Genetic Drift

Ecology, Genetics, Zoology

Genetic Drift Definition

Genetic drift is a change in allele frequency in a population, due to the forces of chance. Oftentimes, mutations in cells can have no effect. These changes in genetics can increase or decrease in a population, simply due to chance. Although variations of genes (also known as alleles) can be selected for because they help or hinder an organism with reproducing. When the allele is not responsible for the change in its frequency in a population, genetic drift is acting on the allele.

Random genetic drift chart

Genetic drift is much more likely in smaller populations of organisms, as seen in the graphic above. The individual lines in the graph above track the frequency of alleles in a given population. When the population is small, and many alleles exist (as in the top graph), any of the alleles can quickly become fixed or extinct in the population. When there are many organisms in the population (see bottom graph), there is less of a chance of losing an entire allele, because many organisms carry the allele and it is less likely they will all be wiped out.

Genetic drift can easily be confused with natural selection. The difference is whether or not the allele is actively participating in the change in allele frequencies. If the allele affects an organism in a way that causes more reproduction of the DNA, the allele will increase in frequency. If it causes harm, it will decrease. This is caused by the allele’s direct effects on the organism and the environment. This is natural selection. When the allele is increased or decreased simply because it was present in the organisms that survived, this is genetic drift.

Types of Genetic Drift

Population Bottleneck

A population bottleneck is a type of genetic drift in which a population’s size severely decreases. This could be caused by predators, disease, or competition from another source. As the population becomes smaller, certain alleles change in frequency in the population simply because the organisms that carry them get killed or eaten. The other alleles increase in frequency, simply because they are the only alleles left. This type of genetic drift can be seen when people don’t take their entire course of antibiotics.

Antibiotics kill harmful bacteria in your system, regardless of what alleles they have. The populations of harmful bacterial that cause symptoms are severely reduced until they no longer cause symptoms. If someone quits their antibiotic regiment before all the bacteria are killed, a small population is left. This much smaller population could have allele frequencies that are very different from the original population of bacteria. These changes do not reflect the success or failure of the different alleles, but rather the effects of random selection of bacteria. The new alleles will dominate the population until selection or more genetic drift cause the allele frequencies to change.

Founder Effect

In another type of genetic drift known as the founder effect, a new population is formed, or founded, in a new location. If this new population does not interact and reproduce with the main population, the allele frequencies in this population will be much different from that of the parent population. Many island contain species that only exist on a single island because of the founder effect. For instance, if only two birds of a species land on an island, their alleles alone will account for the diversity present. While these alleles will dominate at first, mutations will arise in the population that will lead to new adaptations. Because the two populations of birds are separated, they cannot share this new adaptation. With enough time, the two populations can diverge to a point which they can no longer interbreed. This is one way that separate species are created.

Examples of Genetic Drift

In a hypothetical population

A population of rabbits lives in the woods. The rabbits have many different coat colors: black, brown, tan, white, grey, and even red. In the population, the different alleles that create coat color are equally distributed. A disease comes into the rabbit population and kills 90% of the rabbits. The only rabbits that are left are red and grey rabbits, simply by chance. The genes have thus “drifted” from 6 alleles to only 2. This is an example of a bottleneck effect.

In real life

Genetic drift happens all the time in populations, although it is not easily seen. Often, mutations arise that have little effect on the organism. These mutations get passed on if the organism reproduces, and do not get passed on if the organism does not survive. Although genetic drift used to be thought of in only small populations, even large populations experience genetic drift of certain alleles, because a small number of individuals carry the alleles. Whether or not these alleles are duplicated is not a function of natural selection, but of chance. Many alleles come or go in populations without affecting great change.

Related Biology Terms

  • Founder Effect – When a small number of individuals establish a population that is geographically isolated from the main population, changing the allele frequency in the smaller population.
  • Population Bottleneck – When a population is reduced, and thus certain alleles are lost or increased in frequency purely by chance.
  • Natural Selection – The process by which allele frequencies are changed in response to the force the environment puts on the phenotypes they create.
  • Allele Frequency – The number of a certain allele in a population compared to the overall number of alleles.

Quiz

1. An allele arises in a population that helps an organism digest food. While the allele starts in only one organism, it increases in allele frequency because it allows organisms that carry it to reproduce more. What is this an example of?
A. Natural Selection
B. Genetic Drift
C. Founder Effect

Answer to Question #1

2. A population of white, black, and tan mice live in a laboratory. A scientist separates two rats at random out of the population and starts a new rat colony. The two rats he pick are both tan. The new colony, after the two tan rats reproduce, is entirely tan. What is this an example of?
A. Founder Effect
B. Natural Selection
C. Population Bottleneck

Answer to Question #2

3. Why is genetic drift more frequent of an occurrence than scientists once thought?

A. Many populations do not have enough members to avoid genetic drift.
B. There are many more events showing the founder effect than once thought.
C. Oftentimes, neutral mutations get passed on.

Answer to Question #3
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