Genetic Drift: Definition, Examples & Types

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  • 0:00 Genetic Drift Defined
  • 0:45 Types of Genetic Drift
  • 1:30 Population Bottleneck Examples
  • 2:10 Founder Effect Examples
  • 3:10 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, 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.

Genetic Drift Defined

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

Types of Genetic Drift

There are two major types of genetic drift: 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.

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

Population Bottleneck Examples

While natural disasters can 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 1800s. 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 Examples

When a group of individuals leave a population, the genetic makeup of the new group is unlikely to be similar to the gene pool of the larger population that they left.

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