Genetic rescue

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Genetic rescue is seen as a mitigation strategy designed to restore genetic diversity and reduce extinction risks in small, isolated and frequently inbred populations. [1] 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'. [1] 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. [2]

Contents

History

The conceptual foundation of genetic rescue can be traced back to the work of geneticist Sewall Wright, who studied the effect of immigration among populations linked by gene flow. [3] More recently, genetic rescue has been defined by scientific reviews as: "when population fitness, inferred from some demographic vital rate or phenotypic trait, increases by more than can be attributed to the demographic contribution of immigrants." [4] [5] Genetic mixing leading to fitness recovery could be described as "genetic rescue", but perpetuates the unclear differences between genetic rescue and pollution.

Genetic processes

When a species' population becomes too small, they are subject to genetic processes such as inbreeding depression from a lack of gene flow, allelic fixation from genetic drift, and loss of diversity. In combination these can lead to a decrease in population fitness, and increase the risk of extinction. [3] Genetic rescue is a conservation tool which tries to address these genetic factors by moving genes from one population to another to increase the overall genetic diversity and minimize inbreeding. [6] This conservation technique intended to increase the fitness of a small, imperiled population [2] [3] through the introduction of beneficial alleles through migration. [2] It is often used for populations of species that are at a high risk of extinction. A successful genetic rescue occurs when the addition of new genes effectively introduces genetic diversity that leads to increased population size and growth rate, as well as a greater population fitness. [2] An unsuccessful genetic rescue may occur if the addition of new genes causes outbreeding depression, which decreases their population fitness. [3] Too much gene flow may lead to genetic swamping through extensive hybridization. [2] Genetic rescue can occur through multiple pathways, including heterosis and adaptive evolution. [2] It is closely related to, but distinctly different from the concepts of genetic pollution, evolutionary rescue, genetic restoration, and assisted gene flow. [2]

Gene flow

Gene flow (migration) is the introduction of new individuals (and genes) into a target population. [7] Predicting the impact of a migrant on a population will depend on combination of complex genetic and non-genetic factors. Whether migration increases population fitness will depend if the genes brought in are adapted to local conditions and if they decrease levels of inbreeding in the target population. An Introduced individuals can also positively or negatively affect genetic rescue through behaviors such as mate choice, dominance hierarchies, and infanticide. [3]

Genetic drift

Genetic drift is the fixation of alleles by chance, hence reducing the overall diversity in the population. Genetic rescue can restore diversity by adding new genes to a population, counteracting fixation. [8]

Selection and local adaptation

Natural selection occurs when variations in heritable traits determines reproductive success of an individual, and thereby determines the persistence of that trait in that population. [9] Genetic rescue may introduce traits that are advantageous to the target population or reduced the frequency of disadvantageous traits, increasing the net fitness of a population to ensure the continued survival as a species.

Controversy

Genetic rescue can be a controversial tool because it is hard to predict how a population will be affected by a migration event. Genetic rescue has the possibility of actually lowering the fitness of a population by swamping the population or increasing rare deleterious alleles. [10] This instance may simply be termed genetic pollution instead of being referred to genetic rescue. Rescue may also only be a short-term solution, as shown by the case of the Isle Royale Wolves. In that case, genetic rescue of the wolves resulted in a large initial increase in population fitness followed by a large decline in subsequent years. [10] Many conservationists argue that genetic rescue could create unforeseen problems for species at risk, and that it overlooks the underlying problems that push so many species to the brink of extinction, including habitat loss due to human development. [11]

As with the term genetic pollution, 'genetic rescue' has political connotations. Some of the more controversial practices which can be considered genetic rescue include

Examples

Florida panther

A case of successful genetic rescue can be observed in the Florida panther population. Habitat loss and other anthropogenic influences led to small, inbred population which increased the decline of this population ( Puma concolor cougar), . [16] Inbreeding depression resulted in kinked tails and cowlicks, sperm defects, and heart abnormalities. [16] The population reached a low of approximately 22 panthers. [3] Fearing inevitable extinction, eight panthers from Texas were translocated to Florida in the mid 1990s. [16] This effort was deemed successful after analysis showed a 4% annual population growth rate following the immigration event. [3] Additionally, researchers found that the resulting hybrid kittens were three times more likely to survive to adulthood than “purebred” kittens. [16] The Florida panther population increased from around 25 to over 100 individuals in roughly a decade. [4]

