What is the typical effect of genetic drift on genetic variation in small populations?

• A. It increases genetic variation.
• B. It has no effect on genetic variation.
• C. It only affects mutation rates.
• D. It reduces genetic variation over time and increases the chance of fixation.

Answer: (D)

Genetic drift in small populations causes random shifts in allele frequencies from one generation to the next. Because the population is small, these random fluctuations have a large impact, so over time some alleles can become fixed (frequency 1) while others are lost (frequency 0). This process reduces the overall genetic variation within the population. With fewer alleles present, the population is less able to respond to new environmental pressures. Drift acts on existing variation, not by creating new mutations, and in the absence of selection it tends to push toward fewer alleles being maintained. This is especially pronounced in bottlenecks or founder events, where a tiny group starts off with only a subset of the original genetic diversity. So, the typical effect is a loss of genetic variation over time and an increased chance of fixation.