How do population declines reduce genetic diversity within a species?

Population declines reduce genetic diversity primarily through the effects of genetic drift and increased inbreeding.

When a population shrinks, it becomes susceptible to random sampling errors. Because the number of reproducing individuals is small, certain alleles (forms of a gene) may be lost by chance alone from one generation to the next. Over time, this process can lead to the fixation of a single allele at a locus, effectively eliminating genetic variation.12

Additionally, rapid population declines often force mating between closely related individuals. This increase in inbreeding further reduces genetic diversity by raising the probability that offspring will inherit identical alleles derived from a common ancestor.34

These declines are particularly dangerous because genetic variation is essential for a species to adapt to environmental changes, diseases, and predators. Without this diversity, populations face a higher risk of extinction, as they lack the raw material necessary for evolutionary responses to survive.142