Inbreeding depression
Inbreeding depression is the reduced fitness
Fitness (biology)
Fitness is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment...

 in a given population
A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...

 as a result of breeding of related individuals. It is often the result of a population bottleneck
Population bottleneck
A population bottleneck is an evolutionary event in which a significant percentage of a population or species is killed or otherwise prevented from reproducing....

. In general, the higher the genetic variation within a breeding population, the less likely it is to suffer from inbreeding
Inbreeding is the reproduction from the mating of two genetically related parents. Inbreeding results in increased homozygosity, which can increase the chances of offspring being affected by recessive or deleterious traits. This generally leads to a decreased fitness of a population, which is...


Inbreeding depression seems to be present in most groups of organisms, but is perhaps most important in hermaphroditic species. The majority of plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s are hermaphroditic and thus are capable of the most severe degree of inbreeding depression.


Breeding between closely related individuals, called inbreeding
Inbreeding is the reproduction from the mating of two genetically related parents. Inbreeding results in increased homozygosity, which can increase the chances of offspring being affected by recessive or deleterious traits. This generally leads to a decreased fitness of a population, which is...

, may on one hand result in more recessive
In genetics, the term "recessive gene" refers to an allele that causes a phenotype that is only seen in a homozygous genotype and never in a heterozygous genotype. Every person has two copies of every gene on autosomal chromosomes, one from mother and one from father...

 deleterious traits manifesting themselves, because the genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....

s of pair-mates are more similar: recessive traits can only occur in offspring if present in both parents' genomes, and the more genetically similar the parents are, the more often recessive traits appear in their offspring. Consequently, the more closely related the breeding pair is, the more homozygous deleterious gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

s the offspring may have, resulting in very unfit individuals. For allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...

s that confer an advantage in the heterozygous and/or homozygous-dominant state, the fitness of the homozygous-recessive state may even be zero (meaning sterile
Sterility (physiology)
Sterility is the physiological inability to effect sexual reproduction in a living thing, members of whose kind have been produced sexually. The term may be used in reference to* types of organism, such as the mule, a sterile hybrid;...

 or unviable offspring).

Another mechanism responsible for inbreeding depression is overdominance
Overdominance is a condition in genetics where the phenotype of the heterozygote lies outside of the phenotypical range of both homozygote parents. Overdominance can also be described as heterozygote advantage, wherein heterozygous individuals have a higher fitness than homozygous individuals.An...

 of heterozygous alleles. This can lead to reduced fitness of a population with many homozygous genotypes, even if they are not deleterious. Here, even the dominant alleles result in reduced fitness if present homozygously (see also hybrid vigour).

Currently, it is not known which of the two mechanisms is more prevalent in nature. For practical applications, e.g. in livestock
Livestock refers to one or more domesticated animals raised in an agricultural setting to produce commodities such as food, fiber and labor. The term "livestock" as used in this article does not include poultry or farmed fish; however the inclusion of these, especially poultry, within the meaning...

 breeding, the former is probably more significant – it may yield completely unviable offspring (meaning outright failure of a pedigree
Purebreds, also called purebreeds, are cultivated varieties or cultivars of an animal species, achieved through the process of selective breeding...

), while the latter can only result in relatively reduced fitness.

Inbreeding depression and natural selection

Natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....

 cannot effectively remove all deleterious recessive genes from a population for several reasons. First, deleterious genes arise constantly through mutation within a population. Second, in a population where inbreeding occurs frequently, most offspring will have some deleterious traits, so few will be more fit for survival than the others. It should be noted, though, 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
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

. Third, recessive deleterious alleles will be "masked" by heterozygosity, and so in a dominant-recessive trait, heterozygotes will not be selected against.

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 their crossbreeding will not result in homozygosity
Zygosity refers to the similarity of alleles for a trait in an organism. If both alleles are the same, the organism is homozygous for the trait. If both alleles are different, the organism is heterozygous for that trait...

 in most loci
Locus (genetics)
In the fields of genetics and genetic computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map...

 in the offspring. This is known as outbreeding enhancement
Heterosis, or hybrid vigor, or outbreeding enhancement, is the improved or increased function of any biological quality in a hybrid offspring. The adjective derived from heterosis is heterotic....

