Genetic drift refers to the random fluctuations in gene frequencies within a population, particularly in smaller populations. This process can result in significant changes over time, often leading to reduced genetic diversity. A classic example is the Founder Effect, seen in the Amish community of Pennsylvania, where rare genetic disorders are common due to their ancestry tracing back to a small group of founders. Similarly, the Bottleneck Effect, as seen in the Northern Elephant Seal, occurs when a population dramatically reduces in size and then expands again, leading to low genetic diversity. Another example includes the Florida panther, which had a surge of genetic disorders due to inbreeding from a small population. The introduction of outside genes helped increase genetic diversity, demonstrating intervention to counter genetic drift.
Table of Contents
What is Genetic Drift?
Genetic drift is a random process in evolution where allele frequencies within a population change over generations due to chance. It can occur via the bottleneck effect, where a major event reduces population size and alters gene distribution, or the founder effect, where a small group splits off and forms a new population with different genetic composition. Genetic drift can lead to decreased genetic diversity as certain genes become dominant or disappear entirely. Unlike natural selection, which is adaptive, genetic drift is non-adaptive and random. The smaller the population, the more significant the impact of genetic drift.
Causes of Genetic Drift
Genetic drift is primarily caused by two phenomena: the founder effect and the bottleneck effect, both of which are influenced by changes in population size. However, it can also occur as a natural consequence of random variations in gene segregation.
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Founder Effect
This happens when a new population is established by a very small number of individuals from a larger population. The new population is likely to have different genetic variations from the original population due to the small number of individuals carrying only a small fraction of the original population’s genetic diversity.
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Bottleneck Effect
This is when a population is dramatically reduced in size due to a catastrophic event or environmental change, such as disease, famine, or natural disaster. The survivors may have different allele frequencies compared to the original population, purely by chance.
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Random Variations in Gene Segregation
Genetic drift can also be a result of Mendelian segregation and recombination. For instance, when an organism’s genes are passed on to its offspring, only half of its genes get passed on while the other half comes from the other parent. Some alleles might be overrepresented or underrepresented simply due to chance, causing a change in allele frequencies.
Difference Between Genetic Drift and Gene Flow
Genetic drift and gene flow are both mechanisms of evolution, but they occur via different processes and can have different effects on the genetic variation of populations.
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Genetic Drift
This is a random process that changes the frequency of alleles (variations of a gene) in a population over time, due to chance events. It can lead to a decrease in genetic diversity, especially in small populations. Examples of this process are the bottleneck effect and the founder effect, both leading to a potentially significant shift in allele frequencies within a population.
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Gene Flow
Also known as gene migration, this is the transfer of genetic variation from one population to another. If genes are moving from one population to another, they can introduce new alleles and increase genetic diversity. Gene flow can occur as a result of various actions, including migration or the exchange of pollen between populations.
| Aspect | Genetic Drift | Gene Flow |
|---|---|---|
| Definition | A random process that changes the frequency of alleles in a population over time, due to chance. | The transfer of genetic variation from one population to another. |
| Process | Occurs due to chance events, like the bottleneck effect (disasters) or the founder effect (small group splitting off). | Occurs due to the movement of individuals or genetic materials between populations, through migration, mating, etc. |
| Impact on Genetic Diversity | Tends to decrease genetic diversity as it can lead to the dominance or loss of certain alleles. | Usually increases genetic diversity by introducing new alleles to a population. |
| Effect on Population | Changes occur within a single population. | Changes occur between two or more populations. |
| Adaptive or Non-adaptive | Non-adaptive; changes are due to chance and not driven by natural selection. | Can be either; gene flow can spread beneficial alleles (adaptive) but also harmful or neutral ones (non-adaptive). |
So, while genetic drift is about random changes within a population and can reduce diversity, gene flow is about genetic exchange between populations and can increase diversity.
Examples of Genetic Drift in Real Life
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Founder Effect
The Amish population in Pennsylvania is an example of the Founder Effect, a type of genetic drift. The Amish immigrated to the United States in the 18th century, and their population has grown from a small number of founders who were genetically similar. This led to the high frequency of several genetic disorders in the Amish population, such as Ellis-van Creveld syndrome (a rare genetic disorder characterized by short stature and additional abnormalities).
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Bottleneck Effect
The Northern Elephant Seal is a prime example of the bottleneck effect, another form of genetic drift. The population of these seals was intensely hunted and nearly became extinct in the 19th century. The population has since recovered to over 30,000 seals, but the genetic diversity is very low because all the seals are descended from a small population that survived the hunting.
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Cheetahs
Cheetahs have gone through a genetic bottleneck, most likely due to a dramatic population reduction approximately 10,000 years ago. Today, cheetahs are genetically similar to each other, which reduces their ability to adapt to changes in their environment and makes them more susceptible to certain diseases.
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Finches on the Galapagos Islands
The different species of finches found on the Galapagos Islands provide another example of genetic drift. Each island has different environmental conditions and food sources, so over time, the finches on each island have evolved different physical traits due to genetic drift and natural selection. This led to the development of different species.
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The Isle Royale Wolves
The wolves on Isle Royale, a remote island in Lake Superior, provide a vivid example of genetic drift and inbreeding. These wolves descended from a small number of individuals that crossed an ice bridge from mainland Canada in the late 1940s. Over the years, the isolation led to inbreeding and certain genetic traits became more pronounced, including spinal deformities. By 2019, only two highly inbred wolves remained on the island, both displaying these deformities.
