Mutation is a physical change in a gene or chromosome. Migration - the movement of individual organisms into or out of a population - can alter allele frequencies in a population. The movement of genes into or out of a population through migration is called gene flow. Genetic drift is the phenomenon by which allele frequencies in a population change as a result of random events or chance. The Hardy - Weinberg principle states that for a population to remain in genetic equilibrium natural selection must not occur. Stabilizing selection is a type of natural selection in which individuals with the average form of a trait have an advantage in terms of survival and reproduction. Directional selection is a type of natural selection in which individuals with one of the extreme forms of a trait have an advantage in terms of survival and reproduction. Disruptive selection is a type of natural selection in which individuals with either of the extreme forms of a trait have an advantage in terms of survival and reproduction. Sexual selection is the preferential choice of a mate based on the presence of a specific trait.