Isle Royale wolves

A case of unsuccessful genetic rescue can be observed in the Isle Royale wolf population. In 1997, a single wolf arrived on the island and bred with the wolf population of about 25 individuals. [10] Initially, the addition of his genetic variation resulted in a positive effects on the population , shown by a large increase in population fitness. [10] However, the addition of genetic variation by this immigrant was only beneficial in the short term. The population swiftly declined, with only two wolves sighted in 2016. [10] it is possible that the new immigrant brought a new detrimental allele that increased in frequency as he interbred with the original population or that a single individual was insufficient to overcome the negative impact of genetic load. [10]

Greater prairie chicken

The greater prairie chicken is a ground-nesting bird with ecological and evolutionary hurdles that necessitated genetic rescue to avoid extinction. [1] It was widely distributed across the North American great plains but now requires population management in small remnant areas. In Illinois, the greater prairie chicken declined from millions of individuals in the mid 19th century to 46 by 1998. This prompted genetic rescue efforts and movement of individuals from neighboring states to increase Illinois greater prairie chicken numbers. This has been considered an early and successful case of genetic rescue. Although the initial genetic rescue actions seem to have led to an increase in fitness, prairie habitat is now limiting recovery. Exclusively genetic efforts to rescue the species are considered insufficient and more focus on habitat protection may be required to save the species. [1]

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 incest 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 specicide, widespread violence or intentional culling, and human population planning. 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 through sexual reproduction. 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 attributed to the span of survival for a species. It is distinguished from genetic variability, which describes the tendency of genetic characteristics to vary.

<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.

<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 and extinction prevention. Researchers involved in conservation genetics come from a variety of fields including population genetics, natural resources, molecular ecology, biology, evolutionary biology, and systematics. Genetic diversity is one of the three fundamental measures of biodiversity, so it is an important consideration in the wider field of conservation biology.

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

<span class="mw-page-title-main">Molecular ecology</span> Field of evolutionary biology

Molecular ecology is a field of evolutionary biology that is concerned with applying molecular population genetics, molecular phylogenetics, and more recently genomics to traditional 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 field of conservation genetics.

Inbreeding depression is the reduced biological fitness which has the potential to result from inbreeding. 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.

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.

<span class="mw-page-title-main">Genetic pollution</span> Problematic gene flow ⇨ wild populations

Genetic pollution is a term for uncontrolled gene flow into wild populations. It is defined as "the dispersal of contaminated altered genes from genetically engineered organisms to natural organisms, esp. by cross-pollination", but has come to be used in some broader ways. It is related to the population genetics concept of gene flow, and genetic rescue, which is genetic material intentionally introduced to increase the fitness of a population. It is called genetic pollution when it negatively impacts the fitness of a population, such as through outbreeding depression and the introduction of unwanted phenotypes which can lead to extinction.

<span class="mw-page-title-main">Population fragmentation</span> Form of population segregation

Population fragmentation is a form of population segregation. It is often caused by habitat fragmentation.

Genetic purging is the reduction of the frequency of a deleterious allele, caused by an increased efficiency of natural selection prompted by inbreeding.

<span class="mw-page-title-main">Reinforcement (speciation)</span> Process of increasing reproductive isolation

Reinforcement is a process of speciation where natural selection increases the reproductive isolation between two populations of species. This occurs as a result of selection acting against the production of hybrid individuals of low fitness. The idea was originally developed by Alfred Russel Wallace and is sometimes referred to as the Wallace effect. The modern concept of reinforcement originates from Theodosius Dobzhansky. He envisioned a species separated allopatrically, where during secondary contact the two populations mate, producing hybrids with lower fitness. Natural selection results from the hybrid's inability to produce viable offspring; thus members of one species who do not mate with members of the other have greater reproductive success. This favors the evolution of greater prezygotic isolation. Reinforcement is one of the few cases in which selection can favor an increase in prezygotic isolation, influencing the process of speciation directly. This aspect has been particularly appealing among evolutionary biologists.

This glossary of evolutionary biology is a list of definitions of terms and concepts used in the study of evolutionary biology, population biology, speciation, and phylogenetics, as well as sub-disciplines and related fields. For additional terms from related glossaries, see Glossary of genetics, Glossary of ecology, and Glossary of biology.

