Genetic viability

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Genetic viability is the ability of the genes present to allow a cell, organism or population to survive and reproduce. [1] [2] The term is generally used to mean the chance or ability of a population to avoid the problems of inbreeding. [1] Less commonly genetic viability can also be used in respect to a single cell or on an individual level. [1]

Contents

Inbreeding depletes heterozygosity of the genome, meaning there is a greater chance of identical alleles at a locus. [1] When these alleles are non-beneficial, homozygosity could cause problems for genetic viability. [1] These problems could include effects on the individual fitness (higher mortality, slower growth, more frequent developmental defects, reduced mating ability, lower fecundity, greater susceptibility to disease, lowered ability to withstand stress, reduced intra- and inter-specific competitive ability) or effects on the entire population fitness (depressed population growth rate, reduced regrowth ability, reduced ability to adapt to environmental change). [3] See Inbreeding depression. When a population of plants or animals loses their genetic viability, their chance of going extinct increases. [4]

Necessary conditions

To be genetically viable, a population of plants or animals requires a certain amount of genetic diversity and a certain population size. [5] For long-term genetic viability, the population size should consist of enough breeding pairs to maintain genetic diversity. [6] The precise effective population size can be calculated using a minimum viable population analysis. [7]   Higher genetic diversity and a larger population size will decrease the negative effects of genetic drift and inbreeding in a population. [3] When adequate measures have been met, the genetic viability of a population will increase. [8]

Causes for decrease

Population bottleneck can decrease genetic viability leading to possible extinction Population bottleneck.svg
Population bottleneck can decrease genetic viability leading to possible extinction

The main cause of a decrease in genetic viability is loss of habitat. [4] [9] [10] This loss can occur because of, for example urbanization or deforestation causing habitat fragmentation. [4] Natural events like earthquakes, floods or fires can also cause loss of habitat. [4] Eventually, loss of habitat could lead to a population bottleneck. [3] In a small population, the risk of inbreeding will increase drastically which could lead to a decrease in genetic viability. [3] [4] [11] If they are specific in their diets, this can also lead to habitat isolation and reproductive constraints, leading to greater population bottleneck, and decrease in genetic viability. [8] Traditional artificial propagation can also lead to decreases in genetic viability in some species. [12] [13]

Genetic viability of particular wolf populations

A small highly inbred population of gray wolves (Canis lupus) residing in Isle Royale National Park, Michigan, USA has been undergoing population decline and is nearing extinction. [14] These gray wolves have been experiencing severe inbreeding depression primarily determined by the homozygous expression of strongly deleterious recessive mutations leading to decreased genetic viability. [14] [15] Reduced genetic viability due to severe inbreeding was expressed as reduced reproduction and survival as well as specific defects such as malformed vertebrae, probable cataracts, syndactyly, an unusual “rope tail,” and anomalous fur phenotypes. A separate inbred Scandinavian population of gray wolves (Canis lupus), also suffering from loss of genetic viability, is experiencing inbreeding depression likely due to the homozygous expression of deleterious recessive mutations. [14]

Population conservation

Habitat protection is associated with more allelic richness and heterozygosity than in unprotected habitats. [16] Reduced habitat fragmentation and increased landscape permeability can promote allelic richness by facilitating gene flow between populations that are isolated or smaller. [16]

The minimum viable population needed to maintain genetic viability is where the loss of genetic variation because of small population size (genetic drift) is equal to genetic variation gained through mutation. [17] When the numbers of one sex is too low, there may be a need for crossbreeding to maintain viability. [18]

Analyzing

When genetic viability seems to be decreasing within a population, a population viability analysis (PVA) can be done to assess the risk of extinction of this species. [19] [20] [21] The result of a PVA could determine whether further action is needed regarding the preservation of a species. [19]

Applications

Genetic viability is applied by wildlife management staff in zoos, aquariums or other such ex situ habitats. [22] They use the knowledge of the animals' genetics, usually through their pedigrees, to calculate the PVA and manage the population viability. [22]

Related Research Articles

<span class="mw-page-title-main">Inbreeding</span> Reproduction by closely related organisms

Inbreeding is the production of offspring from the mating or breeding of individuals or organisms that are closely related genetically. By analogy, the term is used in human reproduction, but more commonly refers to the genetic disorders and other consequences that may arise from expression of deleterious recessive traits resulting from incestuous sexual relationships and consanguinity. Animals avoid inbreeding only rarely.

Small populations can behave differently from larger populations. They are often the result of population bottlenecks from larger populations, leading to loss of heterozygosity and reduced genetic diversity and loss or fixation of alleles and shifts in allele frequencies. A small population is then more susceptible to demographic and genetic stochastic events, which can impact the long-term survival of the population. Therefore, small populations are often considered at risk of endangerment or extinction, and are often of conservation concern.

