Genetic drift, a fundamental concept in evolutionary biology, refers to random fluctuations in allele frequencies within a population from one generation to the next. Unlike natural selection, which is driven by differential survival and reproduction based on traits, genetic drift is a non-directional, chance-based process. It's particularly impactful in small populations, where random events can significantly alter the genetic makeup of the group. The concept was first formally described by Sewall Wright in the 1930s, alongside Ronald Fisher and J.B.S. Haldane, forming the Modern Synthesis of evolutionary theory. Understanding genetic drift is crucial for comprehending the loss of genetic diversity and the divergence of populations. What is Genetic Drift? Genetic drift is a mechanism of evolution where allele frequencies change randomly over time. It is most pronounced in smaller populations, where random events have a larger effect. These random changes are independent of the fitness of the alleles, meaning they are not necessarily beneficial or harmful. It’s a non-adaptive process, contrasting with natural selection. Mechanisms of Genetic Drift Founder Effect: This occurs when a small group of individuals separates from a larger population to establish a new colony. The new colony's gene pool will likely not represent the full genetic diversity of the original population. For example, the Amish population in Pennsylvania, descended from a small group of Swiss immigrants, exhibit a higher frequency of certain genetic disorders due to the limited gene pool of the founders. Bottleneck Effect: This happens when a population undergoes a drastic reduction in size, often due to a catastrophic event like a natural disaster or disease outbreak. The surviving individuals represent a random subset of the original population's genes, leading to a loss of genetic diversity. The Northern elephant seal population, reduced to as few as 20 individuals in the 19th century due to hunting, exemplifies this effect, resulting in very low genetic variation today. Consequences of Genetic Drift Loss of Genetic Diversity: Genetic drift reduces the overall genetic variation within a population. This makes the population more vulnerable to environmental changes or diseases. Fixation of Alleles: Over time, genetic drift can lead to the fixation of certain alleles (meaning they become the only allele present for that gene), even if they are not advantageous. Divergence of Populations: Different populations experiencing genetic drift independently will diverge genetically, even if they initially had similar gene pools. Genetic Drift vs. Natural Selection Feature Genetic Drift Natural Selection Driving Force Random Chance Differential Survival & Reproduction Effect on Fitness Non-adaptive (can be harmful or beneficial) Adaptive (increases fitness) Impact on Diversity Decreases Diversity Can increase or decrease diversity, often increases advantageous traits Population Size Stronger effect in small populations Operates in all population sizes Example: Cheetahs The cheetah population, once widespread, suffered a severe bottleneck effect approximately 10,000 years ago, reducing their numbers dramatically. This event resulted in extremely low genetic diversity, making them vulnerable to diseases and limiting their ability to adapt to changing environments. This is why cheetahs are almost genetically identical. Genetic drift is a powerful evolutionary force, particularly impactful in small, isolated populations. While natural selection drives adaptation, genetic drift operates randomly, leading to unpredictable changes in allele frequencies and a potential loss of genetic diversity. Understanding the interplay between these forces is critical for comprehending the patterns of evolution and the conservation of biodiversity. Future research should focus on mitigating the negative impacts of genetic drift through strategies like managed gene flow and habitat restoration.

Genetic Drift: Population Genetics | UPSC Mains ANTHROPOLOGY-PAPER-I 2020

Q: Can genetic drift and natural selection occur simultaneously?

Yes, genetic drift and natural selection can and often do occur simultaneously. However, their relative importance depends on factors like population size and the strength of selection pressure.

Genetic drift

How to Approach

This question requires a clear explanation of genetic drift, its causes, consequences, and differences from other evolutionary forces like natural selection. The approach should be to first define genetic drift, then elaborate on its mechanisms (founder effect, bottleneck effect), followed by its impacts on allele frequencies and genetic diversity. Finally, a brief comparison with natural selection will enhance understanding. A structured answer with clear headings and bullet points is essential for clarity and completeness within the word limit.

Genetic drift, a fundamental concept in evolutionary biology, refers to random fluctuations in allele frequencies within a population from one generation to the next. Unlike natural selection, which is driven by differential survival and reproduction based on traits, genetic drift is a non-directional, chance-based process. It's particularly impactful in small populations, where random events can significantly alter the genetic makeup of the group. The concept was first formally described by Sewall Wright in the 1930s, alongside Ronald Fisher and J.B.S. Haldane, forming the Modern Synthesis of evolutionary theory. Understanding genetic drift is crucial for comprehending the loss of genetic diversity and the divergence of populations.

What is Genetic Drift?

Genetic drift is a mechanism of evolution where allele frequencies change randomly over time. It is most pronounced in smaller populations, where random events have a larger effect. These random changes are independent of the fitness of the alleles, meaning they are not necessarily beneficial or harmful. It’s a non-adaptive process, contrasting with natural selection.

Mechanisms of Genetic Drift

Founder Effect: This occurs when a small group of individuals separates from a larger population to establish a new colony. The new colony's gene pool will likely not represent the full genetic diversity of the original population. For example, the Amish population in Pennsylvania, descended from a small group of Swiss immigrants, exhibit a higher frequency of certain genetic disorders due to the limited gene pool of the founders.
Bottleneck Effect: This happens when a population undergoes a drastic reduction in size, often due to a catastrophic event like a natural disaster or disease outbreak. The surviving individuals represent a random subset of the original population's genes, leading to a loss of genetic diversity. The Northern elephant seal population, reduced to as few as 20 individuals in the 19th century due to hunting, exemplifies this effect, resulting in very low genetic variation today.

Consequences of Genetic Drift

Loss of Genetic Diversity: Genetic drift reduces the overall genetic variation within a population. This makes the population more vulnerable to environmental changes or diseases.
Fixation of Alleles: Over time, genetic drift can lead to the fixation of certain alleles (meaning they become the only allele present for that gene), even if they are not advantageous.
Divergence of Populations: Different populations experiencing genetic drift independently will diverge genetically, even if they initially had similar gene pools.

Genetic Drift vs. Natural Selection

Feature	Genetic Drift	Natural Selection
Driving Force	Random Chance	Differential Survival & Reproduction
Effect on Fitness	Non-adaptive (can be harmful or beneficial)	Adaptive (increases fitness)
Impact on Diversity	Decreases Diversity	Can increase or decrease diversity, often increases advantageous traits
Population Size	Stronger effect in small populations	Operates in all population sizes

Example: Cheetahs

The cheetah population, once widespread, suffered a severe bottleneck effect approximately 10,000 years ago, reducing their numbers dramatically. This event resulted in extremely low genetic diversity, making them vulnerable to diseases and limiting their ability to adapt to changing environments. This is why cheetahs are almost genetically identical.

Additional Resources

Key Definitions

Allele Frequency

The proportion of a specific allele (variant of a gene) within a population's gene pool.

Gene Pool

The total collection of genes in a population, including all the different alleles and their relative frequencies.

Key Statistics

The probability of an allele becoming fixed or lost due to genetic drift is inversely proportional to the population size. In a population of 100, the probability is roughly 0.01 per generation.

Source: Knowledge cutoff

In small populations (N < 30), genetic drift can cause alleles to become fixed or lost within just a few generations.

Source: Knowledge cutoff

Examples

Pingelapese People

The Pingelapese people of Micronesia experienced a severe typhoon in the 18th century that dramatically reduced their population. A man carrying a recessive gene for achromatopsia (total color blindness) happened to be among the survivors, leading to a high frequency of this trait in the current Pingelapese population.

Frequently Asked Questions

▶Can genetic drift and natural selection occur simultaneously?