75. One hundred Independent populations of Drosophila are established with 10 individuals in each population, of which, one individual is of Aa genotype and the other nine are of AA genotype. If random genetic drift is the only mechanism acting on these populations, then, after a large number of generations, the expected number of populations fixed for the “a” allele
is
(1) 75 (2) 50
(3) 25 (4) 5
Understanding the Scenario
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Number of populations: 100
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Individuals per population: 10
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Genotypes: 1 individual is Aa, 9 are AA
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Alleles per individual: Each fly has 2 alleles (diploid)
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Mechanism: Only genetic drift is acting (no selection, mutation, or migration)
Calculating Initial Allele Frequencies
Let’s break down the allele counts in each population:
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AA individuals: 9 × 2 = 18 A alleles
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Aa individual: 1 A + 1 a = 1 A allele, 1 a allele
Total A alleles: 18 (from AA) + 1 (from Aa) = 19
Total a alleles: 1 (from Aa)
Total alleles per population: 10 individuals × 2 alleles = 20
Initial frequency of the a allele ($p_a$):
pa=120=0.05
Predicting Fixation by Genetic Drift
In the absence of selection, the probability that an allele will become fixed in a population is equal to its initial frequency.
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Probability that “a” allele becomes fixed in one population: 0.05 (5%)
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Number of populations: 100
Expected number of populations fixed for “a” allele:
100×0.05=5
Why Does This Happen?
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Random Sampling: In small populations, allele frequencies fluctuate randomly from generation to generation.
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Eventual Fixation or Loss: Over many generations, each population will either lose the “a” allele or fix it (reach 100% frequency).
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No Selection: Since only drift is acting, the initial frequency entirely determines the chance of fixation.
Key Takeaways
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Small populations are subject to strong genetic drift.
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The probability of fixation of an allele equals its starting frequency.
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In this example, with an initial frequency of 0.05 for the “a” allele, 5 out of 100 populations are expected to become fixed for “a” after many generations.
Correct answer:
(4) 5
Why Is This Important?
Understanding how genetic drift operates in small populations is crucial for:
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Conservation biology: Small populations of endangered species are especially vulnerable to the random loss of genetic diversity.
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Breeding programs: Managing genetic diversity in captive breeding or laboratory populations requires knowledge of drift’s effects.
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Evolutionary theory: Drift explains much of the genetic variation (and loss thereof) seen in nature, especially in isolated or fragmented populations.
Genetic drift is a reminder that in evolution, chance can be just as powerful as natural selection—especially when population sizes are small.
