Over time, a series of random occurrences can cause an allele to become more or less common in a population. This is called . When a population is severely reduced by an environmental disaster such as a fire, the result is a due to the reduced genetic diversity of the survivors. 3. The can occur if a small group of organisms migrates to a new location and becomes isolated from the rest of the population.

Over time, a series of random occurrences can cause an allele to become more or less common in a population. This is called Genetic drift.

When a population is severely reduced by an environmental disaster such as a fire, the result is a Bottleneck effect due to the reduced genetic diversity of the survivors.

The Founder effect can occur if a small group of organisms migrates to a new location and becomes isolated from the rest of the population.

Explanation:

Genetic drift is an evolutive force. It is the random change that occurs in the allelic frequency of a population through generations. Its effects are harder in a small-sized population, meaning that the magnitude of this change is inversely related to the size of the original population.  

Genetic drift results in some alleles loss -including the beneficial ones-, while some other alleles get fixated. Low-frequency alleles are the most likely to be lost. The changes produced by genetic drift accumulate in time and results in a loss of genetic variability within a population.  

Genetic drift affects a population and reduces its size dramatically due to a disaster or pressure -bottleneck effect- or because of a population split -founder effect-. The bottleneck effect most likely affects smaller populations.  

The bottleneck effect -a case of genetic drift-, mostly affects smaller populations after the occurrence of a natural disaster or some human action -such as extensive hunting, for instance-. These events might act as a pressure that reduces significantly the number of individuals in a population. In these situations, some alleles are lost, and the survivors have a different genetic charge than the one of the original population. There might be a reduced genetic variability, with a possibility of developing a peculiar allelic component. If the survivors in the population carried or developed a mutation, probably this mutation passed from generation to generation.  Founder effect refers to the origin of a new population from only a few individuals that are coming from a bigger-sized population. These founder individuals, which are carrying some of the genes of the original population, settle down in a new area and reproduce.  The new and small population might or might not be genetically representative of the original one. Some rare alleles might be exceeded or might be lost by complete. Consequently, when the small population increases in size, it will have a genetically different composition from the original one. In these situations, genetic variability is reduced, and there exists the possibility of developing a peculiar allelic composition. When the number of individuals that originated the new population is low, the founder effect will be very extreme because the genetic drift effects are inversely proportional to the original number of individuals.