the same genotype than genotypes that are less well adapted to their environments. Natural selection was proposed as a mechanism of evolutionary change independently by Charles Darwin and Alfred Russell Wallace. Let's look at how natural selection can change the genetic characteristics of populations. Directional selection occurs when their is a directional change in the environment or a new mutation produces a new phenotype that is more adaptive than existing phenotypes. Figure 16.13 .