The harmful impacts of inbreeding are generally greater in species that naturally outbreed compared to those in inbreeding species, greater in stressful than benign environments, greater for fitness than peripheral traits, and greater for total fitness compared to its individual components. Inbreeding reduces survival and reproduction (i.e., it causes inbreeding depression), and thereby increases the risk of extinction. Inbreeding depression is due to increased homozygosity for harmful alleles and at loci exhibiting heterozygote advantage. Natural selection may remove (purge) the alleles that cause inbreeding depression, especially following inbreeding or population bottlenecks, but it has limited effects in small populations and usually does not completely eliminate inbreeding depression. Inbreeding depression is nearly universal in sexually reproducing organisms that are diploid or have higher ploidies.
Resolving the conundrum of inbreeding depression but no inbreeding avoidance: Estimating sex-specific selection on inbreeding by song sparrows (Melospiza melodia)