Genetic Drift, Effective Population Size, and Neutral Evolution
Random Evolutionary Change
What drift is
Genetic drift is random fluctuation in allele frequencies due to finite population size. Unlike selection, drift is not adaptive and can fix beneficial alleles by chance or eliminate them despite potential advantage.
Effective population size
Effective population size, , is the size of an idealized population that would experience the same amount of drift as the real population. Factors such as unequal sex ratios, variation in reproductive success, and population bottlenecks often make smaller than census size.
Why small populations matter
When is low, drift is stronger, inbreeding increases, and harmful alleles can rise in frequency more easily.
Selection vs. drift
| Feature | Selection | Drift |
|---|---|---|
| Cause | Consistent differences in fitness | Chance sampling |
| Direction | Nonrandom | Random with respect to fitness |
| Strength | Often stronger in large populations | Strongest in small populations |
| Outcome | Adaptive or constrained change | Can fix neutral or harmful alleles |
Which factor most directly increases the strength of genetic drift?
Drift is stronger when fewer breeding individuals contribute to the next generation.
Correct answer: Smaller effective population size
Why can a beneficial allele be lost in a small population?
Chance can outweigh advantage when populations are small.
Correct answer: Because random sampling can remove it before selection consistently increases it.
Neutral theory
Neutral theory emphasizes that many molecular changes are selectively neutral or nearly neutral, so drift explains much of sequence evolution. This does not deny selection; it highlights that much genomic variation is not strongly adaptive.
A population bottleneck is most likely to cause which outcome?
Bottlenecks usually reduce diversity by sampling only a subset of the original variation.
Correct answer: Reduced genetic diversity
Diagnosing drift in a system
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1
Check whether the population is small or has experienced bottlenecks.
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2
Compare observed changes to what selection would predict.
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3
Look for random fluctuations among replicate populations.
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4
Assess diversity loss and allele fixation patterns.