Heterosis and inbreeding depression

Mating among closely related individuals, which occurs in small populations, often results in lower reproductive
success and weaker offspring.

In large populations, a variety of instinctive mechanisms are in place to promote heterosis, which occurs when
offspring have a level of genetic variation that improves their individual evolutionary fitness.

Some species are predisposed to disperse from their place of birth to prevent sibling–sibling or parent-offspring mating, while others are restrained from mating with close relatives through sensory cues such as individual odors.

Many plants have morphological and physiological traits that facilitate cross-pollination and reduce self-pollination.
However, in small populations with few unrelated mates, the urge to breed might be stronger than the mechanisms that promote heterosis.

Under these conditions, rather than forgoing reproduction, breeding among closely-related individuals (or inbreeding) can occur. This breeding among close relatives might result in inbreeding depression, which can occur when closely-related parents give their offspring two copies of a deleterious allele.

Individuals suffering from inbreeding depression typically have fewer offspring or have offspring that are weak or fail to reproduce. Such is the case for some mountain gorillas (Gorilla beringei beringei, EN): genetic studies have shown how birth defects in several small populations can be attributed to inbreeding depression.

Inbreeding depression has also been identified as the reason why some small lion populations are more susceptible to diseases.

Inbreeding depression can result in a vicious cycle of declining population sizes, where such declines can lead to even more inbreeding depression, and eventually extinction.

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