Evolution occurs in various ways, not solely through natural selection. There are instances where well-adapted organisms can perish due to unforeseen events, leading to the loss of advantageous traits. This phenomenon is known as genetic drift, and it can have a profound impact on small populations. Genetic drift refers to the random changes that occur in the genes of a population. It can decrease the genetic variation within a group, making it less adaptable to environmental changes. Genetic drift is more likely to occur in smaller populations, where the loss of a single beneficial trait can have a major effect.
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Genetic Drift vs. Natural Selection
Natural selection and genetic drift are both mechanisms that drive evolution, but they function differently. Natural selection involves the survival and reproduction of individuals with advantageous traits, ensuring the passing on of those traits to subsequent generations. On the other hand, genetic drift is a random event that can cause changes in the genetic makeup of a population, regardless of whether those changes are advantageous or not.
The crucial distinction between the two lies in the fact that natural selection leads to adaptive changes, where the population evolves to be better suited to its environment. In contrast, genetic drift often leads to non-adaptive changes, meaning the population evolves 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.
Therefore, while both natural selection and genetic drift contribute to evolution, natural selection is the more effective mechanism for creating adaptations that increase the chances of survival and reproduction within a specific environment.
Types of Genetic Drift
As mentioned earlier, genetic drift is a common occurrence in populations due to random fluctuations in allele transmission from one generation to the next. There are two types of events that are considered extreme cases of genetic drift: bottlenecks and the founder effect.
Bottleneck events can have a significant impact on the genetic makeup of a population. Imagine a population of organisms where there is a sudden reduction in the number of individuals. This reduction has consequences for the population’s genetic diversity.
In a bottleneck event, the surviving individuals may not accurately represent the original population. By chance, they may possess different genetic traits. Consequently, genetic diversity can be lost as certain alleles are eliminated or become less prevalent.
To illustrate this, consider a bottle filled with candy balls. Initially, the bottle contained five different colors of candy, representing distinct alleles in the population. However, after a bottleneck event, only three colors of candy balls remain. This reduction represents the loss of genetic diversity, as only a subset of the original alleles persists.
Bottleneck events can have long-lasting effects on a population’s genetic makeup, as reduced diversity may impede adaptation to environmental changes.
The northern elephant seal serves as an excellent example of a species that recovered remarkably from a population bottleneck. In the 19th century, the seal population was heavily hunted, resulting in a decline to fewer than 100 individuals. However, a population on Guadalupe Island in Mexico was protected, leading to a rebound in numbers, with an estimated 225,000 individuals in 2010. This success story in conservation biology showcases the species’ recolonization of its former range.
Despite this population recovery, studies indicate that the northern elephant seal still experiences genetic depletion. This means there is minimal genetic variation among individuals in the population, despite its large size and rapid recovery.
Compared to the southern elephant seal, which underwent less intensive hunting, the northern elephant seal exhibits significantly less genetic diversity. This is uncommon, as genetic depletion is typically observed in smaller endangered species.
The genetic depletion of northern elephant seals serves as a reminder that even large populations can encounter genetic issues following a bottleneck event. For conservation efforts to ensure the long-term survival of species, protecting large populations alone may not suffice. Preserving genetic diversity is equally important.
The founder effect is a type of genetic drift that occurs when a small group of a population becomes physically separated from the main group or establishes a new colony. The outcomes of the founder effect are similar to those of a bottleneck. In summary, the new population is significantly smaller, with different allele frequencies and potentially lower genetic variation compared to the original population.
Nevertheless, a bottleneck stems from random, often adverse environmental events, while the founder effect primarily arises from the geographical separation of a portion of the population. The original population typically remains intact with a founder effect.
Examples of Ellis-Van Creveld syndrome in the Amish population and genetic depletion in cheetahs illustrate the lasting 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. Over time, individuals tended to mate with those genetically similar to them, leading to increased homozygosity for the harmful allele.
Similarly, cheetahs experienced two genetic drift events that diminished their genetic diversity. Recent anthropogenic pressures further reduced their population size, making it challenging for cheetahs to adapt to changing environmental conditions.
The consequences of genetic depletion can be severe, as demonstrated in both examples. Amish individuals with Ellis-Van Creveld syndrome experience physical abnormalities that impact their quality of life. Cheetahs, on the other hand, face increased mortality rates in their young and encounter difficulties in captive breeding.
These examples underscore the significance of maintaining genetic diversity within populations. Genetic drift can leave a lasting impact on a population’s genetic makeup, and even large populations can suffer from genetic problems following a bottleneck event. Conservation efforts must prioritize the preservation of genetic diversity to ensure 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 greatly affected by its consequences.
- Genetic drift is one of the primary mechanisms driving evolution, alongside natural selection and gene flow.
- The effects of genetic drift on populations, especially smaller ones, include non-adaptive changes in allele frequency, a reduction in genetic variation, and increased differentiation between populations.
- Evolution driven by natural selection tends to result in adaptive changes that enhance survival and reproduction probabilities, while changes caused by genetic drift are typically non-adaptive.
- A bottleneck arises from random, usually adverse environmental events, while a founder effect is mostly due to the geographical separation of a small portion of the population. Both have similar effects on the population.
- Extreme genetic drift events can have long-term consequences for populations, hindering their ability to adapt to further changes in environmental conditions. Inbreeding often results from genetic drift.
For more detailed information related to this topic, refer to the following sources:
- Alicia Abadía-Cardoso et al., Molecular Population Genetics of the Northern Elephant Seal Mirounga angustirostris, Journal of Heredity, 2017.
- Laurie Marker et al., A Brief History of Cheetah Conservation, 2020.
- Pavel Dobrynin et al., Genomic legacy of the African cheetah, Acinonyx jubatus, Genome Biology, 2014.
- Campbell and Reece, Biology 7th edition, 2005.
Genetic Drift FAQ
Q: What is genetic drift?
A: Genetic drift refers to the random change in allele frequencies within a population.
Q: How does genetic drift differ from natural selection?
A: Genetic drift differs from natural selection primarily because changes driven by genetic drift are random and usually non-adaptive, whereas changes caused by natural selection tend to be adaptive, enhancing survival and reproductive probabilities.
Q: What causes genetic drift?
A: Genetic drift is caused by chance, also known as sample error. The allele frequencies within a population represent a “sample” from the parental gene pool, and they can shift in the next generation purely by chance. A random event, unrelated to natural selection, can prevent a well-suited organism from reproducing and passing on its alleles.
Q: When is genetic drift a major factor in evolution?
A: Genetic drift plays a significant role in evolution when it affects small populations, as its effects are more pronounced. Extreme cases of genetic drift, such as sudden reductions in population size and genetic variability (bottlenecks) or the colonization of a new area by a small portion of the population (founder effect), are also major factors in evolution.
Q: Can you provide an example of genetic drift?
A: One example of genetic drift is the African cheetah. Its genetic makeup is highly reduced, leading to increased mortality and vulnerability to infectious diseases. Studies estimate that two events contributed to this reduction: a founder effect when cheetahs migrated from the Americas to Eurasia and Africa, and a bottleneck coinciding with the large mammal extinctions in the Late Pleistocene.