Outbreeding depression
Encyclopedia
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. (In selective breeding, the concept is opposed to inbreeding depression
). This phenomenon can occur in two ways.
First, selection in one population might produce a large body size, whereas in another population small body size might be more advantageous. Gene flow between these populations may lead to individuals with intermediate body sizes, which may not be adaptive in either population. It might be that, in a certain environment, having either a large or a small body is advantageous, whereas an intermediate-sized body is comparatively disadvantageous.
A second way outbreeding depression can occur is by the breakdown of biochemical or physiological compatibilities between genes in the different breeding populations. Within local, isolated breeding populations, allele
s are selected for their positive, overall effects on the local genetic background. Due to nonadditive gene action, the same genes may have rather different average effects in different genetic backgrounds--hence, the potential evolution of locally coadapted gene complexes. However, on the other hand, according to the overdominance hypothesis in genetics, it is believed that certain combinations of alleles (which can only be obtained by outbreeding) are especially advantageous when paired in a heterozygous individual
, which is one explanation for the existence of hybrid vigor.
A third, but neutral, effect of outbreeding is the loss of allopatric speciation
of a particular group, that lends its distinctness and contributes to the diversity of said types in either group by the exclusive retention of select traits.
Considering the second manner of outbreeding depressing cited above, individuals from Population A will tend to have alleles selected for the quality of combining well with allele combinations common in Population A. However, alleles found in Population A will not have been selected for the quality of crossing well with alleles common in Population B. Therefore outbreeding can undermine vitality by reducing positive epistasis and/or increasing negative epistasis. The sterility
and other fitness-reducing effects often seen in interspecific
hybrids (such as mule
s) can be considered an extreme case of this type of outbreeding depression, involving not only different allele
s of the same gene (as in distinct populations of a single species) but even different orthologous genes.
However, it is critical to understand that reduced inbreeding depression
in first generation hybrids can, in some circumstances, be strong enough to more than make up for outbreeding depression. Because of this and because of the uniformity and predictable outcome of a first generation hybrid (F1 hybrid
) farmers keep purebred strains for the purpose of outcrossing. Crossing the hybrids will give unpredictable outcomes and outbreeding depression will remain or worsen so that is not common practice.
As a general rule of thumb, hybrid vigor (another way of saying a reduction of inbreeding depression) is strongest in first generation hybrids and gets weaker over time. In contrast, outbreeding depression can be relatively weak in the first generation. But outside the context of ruthless selective pressure, outbreeding depression will increase in power through the further generations as co-adapted gene complexes are broken apart without the forging of new co-adapted gene complexes to take their place.
It is important to keep in mind that these two mechanisms of outbreeding depression can be operating at the same time. However, determining which mechanism is more important in a particular population is very difficult.
Selective breeding
Selective breeding is the process of breeding plants and animals for particular genetic traits. Typically, strains that are selectively bred are domesticated, and the breeding is sometimes done by a professional breeder. Bred animals are known as breeds, while bred plants are known as varieties,...
and zoology
Zoology
Zoology |zoölogy]]), is the branch of biology that relates to the animal kingdom, including the structure, embryology, evolution, classification, habits, and distribution of all animals, both living and extinct...
, outbreeding depression refers to cases when offspring
Offspring
In biology, offspring is the product of reproduction, of a new organism produced by one or more parents.Collective offspring may be known as a brood or progeny in a more general way...
from crosses
Cross-fertilization
Cross-fertilization can refer to:*Allogamy, where an ovum from one individual is fertilized with the spermatozoa of another*Heterosis, where different strains are cross-bred to form a hybrid...
between individuals from different populations have lower fitness than progeny from crosses between individuals from the same population. (In selective breeding, the concept is opposed to inbreeding depression
Inbreeding depression
Inbreeding depression is the reduced fitness in a given population as a result of breeding of related individuals. It is often the result of a population bottleneck...
). This phenomenon can occur in two ways.
First, selection in one population might produce a large body size, whereas in another population small body size might be more advantageous. Gene flow between these populations may lead to individuals with intermediate body sizes, which may not be adaptive in either population. It might be that, in a certain environment, having either a large or a small body is advantageous, whereas an intermediate-sized body is comparatively disadvantageous.