, practiced by conservation managers and zoo captive breeders to prevent homozygosity.

However, intermixing two different populations may give rise to unfit polygenic traits in outbreeding depression
Outbreeding depression
A concept in selective breeding and zoology, outbreeding depression refers to cases when offspring from crosses between individuals from different populations have lower fitness than progeny from crosses between individuals from the same population....

, yielding offspring which lack the genetic adaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....

s to specific environmental conditions. These, then, will have a lowered fitness than pure-bred individuals e.g. of a particular subspecies
Subspecies in biological classification, is either a taxonomic rank subordinate to species, ora taxonomic unit in that rank . A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one...

 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
Charles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...

, 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 age of ten. Of the surviving children, three of the six who had long-term marriages did not have any children.
As with animals, this phenomenon tends to occur in isolated, rural populations that are cut off to some degree from other areas of civilization.

A notable example is the Vadoma
The Doma, also Vadoma or Wadoma , are a tribe living in the west of Zimbabwe, especially in the Urungwe and Sipolilo districts on the Zambezi river valley...

 tribe of western Zimbabwe
Zimbabwe is a landlocked country located in the southern part of the African continent, between the Zambezi and Limpopo rivers. It is bordered by South Africa to the south, Botswana to the southwest, Zambia and a tip of Namibia to the northwest and Mozambique to the east. Zimbabwe has three...

, many of whom carry the trait of having only two toes
Ectrodactyly, sometimes referred to as the “Lobster-Claw Syndrome” involves the deficiency or absence of one or more central digits of the hand or foot and is also known as split hand/split foot malformation...

 due to a small gene pool. Another example is fumarase deficiency
Fumarase deficiency
Fumarase deficiency , also known as "Polygamist Down's", is an autosomal recessive metabolic disorder characterized by a deficiency of the enzyme fumarate hydratase, which is indicated by a build up of fumaric acid in the urine....

, a rare genetic disorder that leads to severe mental retardation
Mental retardation
Mental retardation is a generalized disorder appearing before adulthood, characterized by significantly impaired cognitive functioning and deficits in two or more adaptive behaviors...

.Over half of the known cases are in the isolated and adjoining polygamous Mormon
The term Mormon most commonly denotes an adherent, practitioner, follower, or constituent of Mormonism, which is the largest branch of the Latter Day Saint movement in restorationist Christianity...

 communities of Hilldale, Utah and Colorado City, Arizona
Colorado City, Arizona
Colorado City is a town in Mohave County, Arizona, United States, and is located in a region known as the Arizona Strip. According to 2006 Census Bureau estimates, the population of the town was 4,607...


Species not subject to inbreeding depression

Inbreeding depression is not a phenomenon
A phenomenon , plural phenomena, is any observable occurrence. Phenomena are often, but not always, understood as 'appearances' or 'experiences'...

 that will inevitably occur. Given enough time and a sufficiently (but not too) small gene pool, deleterious alleles may be eliminated by natural selection and genetic drift.

Under most circumstances, this is a rare occurrence though, as the gene pool cannot become too large (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
Panmixia means random mating.A panmictic population is one where all individuals are potential partners. This assumes that there are no mating restrictions, neither genetic or behavioural, upon the population, and that therefore all recombination is possible...

 in the early stages of speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...

 was usually thorough. This will result in a very comprehensive elimination of deleterious recessive alleles at least. The second type of inbreeding depression – caused by overdominant heterozygous alleles – is impossible to eliminate by panmixia. However, local conditions 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 size
Effective population size
In population genetics, the concept of effective population size Ne was introduced by the American geneticist Sewall Wright, who wrote two landmark papers on it...


  • Chatham Islands Robin
  • Laysan Duck
    Laysan Duck
    The Laysan Duck , also known as the Laysan Teal because of its small size, is an endangered dabbling duck endemic to the Hawaiian Islands...

