Inbreeding depression
Inbreeding depression is the reduced biological fitness which has the potential to result from inbreeding (the breeding of related individuals). Biological fitness refers to an organism's ability to survive and perpetuate its genetic material. Inbreeding depression is often the result of a population bottleneck. In general, the higher the genetic variation or gene pool within a breeding population, the less likely it is to suffer from inbreeding depression, though inbreeding and outbreeding depression can simultaneously occur.
Inbreeding depression seems to be present in most groups of organisms, but varies across mating systems.
Mechanisms
Inbreeding (i.e., breeding between closely related individuals) results in more
An example of inbreeding depression is shown to the right. In this case, a is the recessive allele which has negative effects. In order for the a phenotype to become active, the gene must end up as homozygous aa because in the geneotype Aa, the A takes dominance over the a and the a 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 as
Overdominance is rare in nature.[3] 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.
Natural selection
Natural selection cannot effectively remove all deleterious recessive genes from a population for several reasons. First, deleterious genes arise constantly through de 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 its phenotype. 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 (see complementation (genetics)).
In general, sexual reproduction in
Management
Hybridization as a conservation effort is be 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.[7][8] Different populations of the same species have different deleterious traits, and therefore their cross breeding is less likely to result in homozygosity at most loci in the offspring. This is known as outbreeding enhancement, which can be performed in extreme cases of severe inbreeding[7] by conservation managers and zoo captive breeders to prevent inbreeding depression.
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). These, then, will have a lowered fitness than pure-bred individuals of a particular subspecies that has adapted to its local environment.
In humans
Inbreeding may have both detrimental and beneficial effects.
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.[14][15][16]
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.[17][18] Indeed, inbreeding "increases the speed of selection of beneficial recessive and co-dominant alleles, e.g. those that protect against diseases."[19]
Factors reducing inbreeding depression
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.
Purging selection
Polyploidy
Many
Selection for heterozygosity
Selection for heterozygosity is rare, as lost loci undergo purifying selection for homozygous loci.
See also
- Backward evolution
- Genetic diversity
- Heterosis (outbreeding enhancement)
- Insular dwarfism
- Island gigantism
- Minimum viable population
- Outbreeding depression
- Population genetics
- Rescue effect
- Royal intermarriage
- Vadoma
- Autogamy depression
References
- ^ Begon, Michael, Colin R. Townsend, and John L. Harper. Ecology: from individuals to ecosystems. 4th ed. Malden, MA: Blackwell Pub., 2006. Print.
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- ^ "Third Cousins Have Greatest Number Of Offspring, Data From Iceland Shows". ScienceDaily. Retrieved 2022-07-26.
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- ^ "Inbreeding May Have Caused Darwin Family Ills, Study Suggests". Science Daily.
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- ^ a b c d e f g Frankham, Ballou, Briscoe (2002). Introduction to conservation genetics. Cambridge university press.
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