Genetic Drift

Definition

During genetic drift, certain genes are randomly selected in a population, which results in a change in allele frequency. In many cases, mutations in DNA do not affect the fitness of an organism. Simply by chance, genetic changes can increase or decrease in a population.

Genetic Drift Explained

Although variations of genes (also known as alleles) can be selected for because they help or hinder an organism, other mutations can have no effect. When the allele itself is not responsible for the change in its frequency in a population, genetic drift is acting on the allele. As you can see, the frequency of these genes can change drastically over time, especially with the smallest populations.

In the largest populations, the allele frequency of each gene stays relatively stable. This happens because the genes are not affecting fitness, and thus do not have a natural selection pressure against or for the allele. In the smallest populations, the frequency of these genes can fluctuate greatly. Some become fixed within the population, while others disappear. These chance events which lead to changes in frequency are called genetic drift.

Genetic Drift Examples

In a hypothetical population

A population of 100 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 98 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. 

This happens 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.

What Causes Genetic Drift?

Genetic drift is much more likely in smaller populations of organisms, as seen in the image found in this article. The individual lines in the graph track the frequency of alleles in a given population. When the population is small and many alleles exist (see the first graph), any of the alleles can quickly become fixed or extinct in the population. 

When there are many organisms in the population (see the last 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 random 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. Competition, disease, or predation leads to these massive decreases in population size. The allele pool is now determined by the organisms which did not die. Some 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. Antibiotics cause a massive reduction in harmful bacteria. This stops symptoms of the disease. A small population will survive if a patient quits their antibiotic early. 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 a 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 islands 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. This new adaptation stays with the founding population. With enough time, the two populations can diverge to a point which they can no longer interbreed. Species often separate in this way.

Genetic Drift vs. Gene Flow

The concept of genetic drift is often confused with the concept of gene flow in biology. Gene flow is the movement of genes between populations, species, or between organisms. For instance, bacterial cells are able to transfer genes between different cells as a method of gaining antibiotic resistance. Populations of organisms exhibit gene flow when individuals from one population migrate and breed with a new population.

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