76. In several populations, each of size N =20, if genetic drift results in a change in the relative frequencies of alleles,
    A. What is the rate of increase per generation in the proportion of populations in which the allele is lost or fixed?
    B. What is the rate of decrease per generation in each allele frequency class between 0 and 1?
    The correct answer for A and B is:
    (1) A-0.25, B-0.125 (2) A-0.025, B-0.0125
    (3) A-0.0125, B-0.025 (4) A-0.125, B-0.25

Genetic drift is a powerful evolutionary force, especially in small populations, where random fluctuations in allele frequencies can rapidly lead to the fixation (100% frequency) or loss (0% frequency) of alleles. Understanding the rates at which these changes occur is crucial for predicting genetic diversity and evolutionary potential in small, isolated groups.

Let’s address a classic population genetics question:
In several populations, each of size N = 20, if genetic drift results in a change in the relative frequencies of alleles:

• A. What is the rate of increase per generation in the proportion of populations in which the allele is lost or fixed?

• B. What is the rate of decrease per generation in each allele frequency class between 0 and 1?

The Mathematics Behind Genetic Drift

The rate of allele fixation or loss per generation in a population of size $N$ can be estimated using the formula:

Rate=12NRate=2N1

This value represents the proportion of populations in which an allele either becomes fixed or is lost in each generation due to genetic drift.

For N = 20:

Rate=12×20=140=0.025Rate=2×201=401=0.025

This means that 2.5% of populations will see an allele either fixed or lost each generation.

The rate of decrease per generation in each allele frequency class between 0 and 1 (i.e., the intermediate frequency classes) is half of the above rate, because the loss from intermediate classes is split between fixation and loss:

Rate=14NRate=4N1Rate=14×20=180=0.0125Rate=4×201=801=0.0125

So 1.25% of populations will leave each intermediate frequency class per generation.

Correct Answer for the Scenario

• A. Rate of increase in fixation or loss per generation: 0.025

• B. Rate of decrease in each intermediate allele frequency class per generation: 0.0125

Thus, the correct answer is:

(2) A-0.025, B-0.0125

Why These Rates Matter

• Rapid Genetic Change: In small populations, genetic drift can quickly lead to the loss of genetic diversity, as alleles are randomly fixed or lost.

• Conservation Implications: Endangered species with small population sizes are at greater risk of losing genetic diversity, which can impact adaptability and long-term survival.

• Population Management: Understanding these rates helps in predicting genetic outcomes and planning conservation strategies to maintain genetic health.

Key Takeaways

• The rate of allele fixation or loss per generation in a population of size 20 is 0.025.

• The rate of decline in each intermediate allele frequency class per generation is 0.0125.

• These rates highlight the strong influence of genetic drift in small populations and underscore the importance of maintaining larger population sizes to preserve genetic diversity.