Genetic drift is a change in the frequency of alleles in a population, due to a random selection of certain genes. Often, mutations in DNA cannot have any effect on the physical form of an organism. These genetic changes can increase or decrease in a population, just by chance.
Genetic drift explained
Although variations in genes (also called alleles) can be selected because they help or hinder an organism, other mutations may not affect. When the allele itself is not responsible for changing its frequency in a population, genetic drift acts on the allele. The graph below shows different trajectories for the same genes over time. As you can see, the frequency of these genes can change dramatically over time, especially with smaller populations.
In larger populations, the allelic frequency of each gene remains relatively stable. This happens because genes do not affect physical form and therefore do not have natural selection pressure against or for the allele. In smaller populations, the frequency of these genes can fluctuate widely. Some settle within the population, while others disappear. These fortuitous events that cause frequency changes are called genetic drift.
Genetic Drift Examples
In a hypothetical population
A population of 100 rabbits lives in the woods. Rabbits have many coat colors: black, brown, tan, white, gray, and even red. In the population, the different alleles that create the color of the coat are also distributed. A disease enters the rabbit population and kills 98 rabbits. The only rabbits that remain are red and gray, just by chance. The genes have thus “derived” from 6 alleles to only 2. This is an example of a bottleneck effect.
In real life
Genetic drift occurs all the time in populations, although it is not easily seen. Often, mutations occur that have little effect on the body. These mutations are passed on if the organism reproduces and are not passed on if the organism does not survive. Although genetic drift was once only seen in small populations, even large populations experience genetic drift from certain alleles. This happens because a small number of individuals carry the alleles. The duplication or not of these alleles is not a function of natural selection, but chance. Many alleles come or go in populations without affecting large changes.
How is genetic drift caused?
Genetic drift is much more likely in small populations of organisms, as shown in the image found in this article. The individual lines of the graph follow 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 attach or die out in the population. When there are many organisms in the population (see last graph), there is less chance of losing an entire allele, because many organisms carry the allele and they are less likely to be all wiped out.
Genetic drift can easily be confused with natural selection. The difference is whether or not the allele actively participates in changing allele frequencies. If the allele affects an organism in a way that results in greater reproduction of DNA, the allele will increase in frequency. If it causes damage, it will decrease. This is due to the direct effects of the allele on the body and the environment. It is natural selection. When the allele is increased or decreased simply because it was present in the random organisms that survived, it is a genetic drift.
Types of genetic drift
- Population bottleneck
- Founder Effect
A population bottleneck is a type of genetic drift in which the size of a population decreases dramatically. Competition, disease or predation lead to these massive reductions in population size. The pool of alleles is now determined by the organisms that are not dead. Some alleles increase in frequency simply because they are the only alleles left. This type of genetic drift can be seen when people do not take their full course of antibiotics.
Antibiotics kill harmful bacteria in your system, regardless of their alleles. Antibiotics cause a massive reduction in harmful bacteria. This stops the symptoms of the disease. A small population will survive if a patient stops their antibiotic prematurely. This much smaller population could have very different allele frequencies from the original bacterial population. 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 greater genetic drift causes the frequency of alleles to change.
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 of this population will be very different from those of the parent population. Many islands contain species that only exist on one island due to the founder effect. For example, if only two birds of a species land on an island, their alleles alone will explain the diversity present.
Although these alleles dominate initially, mutations will occur in the population which will lead to new adaptations. This new adaptation remains with the founding population. With sufficient time, the two populations can diverge to such an extent that they cannot reproduce. Species often separate this way.
Genetic drift vs gene flow
In genetic drift, alleles change frequency within a population due to random sampling. As a result, it does not produce adaptations. Two mechanisms cause genetic drift. The first is the bottle effect. This is a genetic drift in a population after having undergone a catastrophic event such as a flood. The bottleneck occurs when the allele frequency of a major trait in the original population is reduced because many people who carry the allele have died. This causes the majority of the surviving population to die, leaving some individuals to chance as survivors. The other mechanism is called the founder effect. This is when some members of a population separate and create their own group. Due to the random sampling that created the new group, the frequency of alleles can change dramatically depending on the selective pressures that are exerted on the individuals.
Gene flow differs from genetic drift because it is the transfer of alleles or gametes from one population to another. It occurs when a population migrates or is geographically isolated. This is different from the genetic drift observed with the founder effect, where the new group forms in an area that does not have an existing population