Editing Gene flow (section)

===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" />