A second way outbreeding depression can occur is by the breakdown of biochemical or physiological compatibilities between genes in the different breeding populations. Within local, isolated breeding populations, allele
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 are selected for their positive, overall effects on the local genetic background. Due to nonadditive gene action, the same genes may have rather different average effects in different genetic backgrounds--hence, the potential evolution of locally coadapted gene complexes. However, on the other hand, according to the overdominance hypothesis in genetics, it is believed that certain combinations of alleles (which can only be obtained by outbreeding) are especially advantageous when paired in a heterozygous individual
Heterozygote advantage
A heterozygote advantage describes the case in which the heterozygote genotype has a higher relative fitness than either the homozygote dominant or homozygote recessive genotype. The specific case of heterozygote advantage is due to a single locus known as overdominance...
, which is one explanation for the existence of hybrid vigor.
A third, but neutral, effect of outbreeding is the loss of allopatric speciation
Allopatric speciation
Allopatric speciation or geographic speciation is speciation that occurs when biological populations of the same species become isolated due to geographical changes such as mountain building or social changes such as emigration...
of a particular group, that lends its distinctness and contributes to the diversity of said types in either group by the exclusive retention of select traits.
Considering the second manner of outbreeding depressing cited above, individuals from Population A will tend to have alleles selected for the quality of combining well with allele combinations common in Population A. However, alleles found in Population A will not have been selected for the quality of crossing well with alleles common in Population B. Therefore outbreeding can undermine vitality by reducing positive epistasis and/or increasing negative epistasis. The sterility
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;...
and other fitness-reducing effects often seen in interspecific
Interspecific
Interspecific is a term used in biology to describe behaviors, biochemical variations and other issues between individuals of separate species, thereby contrasting with intraspecific...
hybrids (such as mule
Mule
A mule is the offspring of a male donkey and a female horse. Horses and donkeys are different species, with different numbers of chromosomes. Of the two F1 hybrids between these two species, a mule is easier to obtain than a hinny...
s) can be considered an extreme case of this type of outbreeding depression, involving not only different allele
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 of the same gene (as in distinct populations of a single species) but even different orthologous genes.
However, it is critical to understand that reduced inbreeding depression
Inbreeding depression
Inbreeding depression is the reduced fitness in a given population as a result of breeding of related individuals. It is often the result of a population bottleneck...
in first generation hybrids can, in some circumstances, be strong enough to more than make up for outbreeding depression. Because of this and because of the uniformity and predictable outcome of a first generation hybrid (F1 hybrid
F1 hybrid
F1 hybrid is a term used in genetics and selective breeding. F1 stands for Filial 1, the first filial generation seeds/plants or animal offspring resulting from a cross mating of distinctly different parental types....
) farmers keep purebred strains for the purpose of outcrossing. Crossing the hybrids will give unpredictable outcomes and outbreeding depression will remain or worsen so that is not common practice.
As a general rule of thumb, hybrid vigor (another way of saying a reduction of inbreeding depression) is strongest in first generation hybrids and gets weaker over time. In contrast, outbreeding depression can be relatively weak in the first generation. But outside the context of ruthless selective pressure, outbreeding depression will increase in power through the further generations as co-adapted gene complexes are broken apart without the forging of new co-adapted gene complexes to take their place.
It is important to keep in mind that these two mechanisms of outbreeding depression can be operating at the same time. However, determining which mechanism is more important in a particular population is very difficult.
See also
- Heterozygote advantageHeterozygote advantageA heterozygote advantage describes the case in which the heterozygote genotype has a higher relative fitness than either the homozygote dominant or homozygote recessive genotype. The specific case of heterozygote advantage is due to a single locus known as overdominance...
- Dominance versus overdominanceDominance versus overdominanceDominance versus overdominance is a scientific controversy in the field of genetics that has persisted for more than a century. These two alternative hypotheses were first stated in 1908.-Genetic basis of heterosis:...
- Haldane's RuleHaldane's ruleHaldane's rule or Haldane's law was formulated in 1922 by the British evolutionary biologist J.B.S. Haldane. It describes hybrid sterility in species and is extended to describe speciation in evolutionary theory, in two parts: the rule of hybrid sterility and the rule of hybrid inviability...
- HeterosisHeterosisHeterosis, 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....
- Inbreeding depressionInbreeding depressionInbreeding depression is the reduced fitness in a given population as a result of breeding of related individuals. It is often the result of a population bottleneck...