When a small group founds a new population, rare alleles can vanish or surge to dominance purely by chance—no selection required. Watch how population size determines whether genetic diversity survives random drift across generations.
A population bottleneck occurs when a large population suddenly crashes—disease, natural disaster, or habitat loss—leaving only a small number of survivors to rebuild. The surviving group carries only a fraction of the original genetic diversity, and rare alleles are often lost forever. Even if the population later rebounds to millions, it can never recover the genetic variants that disappeared in the bottleneck. This happened to cheetahs around 10,000 years ago: today every cheetah is so genetically similar they can accept skin grafts from unrelated individuals, making the entire species vulnerable to the same diseases.
Small populations lose rare alleles to random drift faster than selection can preserve them, which is why founder effects and bottlenecks permanently reduce genetic diversity regardless of later population growth.