Why does inbreeding become more likely when a species has very few surviving individuals?

Inbreeding becomes more likely in small populations because the limited number of available mates increases the probability that individuals will be related. When a species has very few surviving individuals, the pool of potential partners is restricted, often forcing mating between relatives.12

This process is closely linked to the dynamics of small populations:

• Reduced Genetic Base: Inbreeding narrows the genetic base, as related parents share more genes in common. This can lead to inbreeding depression, characterized by reduced fertility, slower growth, and increased susceptibility to disease.1
• Genetic Drift: In small populations, genetic drift—the random change in gene frequencies—has a more pronounced effect. Because there are fewer individuals to pass on genes, alleles can be lost or fixed by chance, further reducing the genetic diversity available within the group.34
• Population Bottlenecks: Events that drastically reduce population size, known as population bottlenecks, can leave a species with a dangerously low number of individuals, making subsequent inbreeding almost inevitable as the population struggles to recover.3

Ultimately, these factors create a cycle where small population size limits mate choice, leading to higher rates of inbreeding and a loss of the genetic variation necessary for a species to adapt to environmental changes.42