Inbreeding depression
[3] Inbreeding (i.e., breeding between closely related individuals) results in more recessive traits manifesting themselves, as the genomes of pair-mates are more similar.
[4] For practical applications, e.g. in livestock breeding, the former is thought to be more significant – it may yield completely unviable offspring (meaning outright failure of a pedigree), while the latter can only result in relatively reduced fitness.
A proposed adaptive advantage of outcrossing is complementation, which is the masking of deleterious recessive alleles[5][6] (see hybrid vigor or heterosis).
However, intermixing two different populations can give rise to unfit polygenic traits in outbreeding depression (i.e. yielding offspring which lack the genetic adaptations to specific environmental conditions).
[11] The biological effects of inbreeding depression in humans can on occasion be confounded by socioeconomic and cultural influences on reproductive behavior.
A small isolated highly inbred population of gray wolves in Isle Royale National Park, Michigan, USA, was considered in 2019 to be at imminent risk of extinction.
[22] This gray wolf population had been experiencing severe inbreeding depression primarily due to the homozygous expression of strongly deleterious recessive mutations.
[22][23] Defects arising from severe inbreeding among the wolves included reduced survival and reproduction, malformed vertebrae, syndactyly, probable cataracts, an unusual “rope tail” and anomalous fur phenotypes.
[22] A separate small inbred population of gray wolves in Scandinavia was also found to suffer from inbreeding depression due to the homozygous expression of deleterious recessive mutations.
[24] Whilst inbreeding depression has been found to occur in almost all sufficiently studied species, some taxa, most notably some angiosperms, appear to suffer lower fitness costs than others in inbred populations.
[25] It must be cautioned that some studies failing to show an absence of inbreeding depression in certain species can arise from small sample sizes or where the supposedly outbred control group is already suffering inbreeding depression, which frequently occurs in populations that have undergone a recent bottleneck, such as those of the naked mole rat.
This can lead to such detrimental mutations being removed from the population, and has been demonstrated to occur rapidly where the recessive alleles have a lethal effect.
[27] For very small populations, drift has a strong influence, which can cause the fixation of sublethal alleles under weak selection.
[25] Polyploidy (having more than two paired sets of each chromosome), which is prevalent in angiosperms, ferns and a select few animal taxa, accounts for this.
This appears to be due to a selection pressure for more heterozygous individuals, which generally are in better condition and so are more likely to become one of the few animals to breed and produce offspring.