<span class="mw-page-title-main">Population bottleneck</span> Effects of a sharp reduction in numbers on the diversity and robustness of a population

A population bottleneck or genetic bottleneck is a sharp reduction in the size of a population due to environmental events such as famines, earthquakes, floods, fires, disease, and droughts; or human activities such as genocide, speciocide, widespread violence or intentional culling. Such events can reduce the variation in the gene pool of a population; thereafter, a smaller population, with a smaller genetic diversity, remains to pass on genes to future generations of offspring. Genetic diversity remains lower, increasing only when gene flow from another population occurs or very slowly increasing with time as random mutations occur. This results in a reduction in the robustness of the population and in its ability to adapt to and survive selecting environmental changes, such as climate change or a shift in available resources. Alternatively, if survivors of the bottleneck are the individuals with the greatest genetic fitness, the frequency of the fitter genes within the gene pool is increased, while the pool itself is reduced.

Population genetics is a subfield of genetics that deals with genetic differences within and among populations, and is a part of evolutionary biology. Studies in this branch of biology examine such phenomena as adaptation, speciation, and population structure.

<span class="mw-page-title-main">Gene flow</span> Transfer of genetic variation from one population to another

In population genetics, gene flow is the transfer of genetic material from one population to another. If the rate of gene flow is high enough, then two populations will have equivalent allele frequencies and therefore can be considered a single effective population. It has been shown that it takes only "one migrant per generation" to prevent populations from diverging due to drift. Populations can diverge due to selection even when they are exchanging alleles, if the selection pressure is strong enough. Gene flow is an important mechanism for transferring genetic diversity among populations. Migrants change the distribution of genetic diversity among populations, by modifying allele frequencies. High rates of gene flow can reduce the genetic differentiation between the two groups, increasing homogeneity. For this reason, gene flow has been thought to constrain speciation and prevent range expansion by combining the gene pools of the groups, thus preventing the development of differences in genetic variation that would have led to differentiation and adaptation. In some cases dispersal resulting in gene flow may also result in the addition of novel genetic variants under positive selection to the gene pool of a species or population

<span class="mw-page-title-main">Genetic diversity</span> Total number of genetic characteristics in a species

Genetic diversity is the total number of genetic characteristics in the genetic makeup of a species. It ranges widely, from the number of species to differences within species, and can be correlated to the span of survival for a species. It is distinguished from genetic variability, which describes the tendency of genetic characteristics to vary.

Population viability analysis (PVA) is a species-specific method of risk assessment frequently used in conservation biology. It is traditionally defined as the process that determines the probability that a population will go extinct within a given number of years. More recently, PVA has been described as a marriage of ecology and statistics that brings together species characteristics and environmental variability to forecast population health and extinction risk. Each PVA is individually developed for a target population or species, and consequently, each PVA is unique. The larger goal in mind when conducting a PVA is to ensure that the population of a species is self-sustaining over the long term.

<span class="mw-page-title-main">Habitat fragmentation</span> Discontinuities in an organisms environment causing population fragmentation.

Habitat fragmentation describes the emergence of discontinuities (fragmentation) in an organism's preferred environment (habitat), causing population fragmentation and ecosystem decay. Causes of habitat fragmentation include geological processes that slowly alter the layout of the physical environment, and human activity such as land conversion, which can alter the environment much faster and causes the extinction of many species. More specifically, habitat fragmentation is a process by which large and contiguous habitats get divided into smaller, isolated patches of habitats.

The Allee effect is a phenomenon in biology characterized by a correlation between population size or density and the mean individual fitness of a population or species.

In population genetics and population ecology, population size is a countable quantity representing the number of individual organisms in a population. Population size is directly associated with amount of genetic drift, and is the underlying cause of effects like population bottlenecks and the founder effect. Genetic drift is the major source of decrease of genetic diversity within populations which drives fixation and can potentially lead to speciation events.

<span class="mw-page-title-main">Minimum viable population</span> Smallest size a biological population can exist without facing extinction

Minimum viable population (MVP) is a lower bound on the population of a species, such that it can survive in the wild. This term is commonly used in the fields of biology, ecology, and conservation biology. MVP refers to the smallest possible size at which a biological population can exist without facing extinction from natural disasters or demographic, environmental, or genetic stochasticity. The term "population" is defined as a group of interbreeding individuals in similar geographic area that undergo negligible gene flow with other groups of the species. Typically, MVP is used to refer to a wild population, but can also be used for ex situ conservation.

<span class="mw-page-title-main">Conservation genetics</span> Interdisciplinary study of extinction avoidance

Conservation genetics is an interdisciplinary subfield of population genetics that aims to understand the dynamics of genes in a population for the purpose of natural resource management, conservation of genetic diversity, and the prevention of species extinction. Scientists involved in conservation genetics come from a variety of fields including population genetics, research in natural resource management, molecular ecology, molecular biology, evolutionary biology, and systematics. The genetic diversity within species is one of the three fundamental components of biodiversity, so it is an important consideration in the wider field of conservation biology.

