Q.63 Key effects of Genetic Drift are:
- Significant in small populations
- Cause allele frequencies to change at random
- Lead to a loss of genetic variation within populations
- Cause harmful alleles to become fixed
- Significant in extremely large population size
Choose the correct answer from the options given below:
- E only
- A, E only
- A, B, C, D only
- C, D, E only
A, B, C, D only are true key effects of genetic drift.
Statement Analysis
A. TRUE – Genetic drift exerts strongest influence in small populations where random sampling error dominates (1/2N variance per generation). Large populations buffer chance events.
B. TRUE – Drift causes random allele frequency changes independent of fitness—neutral process following binomial sampling. Unlike selection, direction unpredictable.
C. TRUE – Drift progressively reduces genetic variation through allele fixation (p=1) or loss (p=0), increasing homozygosity and inbreeding coefficient over generations.
D. TRUE – In small populations, genetic drift can fix harmful alleles by chance despite negative selection, especially recessive/deleterious mutations (Muller’s ratchet).
E. FALSE – Genetic drift is negligible in extremely large populations (variance ≈ 1/2N → 0 as N → ∞). Only mutation/selection/gene flow matter at infinite population size.
Option Breakdown
Option Evaluation Reason A, B, C, D only Correct Captures all established drift effects; excludes large-population myth. E only Incorrect Directly contradicts fundamental drift theory. A, E only Incorrect E negates core inverse population size relationship. C, D, E only Incorrect E false despite C,D valid. Genetic drift small populations randomly alters allele frequencies, causing genetic variation loss and harmful allele fixation—key evolutionary force tested in NEET/GATE exams.
Population Size Effect (Statement A)
Drift strength ∝ 1/(2N): N=10 populations lose alleles 50x faster than N=500. Founder/bottleneck effects exemplify extreme small N scenarios.
Random Frequency Changes (Statement B)
Binomial sampling: P(k successes) = C(n,k)·pᵏ(1-p)ⁿ⁻ᵏ. Successive generations accumulate variance σ² = pq/(2N), yielding unpredictable trajectories.
Genetic Variation Loss (Statement C)
Fixation probability = initial allele frequency (neutral case). Time to fixation/loss: ~4N generations. Homozygosity ↑, heterozygosity H_t = H_0(1-1/2N)ᵗ.
Harmful Allele Fixation (Statement D)
Selection-drift balance fails when |s| < 1/(2N). Recessive lethals fix despite s=-1; background selection accelerates linked neutral loss.
Large Population Myth (Statement E)
Infinite alleles model: Drift → 0 as N → ∞. Only mutation-selection-gene flow govern dynamics. Large N preserves variation.
Exam Strategy
A,B,C,D only tests inverse N relationship + consequences; E common distractor confusing drift with mutation effects.
