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Genetic Drift Definition
Any population can be subjected to genetic drift, but its effects are stronger in small populations. The dramatic reduction of a beneficial allele or genotype can decrease the overall fitness of a small population because there are few individuals with these alleles to begin with. It is less likely that a large population would lose a significant percentage of these beneficial alleles or genotypes. Genetic drift can reduce the genetic variation within a population (through the removal of alleles or genes) and the changes that this drift produces are generally non-adaptive.
Genetic drift is a random change in allele frequencies within a population. It is one of the main mechanisms that drive evolution.
Another effect of genetic drift occurs when species are divided into several different populations. In this situation, as the allele frequencies within one population shift due to genetic drift, the genetic differences between this population and the other ones can increase.
Usually, populations of the same species already differ in some traits as they adapt to local conditions. But since they are still from the same species, they share many of the same traits and genes. If one population loses a gene or allele that was shared with other populations, it now differs more from the other populations. If the population continues to diverge and isolate from the other ones, this can eventually lead to speciation.
Genetic Drift vs. Natural Selection
Natural selection and genetic drift are both mechanisms that can drive evolution, meaning that both can cause changes to the genetic composition within populations. However, there are important differences between them. When evolution is driven by natural selection it means that the individuals better suited to a particular environment are more likely to survive and will contribute more offspring with the same traits.
Genetic drift, on the other hand, means that a random event happens and the surviving individuals are not necessarily better suited to that particular environment, as better suited individuals may have died by chance. In this case, the surviving less suited individuals will contribute more to the next generations, thus the population will evolve with less adaptation to the environment.
Therefore, evolution driven by natural selection leads to adaptive changes (that increase the survival and reproductive probabilities), while changes caused by genetic drift are usually non-adaptive.
Types of Genetic Drift
As mentioned, genetic drift is common among populations, as there are always random fluctuations in the transmission of alleles from one generation to the next. There are two types of events that are considered more extreme cases of genetic drift: bottlenecks and the founder effect.
Bottleneck
When there is a sudden reduction in the size of a population (usually caused by adverse environmental conditions), we call this type of genetic drift a bottleneck.
Think of a bottle filled with candy balls. The bottle originally had 5 different colors of candy, but only three colors passed through the bottleneck by chance (technically called a sampling error). These candy balls represent the individuals from a population, and the colors are alleles. The population went through a bottleneck event (such as a wildfire) and now the few survivors only carry 3 of the 5 original alleles the population had for that gene (see Fig. 1).
In conclusion, the individuals who survived a bottleneck event did so by chance, unrelated to their traits.
Figure 1. A bottleneck event is a type of genetic drift where there is a sudden decrease in the size of a population, causing a loss in alleles in the population's gene pool.
Northern elephant seals (Mirounga angustirostris) were widely distributed along the Pacific Coast of Mexico and the United States in the early 19th Century. They were then heavily hunted by humans, reducing the population to less than 100 individuals by the 1890s. In Mexico, the last elephant seals persisted on Guadalupe Island, which was declared a reserve for the species’ protection in 1922. Astonishingly, the number of seals rapidly increased to an estimated size of 225,000 individuals by 2010, with extensive recolonization of much of its former range. Such a rapid recovery in population size is rare among endangered species of large vertebrates.
Although this is a great accomplishment for conservation biology, studies show that there is not much genetic variation among individuals. Compared to the southern elephant seal (M. leonina), which was not subjected to as much intensive hunting, they are highly depleted from a genetic standpoint. Such genetic depletion is more commonly seen in endangered species of much smaller sizes.Genetic Drift Founder effect
A founder effect is a type of genetic drift where a small fraction of a population gets physically separated from the main population or colonizes a new area.
The results of a founder effect are similar to those of a bottleneck. In summary, the new population is significantly smaller, with different allele frequencies and probably lower genetic variation, compared to the original population (Fig. 2). However, a bottleneck is caused by a random, usually adverse environmental event, while a founder effect is mostly caused by the geographical separation of part of the population. With the founder effect, the original population usually persists.
Figure 2. Genetic drift can also be caused by a founder event, where a small fraction of a population gets physically separated from the main population or colonizes a new area.
The Ellis-Van Creveld syndrome is common in the Amish population of Pennsylvania, but rare in most other human populations (an approximate allele frequency of 0.07 among the Amish compared to 0.001 in the general population). The Amish population originated from a few colonizers (about 200 founders from Germany) who probably carried the gene with a high frequency. The symptoms include having extra fingers and toes (called polydactyly), short stature, and other physical abnormalities.
The Amish population has remained relatively isolated from other human populations, usually marrying members of their own community. As a result, the frequency of the recessive allele responsible for the Ellis-Van Creveld syndrome increased among Amish individuals.
