AP Bio Study Guide

Big Idea 1.A

What is genetic drift?

Genetic drift is a random increase or decrease in allele frequency. When populations are small (less than 100 individuals), the effect of genetic drift can be very strong and can dramatically influence evolution.

Two examples of genetic drift include:

  • The founder effect. This occurs when a group of migrating individuals are, by chance, not the same as their original population. The founders' traits are then more common in future generations of the new population.
  • The bottleneck effect. This occurs when a population undergoes a dramatic decrease in size, due to a natural disaster. This causes a loss genetic variation, and the survivors may have a different allele frequency than the original population.