Evolution can happen different ways, not just through natural selection. Sometimes, organisms that are well-suited to their environment can die off due to unexpected events, which can result in the loss of their helpful traits. This is called genetic drift, and it can have a big impact on a small population. Genetic drift is a random change in the genes that make up a population. It can reduce the genetic variation within a group, making it less adaptable to changes in its environment. Genetic drift is more likely to occur in small populations, where the loss of even one helpful trait can have a big effect.

When different populations of the same species get separated from each other, genetic drift can cause them to become even more different. As the genes in one population change, the differences between that group and others can increase. If this goes on long enough, the population can become a new species altogether.

Genetic Drift vs. Natural Selection

Natural selection and genetic drift are both mechanisms that can drive evolution, but they work in different ways. Natural selection means that the individuals with more advantageous traits are more likely to survive and pass on those traits to their offspring. Genetic drift, on the other hand, is a random event that can cause changes in the genetic makeup of a population, without regard to whether those changes are advantageous or not.

The key difference between the two is that natural selection leads to adaptive changes, meaning that the population is evolving to be more suited its environment. Genetic drift, on the other hand, often leads toaptive changes, meaning that the population is evolving without necessarily becoming better suited to its environment. This is because the individuals who survive genetic drift are not necessarily the ones with the most advantageous traits.

So, while both natural selection and genetic drift can drive evolution, natural selection is the more effective mechanism for creating adaptations that increase the chances of survival and reproduction in a given environment.

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

Bottleneck events can have a big impact on a population's genetic makeup. Imagine a population of organisms where there is a sudden reduction in the number of to This it consequences for the population's genetic diversity.

In a bottleneck event, the surviving individuals are not necessarily representative of the original population. They may have different genetic traits simply by chance. This can lead to a loss of genetic diversity, as certain alleles may be lost or become much less common.

To illustrate this, imagine a bottle full of candy balls. Originally, the bottle had five different colors of candy, each representing a different allele in the population. But after a bottleneck event, only three colors of candy balls make it through. This represents the loss of genetic diversity in the population, as only a subset of the original alleles are now present.

Bottleneck events can have long-lasting effects on a population's genetic makeup, as the reduced diversity may make it harder for the population to adapt to changes in its environment.

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The northern elephant seal is a species that has experienced a remarkable recovery from a population bottleneck. After being heavily hunted by humans in the 19th century, the population was reduced to less than 100 individuals. However, a population on Guadalupe Island in Mexico was protected and the population has since rebounded to an estimated 225,000 individuals by 2010. This is a great success story for conservation biology, as the species has recolonized much of its former range.

However, despite the population recovery, studies have shown that the northern elephant seal still suffers from genetic depletion. This means that there is not much genetic variation among individuals in the population. This is surprising, given the large population size and rapid recovery.

Compared to the southern elephant seal, which was not subjected to as much intensive hunting, the northern elephant seal has much less genetic diversity. This is unusual, as genetic depletion is more commonly seen in endangered species of much smaller sizes.

The genetic depletion of northern elephant seals is a reminder that even large populations can suffer from genetic problems if they experience a bottleneck event. This has important implications for conservation biology, as it suggests that protecting large populations alone may not be enough to ensure their long-term survival.

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.

The examples of Ellis-Van Creveld syndrome in the Amish population and the genetic depletion in cheetahs illustrate the long-term effects of genetic drift on populations. In both cases, populations experienced a bottleneck event that reduced their genetic diversity.

In the case of the Amish population, the founders carried a recessive allele responsible for Ellis-Van Creveld syndrome at a high frequency. As the population grew, individuals tended to mate with those who were genetically similar, leading to increased homozygosity for the harmful allele.

Similarly, cheetahs experienced two genetic drift events that reduced their genetic diversity. Anthropogenic pressures in recent times have further reduced their population size, making it difficult for cheetahs to adapt to changing environmental conditions.

The consequences of genetic depletion can be severe, as seen in both examples. In the Amish population, individuals with Ellis-Van Creveld syndrome experience physical abnormalities that can affect their quality of life. In cheetahs, genetic depletion has led to increased mortality in juveniles and difficulties with captive breeding.

These examples highlight the importance of maintaining genetic diversity in populations. Genetic drift can have long-lasting effects on a population's genetic makeup, and even large populations can if they experience a bottleneck event efforts must take into account the potential for genetic depletion and aim to preserve genetic diversity for the long-term survival of species.

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.

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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