<span class="mw-page-title-main">Molecular ecology</span> Subdiscipline of ecology

Molecular ecology is a subdiscipline of ecology that is concerned with applying molecular genetic techniques to ecological questions. It is virtually synonymous with the field of "Ecological Genetics" as pioneered by Theodosius Dobzhansky, E. B. Ford, Godfrey M. Hewitt, and others. These fields are united in their attempt to study genetic-based questions "out in the field" as opposed to the laboratory. Molecular ecology is related to the fields of population genetics and conservation genetics.

Inbreeding depression is the reduced biological fitness that has the potential to result from inbreeding. The loss of genetic diversity that is seen due to inbreeding, results from small population size. 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.

<span class="mw-page-title-main">Captive breeding</span> Of wild organisms, by humans

Captive breeding, also known as captive propagation, is the process of keeping plants or animals in controlled environments, such as wildlife reserves, zoos, botanic gardens, and other conservation facilities. It is sometimes employed to help species that are being threatened by the effects of human activities such as climate change, habitat loss, fragmentation, overhunting or fishing, pollution, predation, disease, and parasitism.

Extinction vortices are a class of models through which conservation biologists, geneticists and ecologists can understand the dynamics of and categorize extinctions in the context of their causes. This model shows the events that ultimately lead small populations to become increasingly vulnerable as they spiral toward extinction. Developed by M. E. Gilpin and M. E. Soulé in 1986, there are currently four classes of extinction vortices. The first two deal with environmental factors that have an effect on the ecosystem or community level, such as disturbance, pollution, habitat loss etc. Whereas the second two deal with genetic factors such as inbreeding depression and outbreeding depression, genetic drift etc.

Canine reproduction is the process of sexual reproduction in domestic dogs, wolves, coyotes and other canine species.

A genetic isolate is a population of organisms that has little to no genetic mixing with other organisms of the same species due to geographic isolation or other factors that prevent reproduction. Genetic isolates form new species through an evolutionary process known as speciation. All modern species diversity is a product of genetic isolates and evolution.

Genetic purging is the increased pressure of natural selection against deleterious alleles prompted by inbreeding.

Genetic rescue is seen as a mitigation strategy designed to restore genetic diversity and reduce extinction risks in small, isolated and frequently inbred populations. It is largely implemented through translocation, a type of demographic rescue and technical migration that adds individuals to a population to prevent its potential extinction. This demographic rescue may be similar to genetic rescue, as each increase population size and/or fitness. This overlap in meaning has led some researchers to consider a more detailed definition for each type of rescue that details 'assessment and documentation of pre- and post-translocation genetic ancestry'. Not every example of genetic rescue is clearly successful and the current definition of genetic rescue does not mandate that the process result in a 'successful' outcome. Despite an ambiguous definition, genetic rescue is viewed positively, with many perceived successes.

References

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  2. ↑ Luo L, Zhang YM, Xu S (March 2005). "A quantitative genetics model for viability selection". Heredity. 94 (3): 347–355. doi: 10.1038/sj.hdy.6800615 . PMID   15536483.
  3. 1 2 3 4 5 Lacy RC (1997-05-21). "Importance of Genetic Variation to the Viability of Mammalian Populations". Journal of Mammalogy. 78 (2): 320–335. doi: 10.2307/1382885 . JSTOR   1382885.
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  9. ↑ Vonholdt BM, Stahler DR, Smith DW, Earl DA, Pollinger JP, Wayne RK (January 2008). "The genealogy and genetic viability of reintroduced Yellowstone grey wolves". Molecular Ecology. 17 (1): 252–74. Bibcode:2008MolEc..17..252V. doi:10.1111/j.1365-294X.2007.03468.x. PMID   17877715. S2CID   9222840.
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  12. ↑ Reisenbichler RB, Rubin SP (August 1, 1999). "Genetic changes from artificial propagation of Pacific salmon affect the productivity and viability of supplemented populations". ICES Journal of Marine Science. 56 (4): 459–466. Bibcode:1999ICJMS..56..459R. doi: 10.1006/jmsc.1999.0455 .
  13. ↑ McClure MM, Utter FM, Baldwin C, Carmichael RW, Hassemer PF, Howell PJ, et al. (May 2008). "Evolutionary effects of alternative artificial propagation programs: implications for viability of endangered anadromous salmonids". Evolutionary Applications. 1 (2): 356–75. Bibcode:2008EvApp...1..356M. doi:10.1111/j.1752-4571.2008.00034.x. PMC   3352443 . PMID   25567637.
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  18. ↑ Piyasatian N, Kinghorn BP (2003). "Balancing genetic diversity, genetic merit and population viability in conservation programmes". Journal of Animal Breeding and Genetics. 120 (3): 137–149. doi:10.1046/j.1439-0388.2003.00383.x.
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