The rescue effect is a phenomenon which was first described by Brown and Kodric-Brown, and is commonly used in metapopulation dynamics and many other disciplines in ecology. This populational process explains how the migration of individuals can increase the persistence of small isolated populations by helping to stabilize a metapopulation, thus reducing the chances of extinction. In other words, immigration can lead to the recolonization of previously extinct patches, promoting the long-term persistence of the network of populations.

References

  1. 1 2 3 4 Mussmann, S. M.; Douglas, M. R.; Anthonysamy, W. J. B.; Davis, M. A.; Simpson, S. A.; Louis, W.; Douglas, M. E. (2017-02-22). "Genetic rescue, the greater prairie chicken and the problem of conservation reliance in the Anthropocene". Royal Society Open Science. 4 (2): 160736. doi:10.1098/rsos.160736. ISSN   2054-5703. PMC   5367285 . PMID   28386428.
  2. 1 2 3 4 5 6 7 Whiteley, Andrew R.; Fitzpatrick, Sarah W.; Funk, W. Chris; Tallmon, David A. (2015-01-01). "Genetic rescue to the rescue". Trends in Ecology & Evolution. 30 (1): 42–49. doi:10.1016/j.tree.2014.10.009. ISSN   0169-5347. PMID   25435267.
  3. 1 2 3 4 5 6 7 Tallmon, David A.; Luikart, Gordon; Waples, Robin S. (September 2004). "The alluring simplicity and complex reality of genetic rescue". Trends in Ecology & Evolution. 19 (9): 489–496. doi:10.1016/j.tree.2004.07.003. ISSN   0169-5347. PMID   16701312.
  4. 1 2 Hedrick, Philip W.; Adams, Jennifer R.; Vucetich, John A. (2011-11-09). "Reevaluating and Broadening the Definition of Genetic Rescue". Conservation Biology. 25 (6): 1069–1070. doi:10.1111/j.1523-1739.2011.01751.x. ISSN   0888-8892. PMID   22070251. S2CID   37780553.
  5. ↑ TALLMON, D; LUIKART, G; WAPLES, R (2004-09-01). "The alluring simplicity and complex reality of genetic rescue". Trends in Ecology & Evolution. 19 (9): 489–496. doi:10.1016/j.tree.2004.07.003. ISSN   0169-5347. PMID   16701312.
  6. ↑ "Avoiding genetic rescue not justified on genetic grounds". ConservationBytes.com. 2015-03-11. Retrieved 2018-05-15.
  7. ↑ "Gene flow". evolution.berkeley.edu. Retrieved 2018-05-15.
  8. ↑ "Genetic drift". evolution.berkeley.edu. Retrieved 2018-05-15.
  9. ↑ "Natural selection". evolution.berkeley.edu. Retrieved 2018-05-15.
  10. 1 2 3 4 5 6 Hedrick, Philip W.; Garcia-Dorado, Aurora (December 2016). "Understanding Inbreeding Depression, Purging, and Genetic Rescue". Trends in Ecology & Evolution. 31 (12): 940–952. doi:10.1016/j.tree.2016.09.005. ISSN   1872-8383. PMID   27743611.
  11. ↑ Poppick, Laura. "Threatened Species? Science to the (Genetic) Rescue!". Smithsonian. Retrieved 2018-06-06.
  12. ↑ "What is Genetic Rescue? | Revive & Restore". reviverestore.org. 2013-10-08. Retrieved 2018-06-06.
  13. ↑ Adams, Jonathan M.; Piovesan, Gianluca; Strauss, Steve; Brown, Sandra (August 2002). "The Case for Genetic Engineering of Native and Landscape Trees against Introduced Pests and Diseases". Conservation Biology. 16 (4): 874–879. doi:10.1046/j.1523-1739.2002.00523.x. ISSN   0888-8892. S2CID   86697592.
  14. ↑ "Using genes to rescue animal and plants from extinction | Cornell Chronicle". news.cornell.edu. Retrieved 2018-06-06.
  15. ↑ Baskett, Marissa L.; Gomulkiewicz, Richard (2011-02-22). "Introgressive hybridization as a mechanism for species rescue". Theoretical Ecology. 4 (2): 223–239. doi: 10.1007/s12080-011-0118-0 . ISSN   1874-1738.
  16. 1 2 3 4 Pimm, S. L.; Dollar, L.; Bass, O. L. (2006-05-01). "The genetic rescue of the Florida panther". Animal Conservation. 9 (2): 115–122. doi:10.1111/j.1469-1795.2005.00010.x. ISSN   1469-1795.
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