Inbreeding depression is the reducedbiological fitness caused by loss ofgenetic diversity as a consequence ofinbreeding, the breeding of individuals closely related genetically.[2] This loss of genetic diversity results from small population size, often stemming from apopulation bottleneck.
Biologicalfitness refers to an organism's ability to survive and perpetuate itsgenetic material. In general, the higher thegenetic variation orgene pool within a breeding population, the less likely it is to suffer from inbreeding depression, though inbreeding andoutbreeding depression can simultaneously occur.
Inbreeding depression seems to be present in most populations of organisms, but varies across mating systems. Remarkably,hermaphroditic species often exhibit lower degrees of inbreeding depression thanoutcrossing species, as repeated generations ofselfing is thought topurge deleteriousalleles from populations. For example, the outcrossing nematode (roundworm)Caenorhabditis remanei has been demonstrated to suffer severely from inbreeding depression, unlike its hermaphroditic relativeC. elegans, which experiences outbreeding depression.[3]
Inbreeding (i.e., breeding between closely related individuals) results in morerecessive traits manifesting themselves, as thegenomes of pair-mates are more similar. Recessive traits can only occur in an offspring if present in both parents' genomes. The more genetically similar the parents are, the more often recessive traits appear in their offspring. This normally has a positive effect, as most genes are undergoing purifying selection (the homozygous state is favored). However, for very closely related individuals, there is an increased likelihood ofhomozygous deleteriousgenes in the offspring which can result in less-fit individuals.[4] For thealleles that confer an advantage in theheterozygous and/or homozygous-dominant state, the fitness of the homozygous-recessive state may even be zero (meaningsterile or unviable offspring).
In inbred populations, especially small ones, genetic drift causes alleles to become fixed in a population.[5] Drift load is the decline in fitness in a population due to the fixation of deleterious alleles.[6][7]
An example of inbreeding depression is shown in the image. In this case,a is the recessive allele which has negative effects. In order for thea phenotype to become active, the gene must end up as homozygousaa because in the geneotype Aa, the A takes dominance over thea and thea does not have any effect. Some recessive genes result in detrimental phenotypes by causing the organism to be less fit to its natural environment.
Another mechanism responsible for inbreeding depression is the fitness advantage of heterozygosity, which is known asoverdominance. This can lead to reduced fitness of a population with many homozygous genotypes, even if they are not deleterious or recessive. Here, even the dominant alleles result in reduced fitness if present homozygously (see alsohybrid vigour).
Overdominance is rare in nature.[4] For practical applications, e.g. inlivestock breeding, the former is thought to be more significant – it may yield completely unviable offspring (meaning outright failure of apedigree), while the latter can only result in relatively reduced fitness.
Natural selection cannot effectively remove all deleterious recessive genes from a population for several reasons. First, deleterious genes arise constantly throughde novo mutation within a population. Second, most offspring will have some deleterious traits, so few will be more fit for survival than the others. Different deleterious traits are extremely unlikely to equally affect reproduction – an especially disadvantageous recessive trait expressed in a homozygous recessive individual is likely to eliminate itself, naturally limiting the expression of itsphenotype. Third, recessive deleterious alleles will be "masked" by heterozygosity, and so in a dominant-recessive trait, heterozygotes will not be selected against.
When recessive deleterious alleles occur in the heterozygous state, where their potentially deleterious expression is masked by the corresponding wild-type allele, this masking phenomenon is referred to as complementation (seecomplementation (genetics)).
In general, sexual reproduction ineukaryotes has two fundamental aspects:genetic recombination duringmeiosis, andoutcrossing. It has been proposed that these two aspects have two natural selective advantages respectively. A proposed adaptive advantage of meiosis is that it facilitates recombinational repair of DNA damages that are otherwise difficult to repair (seeDNA repair as the adaptive advantage of meiosis). A proposed adaptive advantage of outcrossing is complementation, which is the masking of deleterious recessive alleles[8][9] (seehybrid vigor or heterosis). The selective advantage of complementation may largely account for the avoidance of inbreeding (seekin recognition).
However animals often do not avoid inbreeding.[10] Among animals, inbreeding avoidance is highly variable.[11] Inbreeding avoidance through mate selection appears to evolve only when there is both a risk of inbreeding depression and there also are frequent encounters between potential sexual partners that are related to each other.[11]
Hybridization as a conservation effort is appropriate if the population has lost "substantial genetic variation through genetic drift and the detrimental effects of inbreeding depression are apparent" and a similar population should be used.[12][13] Different populations of the same species have different deleterious traits, and therefore their cross breeding is less likely to result inhomozygosity at mostloci in the offspring. This is known asoutbreeding enhancement, which can be performed in extreme cases of severe inbreeding[12] by conservation managers and zoo captive breeders to prevent inbreeding depression.