     (data equivocal; severe population fluctuations probably natural)
  • Mauritius Kestrel
    Mauritius Kestrel
    The Mauritius Kestrel is a bird of prey from the family Falconidae endemic to the forests of Mauritius, where it is restricted to the southwestern plateau's forests, cliffs, and ravines.It is the most distinct of the Indian Ocean kestrels...

  • Naked Mole Rat
    Naked Mole Rat
    The naked mole rat , also known as the sand puppy or desert mole rat, is a burrowing rodent native to parts of East Africa and the only species currently classified in the genus Heterocephalus...

     (mammal displaying eusocial reproductive structure and low genetic variation)
  • Stegodyphus
    Stegodyphus is a genus of araneomorph spiders in the family Eresidae. The 21 species are distributed from Africa to Europe and Asia, with two species Stegodyphus is a genus of araneomorph spiders in the family Eresidae. The 21 species are distributed from Africa to Europe and Asia, with two species...

    and some other social spider
    Social spider
    A social spider is a spider species whose individuals form relatively long-lasting aggregations. Whereas most spiders are solitary and even aggressive toward conspecifics, some hundreds of species in several families show a tendency to live in groups, often referred to as colonies, and continue to...

    s (live in highly inbred colonies)
  • Thai Ridgeback
    Thai Ridgeback
    The Thai Ridgeback is an ancient breed of dog. The breed was formerly unknown outside of Thailand, but is gaining notice in the Western world. The Thai Ridgeback is one of only three breeds that has a ridge of hair that runs along its back in the opposite direction to the rest of the coat...

    , a dog breed
    Dog breed
    Dog breeds are groups of closely related and visibly similar domestic dogs, which are all of the subspecies Canis lupus familiaris, having characteristic traits that are selected and maintained by humans, bred from a known foundation stock....

  • Dandelion (reproduces asexually through apomixis
    In botany, apomixis was defined by Winkler as replacement of the normal sexual reproduction by asexual reproduction, without fertilization. This definition notably does not mention meiosis...


  • Nihoa Carnation
    Nihoa Carnation
    Schiedea verticillata, known as the Nihoa Carnation, is an endangered species of carnation, endemic to the island of Nihoa in the Northwestern Hawaiian Islands, where it was discovered in 1923 by the Tanager Expedition. It has been listed as endangered since 1996.It has stems long, erect or...

  • Toromiro
    Sophora toromiro, commonly known as Toromiro, is a species of flowering tree in the legume family, Fabaceae, that is endemic to Easter Island. Heavy deforestation had eliminated most of the island's forests by the first half of the 17th century , and the once common toromiro became rare and...

See also

  • Inbreeding
    Inbreeding is the reproduction from the mating of two genetically related parents. Inbreeding results in increased homozygosity, which can increase the chances of offspring being affected by recessive or deleterious traits. This generally leads to a decreased fitness of a population, which is...

  • Hybrid vigour
  • Minimum viable population
    Minimum Viable Population
    Minimum viable population is a lower bound on the population of a species, such that it can survive in the wild. This term is used in the fields of biology, ecology, and conservation biology...

  • Population genetics
    Population genetics
    Population genetics is the study of allele frequency distribution and change under the influence of the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes into account the factors of recombination, population subdivision and population...

  • Backward evolution
  • Vadoma
    The Doma, also Vadoma or Wadoma , are a tribe living in the west of Zimbabwe, especially in the Urungwe and Sipolilo districts on the Zambezi river valley...

  • Insular dwarfism
    Insular dwarfism
    Insular dwarfism, a form of phyletic dwarfism, is the process and condition of the reduction in size of large animals – typically mammals – when their population's range is limited to a small environment, primarily islands. This natural process is distinct from the intentional creation of dwarf...

  • Island gigantism
    Island gigantism
    Island gigantism or insular giantism is a biological phenomenon in which the size of animals isolated on an island increases dramatically in comparison to their mainland relatives....

External links

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