Question 1

How is genetic drift different from natural selection?

Accepted Answer

Natural selection changes allele frequencies systematically based on differential reproductive fitness—advantageous alleles tend to increase in frequency across generations in a predictable direction. Genetic drift changes allele frequencies randomly due to chance sampling during reproduction—even a disadvantageous allele can increase in frequency or even fix by luck, especially in small populations. In large populations, selection dominates; in small populations, drift can overpower selection, allowing deleterious mutations to accumulate (genetic erosion) and beneficial mutations to be lost by chance.

Question 2

What is allele fixation and how quickly does it happen?

Accepted Answer

Fixation occurs when one allele reaches a frequency of 1 (100%)—it is now the only allele at that locus in the entire population, and all other alleles have been lost. In a neutral model (no selection), the expected time to fixation starting from frequency p is -4N(p·ln(p) + (1-p)·ln(1-p)) generations—roughly 4N generations for a new mutation (p=1/2N). In a population of 100, fixation takes ~400 generations; in a population of 10,000, ~40,000 generations. Once fixed, the allele can only be changed by new mutation.

Question 3

What is the effective population size (Ne)?

Accepted Answer

The effective population size Ne is the size of an idealized Wright-Fisher population that would undergo the same rate of genetic drift as the actual population. Real populations deviate from the ideal due to unequal sex ratios, variance in reproductive success, overlapping generations, and population size fluctuations. Ne is almost always smaller than the census population size N—often dramatically so. A bottleneck of even a few generations can reduce Ne for many generations afterward, explaining why species with large current census sizes can still show low genetic diversity if they recently passed through a bottleneck (e.g., cheetahs, northern elephant seals).

🧬 Genetic Drift

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