Editing Gene flow (section)

== Human assisted gene-flow ==

=== Genetic rescue ===
Gene flow can also be used to assist species which are threatened with extinction. When a species exist in small populations there is an increased risk of inbreeding and  greater susceptibility to loss of diversity due to drift. These populations can benefit greatly from the introduction of unrelated individuals<ref name=":2" /> who can increase diversity<ref>{{cite journal | vauthors = Hasselgren M, Angerbjörn A, Eide NE, Erlandsson R, Flagstad Ø, Landa A, Wallén J, Norén K | display-authors = 6 | title = Vulpes lagopus) population | journal = Proceedings. Biological Sciences | volume = 285 | issue = 1875 | pages = 20172814 | date = March 2018 | pmid = 29593110 | pmc = 5897638 | doi = 10.1098/rspb.2017.2814 }}</ref> and reduce the amount of inbreeding, and potentially increase population size.<ref>{{cite journal| vauthors = Hedrick PW, Fredrickson R |date=2010|title=Genetic rescue guidelines with examples from Mexican wolves and Florida panthers |journal=Conservation Genetics|language=en|volume=11|issue=2|pages=615–626|doi=10.1007/s10592-009-9999-5|bibcode=2010ConG...11..615H |s2cid=23194498|issn=1566-0621}}</ref> This was demonstrated in the lab with two bottleneck strains of ''Drosophila melanogaster'', in which crosses between the two populations reversed the effects of inbreeding and led to greater chances of survival in not only one generation but two.<ref>{{cite journal | vauthors = Heber S, Briskie JV, Apiolaza LA | title = A test of the 'genetic rescue' technique using bottlenecked donor populations of Drosophila melanogaster | journal = PLOS ONE | volume = 7 | issue = 8 | pages = e43113 | date = 13 August 2012 | pmid = 22912802 | pmc = 3418252 | doi = 10.1371/journal.pone.0043113 | bibcode = 2012PLoSO...743113H | doi-access = free }}</ref>

===Genetic pollution===

{{Main|Genetic pollution}}

Human activities such as movement of species and modification of landscape can result in genetic pollution, [[Hybrid (biology)|hybridization]], [[introgression]] and genetic swamping.  These processes can lead to homogenization or replacement of local [[genotypes]] as a result of either a numerical and/or [[Fitness (biology)|fitness]] advantage of introduced plant or animal.<ref>{{cite book |title=Grass cultivars: their origins, development, and use on national forests and grasslands in the Pacific Northwest |vauthors=Aubry C, Shoal R, Erickson V |publisher=USDA Forest Service; Native Seed Network (NSN), Institute for Applied Ecology |year=2005 |location=Corvallis, OR |pages=26–27 |chapter=Glossary |chapter-url=https://www.fs.usda.gov/wildflowers/Native_Plant_Materials/documents/cultivars_maindoc_040405_appendices.pdf |archive-url=https://web.archive.org/web/20230530092809/http://www.fs.usda.gov/wildflowers/Native_Plant_Materials/documents/cultivars_maindoc_040405_appendices.pdf |archive-date=2023-05-30 |url-status=dead |access-date=2023-11-16 }}</ref> Nonnative species can threaten native plants and animals with extinction by hybridization and introgression either through purposeful introduction by humans or through habitat modification, bringing previously isolated species into contact. These phenomena can be especially detrimental for rare species coming into contact with more abundant ones which can occur between island and mainland species. Interbreeding between the species can cause a 'swamping' of the rarer species' gene pool, creating hybrids that supplant the native stock. This is a direct result of evolutionary forces such as natural selection, as well as genetic drift, which lead to the increasing prevalence of advantageous traits and homogenization. The extent of this phenomenon is not always apparent from [[morphology (biology)|outward appearance]] alone. While some degree of gene flow occurs in the course of normal evolution, hybridization with or without introgression may threaten a rare species' existence.<ref>{{cite journal| vauthors = Rhymer JM, Simberloff D |year=1996|title=Extinction by Hybridization and Introgression|journal=Annual Review of Ecology and Systematics|volume=27|issue=1|pages=83–109|doi=10.1146/annurev.ecolsys.27.1.83|jstor=2097230|bibcode=1996AnRES..27...83R }}</ref><ref>{{cite book| vauthors = Potts BM, Barbour RC, Hingston AB |date=September 2001|title=Genetic Pollution from Farm Forestry using eucalypt species and hybrids; A report for the RIRDC/L&WA/FWPRDC; Joint Venture Agroforestry Program|url=http://www.rirdc.gov.au/reports/AFT/01-114.pdf|url-status=dead|publisher=Australian Government, Rural Industrial Research and Development Corporation|isbn=978-0-642-58336-9|issn=1440-6845|archive-url=https://web.archive.org/web/20040102175403/http://www.rirdc.gov.au/reports/AFT/01-114.pdf|archive-date=2004-01-02|work=RIRDC Publication No 01/114; RIRDC Project No CPF - 3A}}</ref> For example, the [[Mallard]] is an abundant species of duck that interbreeds readily with a wide range of other ducks and poses a threat to the integrity of some species.<ref>{{cite journal | vauthors = Bulgarella M, Quenu M, Shepherd LD, Morgan-Richards M | title = The ectoparasites of hybrid ducks in New Zealand (Mallard x Grey Duck) | journal = International Journal for Parasitology: Parasites and Wildlife | volume = 7 | issue = 3 | pages = 335–342 | date = December 2018 | pmid = 30258780 | pmc = 6154467 | doi = 10.1016/j.ijppaw.2018.09.005 | bibcode = 2018IJPPW...7..335B | url = }}</ref>{{Failed verification|date=November 2023}}

