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Genetic Drift: Definition, Examples & Types

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Sarah Friedl

Sarah has a Master's degree in Zoology and a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Genetic drift reduces genetic variability of a population by decreasing the size of the population. The change in population size and variability often leads to new species and unique populations.

Definition

Populations of organisms are constantly changing and adapting to their environment. Drastic changes in environmental conditions can sometimes cause drastic changes to the gene pool of the population. Genetic drift is when chance events cause changes in frequencies of alleles in a population. Alleles are the genetic variations in a population, and they are the driving force behind the evolution of that population.

The smaller the population, the greater the impact genetic drift will have. The impact is greater because there are fewer individuals, and the gene pool is smaller. If the effects of genetic drift are strong enough, the allele may be completely removed from the population, reducing the amount of variation in the population's gene pool.

Types

Two major types of genetic drift are population bottlenecks and the founder effect.

A population bottleneck is when a population's size becomes very small very quickly. This is usually due to a catastrophic environmental event, hunting a species to near extinction, or habitat destruction. When the size of the population is reduced so quickly, many alleles are lost and the genetic variation of the population decreases.

Bottlenecks quickly reduce population size through chance events
population bottleneck

The founder effect is when a few individuals in a population colonize a new location that is separate from the old population. This also greatly reduces the population size, as well as reduces the genetic variability of that population.

A new colony has a less diverse genetic makeup than the original population
founder effect

Examples

Population Bottlenecks

While natural disasters may cause genetic drift, humans are often at fault. American bison were once very prevalent in North America, but were hunted close to extinction in the late 1800's. Though the population is recovering, the rapid decrease in population size has led to a population with very little genetic variability.

Similarly, the number of the greater prairie chicken, a bird found in the prairies of Illinois, was drastically reduced during the 19th and 20th centuries due to hunting and habitat destruction. The number of birds went from millions to fewer than 50, and as a result more than 30% of the alleles were lost forever.

Founder Effect

When a group of individuals leaves a population, the genetic makeup of the new group is not likely be similar to the gene pool of the larger population they left.

Two examples of this are the Galapagos Islands in South America and the island of Madagascar in Africa. A few individuals from large population groups relocated to these islands, and as the islands migrated farther from the main land the colonizing populations became more isolated. The small amount of genetic variation on the island led to new species that are now endemic - only found in one place.

Human genetic diseases are often the result of the founder effect. In 1814, a small population of British colonists founded a new colony on a group of islands in the Atlantic Ocean. One of the founders had an allele for a form of blindness, and the frequency of the allele on the island is now ten times higher than the population the colonists originally came from.

Summary

Genetic drift reduces genetic variability of a population by decreasing the population size. The bottleneck and founder effects are the two main types of genetic drift that reduce population size and genetic variability. These changes can drastically alter the genetic makeup of a population, especially if the population size is small. If the changes are drastic enough, certain alleles may go extinct, being lost from the population forever.

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