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Florida Panthers
The Florida panther population had become so small and isolated by the mid-1990s that individuals were highly inbred, leading to physical abnormalities like kinked tails and heart defects. This is an example of genetic drift known as the bottleneck effect. To combat this, eight female pumas from Texas were introduced into the population. The introduction of these individuals led to increased genetic variation and improved health in subsequent generations.
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Pingelap Atoll, Micronesia
A typhoon in the late 18th century greatly reduced the population on the island of Pingelap in Micronesia to as few as 20 individuals. One of these survivors was a carrier for the gene that causes achromatopsia, complete color blindness. This is a very rare disorder, but now about 10% of the population on Pingelap has achromatopsia, much higher than the global average, due to the founder effect.
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Samso, Denmark
Samso Island in Denmark is another real-life example of the founder effect. Researchers have found that the rate of multiple sclerosis (MS) on Samso is roughly seven times higher than the rest of Denmark and Northern Europe. They believe a single common ancestor who lived about 200 years ago introduced the high prevalence of the gene associated with MS.
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Northern European Descent and Cystic Fibrosis
Cystic fibrosis is more common in people of Northern European descent due to genetic drift. One theory suggests that having one copy of the cystic fibrosis gene offered some protection against the cholera epidemics that swept through Europe centuries ago. This protective effect could have led to an increase in carriers, and therefore an increase in the number of individuals with cystic fibrosis.
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Przewalski’s Horse
Przewalski’s horse, a rare and endangered horse native to the steppes of central Asia, has experienced a severe bottleneck effect. Once extinct in the wild, all individuals alive today have descended from 12 individuals that were bred in captivity. This has led to a very low genetic diversity among the current population, making them more susceptible to diseases.
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Tasmanian Devils
The Tasmanian Devil, a carnivorous marsupial found in Tasmania, has experienced a severe population bottleneck due to Devil Facial Tumor Disease (DFTD), a form of transmissible cancer. The loss of individuals to DFTD, coupled with already low genetic diversity, has resulted in a genetic drift that further reduces the species’ ability to combat the disease.
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Royal Families and Hemophilia
The European royal families of the 19th and 20th centuries provide an example of the founder effect. Queen Victoria of England was a carrier of hemophilia, a rare disorder that impairs the body’s ability to make blood clots. As her descendants married into other royal families across Europe, the disease was passed along and became more common in these populations than in the general population.
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The Galápagos Tortoises
The Chelonoidis abingdonii species of the Galápagos tortoise from Pinta Island is an example of a severe bottleneck. By the 1970s, there was only one individual, named Lonesome George, left of his species. Efforts to get George to breed with females of closely related species were unsuccessful, and he died in 2012, rendering the species extinct.
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Northern Atlantic Cod
Overfishing led to a severe bottleneck in Northern Atlantic cod populations. As a result, there has been a loss of genetic diversity and adaptations in these populations, impacting the ability of the species to recover.
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Pandas
Pandas went through a severe genetic bottleneck event 43,000 years ago, and the population never fully recovered. Their numbers dwindled even more due to habitat loss, and the low genetic diversity puts them at a higher risk of extinction.
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Antarctic Fur Seals
These seals were hunted to near extinction in the 18th and 19th centuries. A small group of less than 100 individuals was left on South Georgia Island, and all current Antarctic fur seals are descended from this small group. This severe bottleneck has resulted in extremely low genetic diversity in the species today.
Summary
Here’s a table that summarizes several examples of genetic drift:
| Example | Type of Genetic Drift | Description |
|---|---|---|
| Pennsylvania Amish | Founder Effect | A small group of genetically similar individuals founded the Amish population, leading to high frequencies of certain genetic disorders. |
| Northern Elephant Seal | Bottleneck Effect | After nearly becoming extinct due to hunting, the population recovered, but with very low genetic diversity due to the small surviving population. |
| Cheetahs | Bottleneck Effect | Cheetahs experienced a dramatic population reduction about 10,000 years ago, leading to extremely low genetic diversity. |
| Galapagos Finches | Founder Effect | Different species of finches on each island have evolved distinct traits due to genetic drift and natural selection. |
| Isle Royale Wolves | Founder Effect | Due to isolation and inbreeding, certain genetic traits (such as spinal deformities) became more pronounced in this population. |
| Florida Panthers | Bottleneck Effect | A small and isolated population led to high inbreeding and physical abnormalities. Introduction of outside individuals increased genetic variation. |
| Pingelap Atoll, Micronesia | Founder Effect | After a typhoon reduced the population, a higher prevalence of achromatopsia (color blindness) was observed, due to a carrier among the survivors. |
| Samso, Denmark | Founder Effect | Multiple sclerosis is seven times more common on Samso Island, likely due to a common ancestor who introduced the associated gene. |
| Przewalski’s Horse | Bottleneck Effect | After going extinct in the wild, all current individuals descended from 12 captive individuals, leading to low genetic diversity. |
| Tasmanian Devils | Bottleneck Effect | A population reduction due to a transmissible cancer led to reduced genetic diversity, making it harder for the species to combat the disease. |
Conclusion
Genetic drift is a fundamental mechanism of evolution that causes random changes in allele frequencies in populations over time. While often overshadowed by natural selection, genetic drift can play a crucial role, especially in small populations, leading to reduced genetic diversity. This reduced diversity can make populations more susceptible to diseases and less adaptable to environmental changes. Examples like the Amish community, Northern Elephant Seal, Cheetahs, and others provide real-life illustrations of this process. Ultimately, understanding genetic drift can aid in the management and conservation of endangered species, allowing us to implement strategies to maintain or increase their genetic diversity.