===Urbanization===

There are two main models for how [[urbanization]] affects gene flow of urban populations. The first is through [[habitat fragmentation]], also called urban fragmentation, in which alterations to the landscape that disrupt or fragment the habitat decrease genetic diversity. The second is called the urban facilitation model, and suggests that in some populations, gene flow is enabled by anthropogenic changes to the landscape. Urban facilitation of gene flow connects populations, reduces isolation, and increases gene flow into an area which would otherwise not have this specific genome composition.<ref name = "test">{{cite journal | vauthors = Miles LS, Rivkin LR, Johnson MT, Munshi-South J, Verrelli BC | title = Gene flow and genetic drift in urban environments | journal = Molecular Ecology | volume = 28 | issue = 18 | pages = 4138–4151 | date = September 2019 | pmid = 31482608 | doi = 10.1111/mec.15221 | s2cid = 201831767 | doi-access =  | bibcode = 2019MolEc..28.4138M }}</ref>

Urban facilitation can occur in many different ways, but most of the mechanisms include bringing previously separated species into contact, either directly or indirectly. Altering a habitat through urbanization will cause habitat fragmentation, but could also potentially disrupt barriers and create a pathway, or corridor, that can connect two formerly separated species. The effectiveness of this depends on individual species’ dispersal abilities and adaptiveness to different environments to use anthropogenic structures to travel. Human-driven [[climate change]] is another mechanism by which southern-dwelling animals might be forced northward towards cooler temperatures, where they could come into contact with other populations not previously in their range. More directly, humans are known to introduce non-native species into new environments, which could lead to [[Hybridisation (biology)|hybridization]] of similar species.<ref>{{cite journal | vauthors = Crispo E, Moore JS, Lee-Yaw JA, Gray SM, Haller BC | title = Broken barriers: human-induced changes to gene flow and introgression in animals: an examination of the ways in which humans increase genetic exchange among populations and species and the consequences for biodiversity | journal = BioEssays | volume = 33 | issue = 7 | pages = 508–18 | date = July 2011 | pmid = 21523794 | doi = 10.1002/bies.201000154 | s2cid = 205470356 }}</ref>

This urban facilitation model was tested on a human health pest, the Western black widow spider (''Latrodectus hesperus''). A study by Miles et al. collected genome-wide [[single nucleotide polymorphism]] variation data in urban and rural spider populations and found evidence for increased gene flow in urban Western black widow spiders compared to rural populations. In addition, the genome of these spiders was more similar across rural populations than it was for urban populations, suggesting increased diversity, and therefore adaptation, in the urban populations of the Western black widow spider. Phenotypically, urban spiders are larger, darker, and more aggressive, which could lead to increased survival in urban environments. These findings demonstrate support for urban facilitation, as these spiders are actually able to spread and diversify faster across urban environments than they would in a rural one. However, it is also an example of how urban facilitation, despite increasing gene flow, is not necessarily beneficial to an environment, as Western black widow spiders have highly toxic venom and therefore pose risks for human health.<ref name = "hi">{{cite journal | vauthors = Miles LS, Johnson JC, Dyer RJ, Verrelli BC | title = Urbanization as a facilitator of gene flow in a human health pest | journal = Molecular Ecology | volume = 27 | issue = 16 | pages = 3219–3230 | date = July 2018 | pmid = 29972610 | doi = 10.1111/mec.14783 | doi-access = free | bibcode = 2018MolEc..27.3219M }}</ref>

Another example of urban facilitation is that of migrating bobcats (''Lynx rufus'') in the northern US and southern Canada. A study by Marrote et al. sequenced fourteen different [[microsatellite]] loci in bobcats across the Great Lakes region, and found that longitude affected the interaction between anthropogenic landscape alterations and bobcat population gene flow. While rising global temperatures push bobcat populations into northern territory, increased human activity also enables bobcat migration northward. The increased human activity brings increased roads and traffic, but also increases road maintenance, plowing, and snow compaction, inadvertently clearing a path for bobcats to travel by. The anthropogenic influence on bobcat migration pathways is an example of urban facilitation via opening up a corridor for gene flow. However, in the bobcat's southern range, an increase in roads and traffic is correlated with a decrease in forest cover, which hinders bobcat population gene flow through these areas. Somewhat ironically, the movement of bobcats northward is caused by human-driven global warming, but is also enabled by increased anthropogenic activity in northern ranges that make these habitats more suitable to bobcats.<ref>{{cite journal | vauthors = Marrotte RR, Bowman J, Wilson PJ | title = Climate connectivity of the bobcat in the Great Lakes region | journal = Ecology and Evolution | volume = 10 | issue = 4 | pages = 2131–2144 | date = February 2020 | pmid = 32128144 | pmc = 7042766 | doi = 10.1002/ece3.6049 | doi-access = free | bibcode = 2020EcoEv..10.2131M }}</ref>

Consequences of urban facilitation vary from species to species. Positive effects of urban facilitation can occur when increased gene flow enables better adaptation and introduces beneficial alleles, and would ideally increase biodiversity. This has implications for conservation: for example, urban facilitation benefits an endangered species of tarantula and could help increase the population size. Negative effects would occur when increased gene flow is maladaptive and causes the loss of beneficial alleles. In the worst-case scenario, this would lead to genomic extinction through a [[hybrid swarm]]. It is also important to note that in the scheme of overall ecosystem health and biodiversity, urban facilitation is not necessarily beneficial, and generally applies to urban adapter pests.<ref name="hi" /> Examples of this include the previously mentioned Western black widow spider, and also the [[cane toad]], which was able to use roads by which to travel and overpopulate Australia.<ref name="test" />