Inbreeding avoidance

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Inbreeding avoidance, or the inbreeding avoidance hypothesis, is a concept in evolutionary biology that refers to the prevention of the harmful effects of inbreeding. The inbreeding avoidance hypothesis posits that certain mechanisms develop within a species, or within a given population of a species, as a result of assortative mating and natural and sexual selection, in order to prevent breeding among related individuals. Although inbreeding may impose certain evolutionary costs, inbreeding avoidance, which limits the number of potential mates for a given individual, can inflict opportunity costs.^[1] Therefore, a balance exists between inbreeding and inbreeding avoidance. This balance determines whether inbreeding mechanisms develop and the specific nature of such mechanisms.^[2]

Inbreeding can result in inbreeding depression, which is the reduction of fitness of a given population due to inbreeding. Inbreeding depression occurs via appearance of disadvantageous traits due to the pairing of deleterious recessive alleles in a mating pair's progeny.^[3] When two related individuals mate, the probability of deleterious recessive alleles pairing in the resulting offspring is higher as compared to when non-related individuals mate because of increased homozygosity. However, inbreeding also gives opportunity for genetic purging of deleterious alleles that otherwise would continue to exist in population and could potentially increase in frequency over time. Another possible negative effect of inbreeding is weakened immune system due to less diverse immunity alleles as a result of outbreeding depression.^[4]

However, a pair of meta-analyses published in 2021 of research across dozens of species found that inbreeding avoidance among animals is rare, only occurs when there is a risk of inbreeding depression, and is more common in species with internal fertilization, higher relatedness coefficients, and developmental co-residence.^[5][6] A 2024 meta-analysis of research across 18 species of fish likewise found a statistical association between inbreeding avoidance and inbreeding depression but also that inbreeding avoidance was more common in species with biparental care.^[7]

A 2007 study showed that inbred mice had significantly reduced survival when they were reintroduced into a natural habitat.^[8]

A review of the genetics of inbreeding depression in wild animal and plant populations, as well as in humans, led to the conclusion that inbreeding depression and its opposite, heterosis (hybrid vigor), are predominantly caused by the presence of recessive deleterious alleles in populations.^[9] Inbreeding, including self-fertilization in plants and automictic parthenogenesis (thelytoky) in hymenoptera, tends to lead to the harmful expression of deleterious recessive alleles (inbreeding depression). Cross-fertilization between unrelated individuals ordinarily leads to the masking of deleterious recessive alleles in progeny.^[10][11]

Many studies have demonstrated that homozygous individuals are often disadvantaged with respect to heterozygous individuals.^[12] For example, a study conducted on a population of South African cheetahs demonstrated that the lack of genetic variability among individuals in the population has resulted in negative consequences for individuals, such as a greater rate of juvenile mortality and spermatozoal abnormalities.^[13] When heterozygotes possess a fitness advantage relative to a homozygote, a population with a large number of homozygotes will have a relatively reduced fitness, thus leading to inbreeding depression. Through these described mechanisms, the effects of inbreeding depression are often severe enough to cause the evolution of inbreeding avoidance mechanisms.^[14]