Inbreeding Depression
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Transcript of Inbreeding Depression
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Inbreeding depression
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Inbreeding depression is the reduced fitness in a given population as a result of breeding ofrelated individuals. It is often the result of a population bottleneck . In general, the higher the
genetic variation within a breeding population, the less likely it is to suffer from inbreeding depression. Inbreeding depression seems to be present in most groups of organisms, but varies
across mating systems. Hermaphroditic species often exhibit lower degrees of inbreeding
depression than outcrossing species, as repeated generations of selfing is thought to purge deleterious alleles from populations. For example, the outcrossing nematode Caenorhabditis
remanei has been demonstrated to suffer severely from inbreeding depression, unlike its
hermaphroditic relative C. elegans, which experiences outbreeding depression.[1]
Inbreeding depression in Delphinium nelsonii. A. Progeny lifespan and the B. overall fitness of progeny cohorts were all lower when progeny were the result of crosses with pollen taken close
to a receptor plant.[2]
Contents
1 Mechanisms
2 Inbreeding depression and natural selection
3 Managing inbreeding depression
4 In humans
5 Species not subject to inbreeding depression
6 See also
7 References
8 External links
Mechanisms
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that 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, recessivedeleterious 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 eukaryotes has two fundamental aspects: recombination during
meiosis, and outcrossing. It has been proposed that these two aspects have two natural selectiveadvantages respectively. A proposed adaptive advantage of meiosis is that it facilitates
recombinational repair of DNA damages that are otherwise difficult to repair (see Meiosis –
section: Theory that DNA repair is the adaptive advantage of meiosis). A proposed adaptive
advantage of outcrossing is complementation, which is the masking of deleterious recessive
alleles
[3][4]
(see hybrid vigor or heterosis). The selective advantage of complementation maylargely account for the general avoidance of inbreeding (see Kin recognition).
Managing inbreeding depression
Introducing alleles from a different population can reverse inbreeding depression. Different
populations of the same species have different deleterious traits, and therefore theircrossbreeding will not result in homozygosity in most loci in the offspring. This is known as
outbreeding enhancement, practiced by conservation managers and zoo captive breeders to
prevent homozygosity.
However, intermixing two different populations may give rise to unfit polygenic traits inoutbreeding depression, yielding offspring which lack the genetic adaptations to specificenvironmental conditions. These, then, will have a lowered fitness than pure-bred individuals
e.g. of a particular subspecies that has adapted to its local environment.
In humans
Although severe inbreeding depression in humans seems to be highly uncommon and not widely
known, there have been several cases of apparent forms of inbreeding depression in human populations. Charles Darwin, through numerous experiments, was one of the first scientists to
demonstrate the effects of inbreeding depression. Darwin had married his first cousin, Emma
Wedgwood. He later became concerned that inbreeding within his own family would adversely
affect the health of his own children. The Darwins had ten children, but three died before the ageof ten. Of the surviving children, three of the six who had long-term marriages did not have any
children.[5][6][7]
As with animals, this phenomenon tends to occur in isolated, rural populations
that are cut off to some degree from other areas of civilization.
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A notable example is the Vadoma tribe of western Zimbabwe, many of whom carry the trait of
having only two toes due to a small gene pool.[8]
Another example is Fumarase deficiency, a rare
genetic disorder that leads to severe mental retardation. Over half of the known cases are in theisolated and adjoining polygamous Reformed Mormon communities of Hilldale, Utah and
Colorado City, Arizona.
Species not subject to inbreeding depression
Inbreeding depression is not a phenomenon that will inevitably occur. Given enough time and a
sufficiently (but not too) small gene pool, deleterious alleles may be eliminated by naturalselection and genetic drift.
Under most circumstances, this is a rare occurrence though, as the gene pool cannot become toogreat (thereby increasing the odds of new deleterious alleles appearing through mutation) nor too
small (resulting in outright inbreeding depression). Among island endemic populations, however,
a high resistance to inbreeding depression is often seen. These derive from very small initial
populations that must have been viable, and panmixia in the early stages of speciation wasusually thorough. This will result in a very comprehensive elimination of deleterious recessive
alleles at least.[9][10][11][verification needed ]
The second type of inbreeding depression — caused by
overdominant heterozygous alleles — is impossible to eliminate by panmixia. However, localconditions may result in an altered selective advantage, so that the fitness of the heterozygous
genotype is lowered.
Example taxa not subject to significant inbreeding depression despite extremely low effective
population sizes:
Animals
Chatham Islands Robin
Laysan Duck (data equivocal; severe population fluctuations probably natural)
Mauritius Kestrel
Naked Mole Rat (mammal displaying eusocial reproductive structure and low geneticvariation
[12][13])
Stegodyphus dumicola and some other social spiders (live in highly inbred colonies)
Thai Ridgeback , a dog breed
Yellow Crazy Ant