Genetic drift is the change in allele frequencies of a population due to random chance events, such as natural disasters.

Genetic drift describes the effect of a change on populations in the absence of positive or negative selection pressures. Through random sampling, or the survival of and reproduction of a random sample of individuals within a population, allele frequencies within a population may change.

Genetic drift in a population can lead to the elimination of an allele from that population by chance. In this example, the brown coat color allele (B) is dominant over the white coat color allele (b). 

A scenario in which populations might experience a strong influence of genetic drift is during a natural disaster that kills a large portion of the population randomly and results in a large portion of the genome suddenly being wiped out is known as the bottleneck effect. 

Another situation is if some portion of the population leaves to start a new population in a new location or if a population gets divided by a physical barrier of some kind. Those individuals are unlikely to be representative of the entire population, which results in the founder effect. The founder effect occurs when the genetic structure changes to match that of the new population’s founding fathers and mothers.

Small populations are more susceptible to the forces of genetic drift. Large populations, on the other hand, are buffered against the effects of chance. 

Key Points

• Genetic drift is the change in the frequency of an allele in a population due to random sampling and random events that influence the survival and reproduction of those individuals.

• The bottleneck effect occurs when a natural disaster or similar event randomly kills a large portion of the population, leaving survivors that have allele frequencies that were very different from the previous population.

• The founder effect occurs when a portion of the population separates from the old population to start a new population with different allele frequencies.

Key Terms

genetic drift: an overall shift of allele distribution in an isolated population, due to random sampling

founder effect: a decrease in genetic variation that occurs when an entire population descends from a small number of founders

random sampling: a subset of individuals (a sample) chosen from a broader set (a population) by chance

selection pressure: a factor that affects mortality or fertility that tends to make a population change genetically.

allele: one of two or more alternative forms of a gene that arise by mutation and are found at the same place on a chromosome.

bottleneck effect: an example of genetic drift that happens when the size of a population is severely reduced

dominant: a relationship between alleles of a gene, in which one allele masks the expression (phenotype) of another allele at the same locus