HiSET Science: Processes Affecting the Gene Pool

Genetic Drift
Genetic drift is a microevolutionary process that causes random changes in allele frequencies that are not the result of natural selection. Genetic drift can result in a loss of genetic diversity. Genetic drift greatly impacts small populations. Two special forms of genetic drift are the genetic bottleneck and the founder effect. A genetic bottleneck occurs when there is a drastic reduction in population due to some change such as overhunting, disease, or habitat loss. When a population is greatly reduced in size, many alleles can be lost. Even if the population size greatly increases again, the lost alleles represent lost genetic diversity. The founder effect occurs when one individual or a few individuals populate a new area such as an island. This new population is limited to the alleles of the founder(s) unless mutations occur or new individuals immigrate to the region.

Gene Flow
Gene flow is a microevolutionary process in which alleles enter a population by immigration and leave a population by emigration.
Gene flow helps counter genetic drift. When individuals from one genetically distinct population immigrate to a different genetically distinct population, alleles and their genetic information are added to the new population. The added alleles will change the gene frequencies within the population. This increases genetic diversity. If individuals with rare alleles emigrate from a population, the genetic diversity is decreased. Gene flow reduces the genetic differences between populations.