Inbreeding is the mating of closely related individuals. While sometimes used in selective breeding, it generally carries substantial negative consequences for animal health and population viability. These consequences stem from fundamental genetic principles that impact both individual animals and the long-term survival of species.
The Genetic Foundation
The primary genetic consequence of inbreeding is an increase in homozygosity, meaning offspring are more likely to inherit identical copies of a gene from both parents. This occurs because related individuals are more likely to carry the same genetic material.
Many genetic traits are determined by dominant and recessive alleles. Harmful alleles are often recessive, their negative effects typically masked by a dominant, functional allele. Increased homozygosity from inbreeding raises the probability of inheriting two copies of the same recessive harmful allele, unmasking the trait. Inbreeding also leads to a loss of genetic diversity within a population, making it less robust and less able to adapt.
Individual Health and Fitness Decline
The expression of harmful recessive alleles due to increased homozygosity often leads to inbreeding depression, a decline in an animal’s overall health and fitness. This includes reduced reproductive efficiency, such as lower conception rates, smaller litter sizes, and increased offspring mortality. For instance, inbred lion populations have exhibited reduced fertility and viability.
Inbred animals frequently show increased susceptibility to diseases due to weakened immune systems. Reduced genetic diversity makes individuals more vulnerable to infections. Physical abnormalities and deformities are also more common in inbred offspring. Studies on various livestock, including cattle, horses, sheep, and swine, demonstrate these effects, which also include lower growth rates, decreased vigor, and shorter lifespans. The Florida panther, an endangered subspecies, has experienced significant inbreeding depression, leading to reduced fertility and viability.
Threats to Species Survival
Beyond individual health, inbreeding poses serious threats to the survival of entire populations and species. Reduced genetic diversity within a population diminishes its capacity to adapt to environmental changes. This reduced adaptability makes populations more vulnerable to new diseases, climate shifts, or changes in resource availability.
Small, isolated populations are particularly susceptible to inbreeding, which can create a “genetic bottleneck.” This bottleneck is a sharp reduction in population size that limits genetic variation, leading to higher rates of inbreeding. A lack of genetic variation means the population has less raw material for natural selection to act upon, hindering its ability to evolve and survive new pressures. This increased inbreeding and reduced genetic diversity contribute to an elevated risk of extinction for endangered species and isolated groups.