However, intermixing two different populations can give rise to unfit polygenic traits inoutbreeding depression (i.e. yielding offspring which lack the geneticadaptations to specific environmental conditions). These, then, will have a lowered fitness than pure-bred individuals of a particularsubspecies that has adapted to its local environment.
Inbreeding may have both detrimental and beneficial effects.[14] The biological effects of inbreeding depression in humans can on occasion be confounded by socioeconomic and cultural influences on reproductive behavior.[15] Studies in human populations have shown that age atmarriage, duration of marriage,illiteracy,contraceptive use, andreproductive compensation are the major determinants of apparent fertility, even amongst populations with a high proportion ofconsanguinous unions.[16] However, several small effects on increased mortality,[17] longer inter-birth intervals[17] and reduced overall productivity[15] have been noted in certain isolated populations, though another study about the Icelandic population suggests no effect on lifespan past the 2nd cousin level.[18]
Charles Darwin was one of the first scientists to demonstrate the effects of inbreeding depression, through numerous experiments on plants. Darwin's wife,Emma, was his first cousin, and he was concerned about the impact of inbreeding on his ten children, three of whom died at age ten or younger; three others had childless long-term marriages.[19][20][21]
Humans do not seek to completely minimize inbreeding, but rather to maintain an optimal amount of inbreeding vs. outbreeding. Close inbreeding reduces fitness through inbreeding depression, but some inbreeding brings benefits.[22][23] Indeed, inbreeding "increases the speed of selection of beneficial recessive and co-dominant alleles, e.g. those that protect against diseases."[24] In general mating between humans who have an equivalent relatedness closer to that of third cousins results in reduced fitness in the children. By contrast outbreeding in humans at worst results in certain autoimmune conditions e.g. asthma.[citation needed]
A small isolated highlyinbred population of gray wolves inIsle Royale National Park, Michigan, USA, was considered in 2019 to be at imminent risk of extinction.[25] This gray wolf population had been experiencing severe inbreeding depression primarily due to thehomozygous expression of strongly deleterious recessivemutations.[25][26] 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.[25] 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.[27]
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.[28] Three mechanisms appear to be responsible for this: purging, differences inploidy, and selection forheterozygosity.[28] 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 thenaked mole rat.[28][29]
Purging selection occurs where the phenotypes ofdeleterious recessivealleles are exposed through inbreeding, and thus can be selected against. This can lead to such detrimental mutations being removed from the population, and has been demonstrated to occur rapidly where therecessive alleles have alethal effect.[28] The efficiency of purging will depend on the relationship between the magnitude of the deleterious effect that is unmasked in the homozygotes and the importance ofgenetic drift, so that purging is weaker for non-lethal than for recessive lethal alleles.[30] For very small populations, drift has a strong influence, which can cause the fixation of sublethal alleles underweak selection.[28] The fixation of a single allele for a specific gene can also reduce fitness whereheterozygote advantage was previously present (i.e., where heterozygous individuals have higher fitness than homozygotes of either allele), although this phenomenon seems to make a usually small contribution to inbreeding depression. Although naturally occurring, purging can be important for population survival, deliberately attempting to purge deleterious mutations from a population is not generally recommended as a technique to improve the fitness of captive bred animals.[31][32][33] In plants, genetic load can be assessed through a test analogous to an inbreeding depression test called anAutogamy depression test.
Manyangiosperms (flowering plants) can self-fertilise for several generations and suffer little from inbreeding depression. This is very useful for species which disperse widely and can therefore find themselves growing in a novel environment with noconspecifics present.[28]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. By having several copies of a chromosome, as opposed to two, homozygosity is less likely to occur in inbred offspring. This means that recessive deleterious alleles are not expressed as frequently as with many copies of a chromosome; it is more likely that at least one will contain a functional allele.[28]
Selection for heterozygosity is rare, as lost loci undergo purifying selection for homozygous loci.[4] Inbreeding depression has also been found to occur more gradually than predicted in some wild populations, such as in the highly inbred population of Scandinavian wolves. This appears to be due to a selection pressure for moreheterozygous individuals, which generally are in better condition and so are more likely to become one of the few animals to breed and produce offspring.[34]