Final answer:
Inbreeding depression is primarily caused by the exposure of deleterious recessive alleles to purifying selection, which happens when related individuals mate and offspring are more likely to inherit the same harmful recessive genes from both parents.
Step-by-step explanation:
The primary cause of inbreeding depression is the exposure of deleterious recessive alleles to purifying selection. Inbreeding depression is the increase in abnormalities and disease in inbreeding populations due to the mating of closely related individuals, also known as inbreeding. When such relatives mate, it increases the probability that offspring will inherit the same harmful recessive genes from both parents, leading to detrimental health effects and reduced biological fitness.
In populations with high genetic diversity, it is less likely for two individuals to carry the same deleterious alleles. However, when inbreeding occurs, the genetic pool becomes more uniform, leading to a higher chance of these alleles being paired and expressed, which can result in harmful phenotypes. Inbreeding depression can be severe in populations that have gone through a genetic bottleneck or where there is a small population size, as there is a limited genetic variety to counteract the effects of deleterious alleles becoming homozygous.