The impact of genetic drift can be strong and long-term. A common consequence is that individuals breed with other very genetically similar individuals, resulting in what is called inbreeding. This increases the chances of an individual inheriting two harmful recessive alleles (from both parents) that were low in frequency in the general population before the drift event. This is the way genetic drift can eventually lead to complete homozygosis in small populations and magnify the negative effects of harmful recessive alleles.
Let’s look at another example of genetic drift. Wild populations of cheetahs have depleted genetic diversity. Although great efforts have been made in cheetah recovery and conservation programs for the past 4 decades, they are still being subjected to the long-term effects of previous genetic drift events that have hindered their ability to adapt to changes in their environment.
Cheetahs (Acinonyx jubatus) currently inhabit a very small fraction of their original range across eastern and southern Africa and Asia. The species is classified as Endangered by the IUCN Red List, with two subspecies listed as Critically Endangered.
Studies estimate two genetic drift events in ancestral populations: one founder effect when cheetahs migrated into Eurasia and Africa from the Americas (more than 100,000 years ago), and the second one in Africa, a bottleneck coinciding with large mammal extinctions in the Late Pleistocene (last glacial retreat 11,084 - 12,589 years ago).Due to anthropogenic pressures over the last century (such as urban development, agriculture, hunting, and stocking for zoos) the cheetah’s population size is estimated to have decreased from 100,000 in 1900 to 7,100 in 2016.The genomes of cheetahs are 95% homozygous on average (compared to 24.08% for outbred domestic cats, which are not endangered, and 78.12% for the mountain gorilla, an endangered species). Among the harmful effects of this impoverishment of their genetic makeup are elevated mortality in juveniles, sperm development abnormalities, difficulties to reach sustainable captive breeding, and high vulnerability to infectious disease outbreaks. Another indication of this loss of genetic diversity is that cheetahs are able to receive reciprocal skin grafts from unrelated individuals without rejection issues (usually, only identical twins accept skin grafts with no major issues).Genetic Drift - Key takeaways
- All populations are subject to genetic drift at any time, but smaller populations are more impacted by its consequences.
- Genetic drift is one of the main mechanisms that drive evolution, along with natural selection and gene flow.
- The main effects that genetic drift might have within populations (especially small populations) are non-adaptive changes in allele frequency, reduction in genetic variation, and increased differentiation between populations.
- Evolution driven by natural selection tends to lead to adaptive changes (that increase the survival and reproductive probabilities) while changes caused by genetic drift are usually non-adaptive.
- A bottleneck is caused by a random, usually adverse, environmental event. A founder effect is mostly caused by the geographical separation of a small part of the population. Both have similar effects on the population.
- Extreme genetic drift events can have a long-term impact on a population and prevent it from adapting to further changes in environmental conditions, with inbreeding being a common consequence of genetic drift.
1. Alicia Abadía-Cardoso et al., Molecular Population Genetics of the Northern Elephant Seal Mirounga angustirostris, Journal of Heredity, 2017.
2. Laurie Marker et al., A Brief History of Cheetah Conservation, 2020.
3. Pavel Dobrynin et al., Genomic legacy of the African cheetah, Acinonyx jubatus, Genome Biology, 2014.
https://cheetah.org/resource-library/
4. Campbell and Reece, Biology 7th edition, 2005.
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Frequently Asked Questions about Genetic Drift
What is genetic drift?
Genetic drift is a random change in allele frequencies within a population.
How is genetic drift different from natural selection?
Genetic drift differs from natural selection mainly because changes driven by the first are random and usually nonadaptive, while changes caused by natural selection tend to be adaptive (they enhance survival and reproductive probabilities).
What causes genetic drift?
Genetic drift is caused by chance, also called sample error. The alleles frequencies within a population are a “sample” of the parents’ gene pool and can shift in the next generation just by chance (a random event, not related to natural selection, can prevent a well-fitted organism to reproduce and pass on its alleles).
When is genetic drift a major factor in evolution?
Genetic drift is a major factor in evolution when it affects small populations, as its effects will be stronger. Extreme cases of genetic drift are also a major factor in evolution, like a sudden reduction in population size and its genetic variability (a bottleneck), or when a small part of a population colonizes a new area (founder effect).
Which is an example of genetic drift?
An example of genetic drift is the African cheetah, whose genetic makeup is extremely reduced and exhibits high mortality and vulnerability to infectious diseases. Studies estimate two events: a founder effect when they migrated into Eurasia and Africa from the Americas, and a bottleneck coinciding with large mammal extinctions in the Late Pleistocene.
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