Genetic drift and gene flow

Genetic drift changes allele frequencies through random sampling. Its effects are especially strong in small populations. Gene flow moves alleles between populations when migrants reproduce. Both processes can change populations without making every change an adaptation. A storm may leave a small, unrepresentative group of survivors by chance. Their allele frequencies can differ from the original population even if those alleles did not affect storm survival. This is a bottleneck example of drift, not necessarily selection.