Genetic drift becomes especially important when a population passes through a stage in which only a small sample of the original gene pool contributes to future generations.

Two common cases are founder effects and population bottlenecks.

Founder effect

A founder effect occurs when a new population is established by a small number of individuals from a larger source population.

If allele $A$ has frequency 0.20 in the source population, a group of only five diploid founders contains just 10 allele copies. By chance, that sample might contain 5 copies of $A$, giving the new population

$$p(A)=\frac{5}{10}=0.50.$$

The large frequency difference does not require selection; it can arise because the founders were an unrepresentative sample.

Population bottleneck

A bottleneck occurs when an existing population is sharply reduced in size by an event such as habitat loss, extreme weather or another demographic disturbance. The surviving individuals may contain only part of the original genetic variation.

Even if population size later recovers numerically, lost alleles do not automatically return. The recovered population can therefore remain genetically less diverse than the original.

Why these are not separate evolutionary forces

Founder effects and bottlenecks are situations that intensify genetic drift, not alternatives to drift.

Their characteristic consequences can include:

• large random allele-frequency shifts;
• loss of rare alleles;
• reduced heterozygosity, meaning a lower fraction of individuals that are heterozygous at a locus;
• increased differences between isolated populations.

Selection may act afterward, but the initial genetic change can be purely stochastic.