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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