Bottleneck and Founder Effects

A bottleneck happens when some disaster — fire, disease, hunting — kills off most of a population at random, without regard to genotype. The survivors are a small, random sample of the original gene pool, so purely by chance some alleles that were common before might now be rare, and alleles that were rare might vanish entirely. The founder effect is the same logic applied to migration instead of death: a few individuals break off to start a new population elsewhere, carrying only a fraction of the source population's alleles with them. Both are forms of genetic drift, random change in allele frequencies that has nothing to do with which alleles help an organism survive.

The key trap is thinking that lost alleles come back once numbers do. They don't. If an allele's frequency drops to zero, no individual carries it anymore, so reproduction among survivors can only shuffle and copy the alleles still present. Getting that allele back requires a new mutation recreating it or gene flow bringing it in from another population — population growth alone just multiplies what's already there.

This connects directly to the bigger picture of Unit 7: drift from bottlenecks and founder events is one more way allele frequencies shift without natural selection acting at all, alongside mutation and gene flow. Because founder populations often drift apart genetically from their source, repeated founder effects across isolated populations can set the stage for speciation, linking this topic forward to how populations diverge into separate species.