Editing Brown rat (section)

=== Adaptations in Wild Populations ===
Like laboratory strains, wild populations in human-occupied environments show significant genetic variation. Urban environments present substantial barriers to movement that may restrict brown rat populations to single city blocks.<ref>{{Cite journal |last1=Feng |first1=Alice Y. T. |last2=Himsworth |first2=Chelsea G. |date=March 2014 |title=The secret life of the city rat: a review of the ecology of urban Norway and black rats (Rattus norvegicus and Rattus rattus) |url=http://link.springer.com/10.1007/s11252-013-0305-4 |journal=Urban Ecosystems |language=en |volume=17 |issue=1 |pages=149–162 |doi=10.1007/s11252-013-0305-4 |bibcode=2014UrbEc..17..149F |issn=1083-8155}}</ref>  These lead to differentiated population structures between regions of the same city, as demonstrated in New York, Vancouver, and Salvador.<ref name="Combs" /> Roadways and districts with low levels of garbage were found to separate populations and restrict [[gene flow]] among groups.<ref name="Combs" />

Particular attention has been given to the adaptations found in the New York City population. New York rats have longer noses and shorter molar rows than the Chinese population; these are hypothesized to be adaptations to a colder climate and a diet including human food, respectively.<ref>{{Cite journal |last1=Puckett |first1=Emily E. |last2=Sherratt |first2=Emma |last3=Combs |first3=Matthew |last4=Carlen |first4=Elizabeth J. |last5=Harcourt-Smith |first5=William |last6=Munshi-South |first6=Jason |date=June 2020 |title=Variation in brown rat cranial shape shows directional selection over 120 years in New York City |journal=Ecology and Evolution |language=en |volume=10 |issue=11 |pages=4739–4748 |doi=10.1002/ece3.6228 |pmid=32551057 |pmc=7297766 |bibcode=2020EcoEv..10.4739P |issn=2045-7758}}</ref> Genetic markers also show differences in regions associated with the metabolism and diet of New York rats.<ref>{{Cite journal |last1=Harpak |first1=Arbel |last2=Garud |first2=Nandita |last3=Rosenberg |first3=Noah A |last4=Petrov |first4=Dmitri A |last5=Combs |first5=Matthew |last6=Pennings |first6=Pleuni S |last7=Munshi-South |first7=Jason |date=2021-01-07 |editor-last=Eyre-Walker |editor-first=Adam |title=Genetic Adaptation in New York City Rats |url=https://academic.oup.com/gbe/article/doi/10.1093/gbe/evaa247/5991490 |journal=Genome Biology and Evolution |language=en |volume=13 |issue=1 |doi=10.1093/gbe/evaa247 |pmid=33211096 |issn=1759-6653|pmc=7851592 }}</ref> However, metabolic differences have also been connected to migrating populations prior to relocation in urban settlements.<ref name=":9" /> Immune response changes from this period are suspected to have enabled the eventual domestication of brown rats in Europe.<ref name="Hulme-Beaman" />

Population [[Population bottleneck|bottlenecks]] are a significant source of adaptation. Such a bottleneck is theorized to have occurred 20,000 years ago in the ancestral Chinese population, and similar reductions due to founder effects have been observed as invasive populations spread to new areas.<ref>{{Cite journal |last1=Deinum |first1=Eva E. |last2=Halligan |first2=Daniel L. |last3=Ness |first3=Rob W. |last4=Zhang |first4=Yao-Hua |last5=Cong |first5=Lin |last6=Zhang |first6=Jian-Xu |last7=Keightley |first7=Peter D. |date=October 2015 |title=Recent Evolution in Rattus norvegicus Is Shaped by Declining Effective Population Size |url=https://academic.oup.com/mbe/article-lookup/doi/10.1093/molbev/msv126 |journal=Molecular Biology and Evolution |language=en |volume=32 |issue=10 |pages=2547–2558 |doi=10.1093/molbev/msv126 |pmid=26037536 |pmc=4576703 |issn=0737-4038}}</ref><ref name=":11">{{Cite journal |last1=Chen |first1=Yi |last2=Zhao |first2=Lei |last3=Teng |first3=Huajing |last4=Shi |first4=Chengmin |last5=Liu |first5=Quansheng |last6=Zhang |first6=Jianxu |last7=Zhang |first7=Yaohua |date=December 2021 |title=Population genomics reveal rapid genetic differentiation in a recently invasive population of Rattus norvegicus |journal=Frontiers in Zoology |language=en |volume=18 |issue=1 |page=6 |doi=10.1186/s12983-021-00387-z |doi-access=free |pmid=33499890 |pmc=7836188 |issn=1742-9994}}</ref> A notable recent instance of bottleneck-induced adaptation is the rise of rodenticide resistance among wild brown rats. Resistance to [[warfarin]] was discovered in urban populations in the mid-20th century, prompting the synthesis of new forms of rodenticide.<ref name=":11" /> Some populations in the United Kingdom have also been found to resist the second-generation rodenticides developed.<ref>{{Cite journal |last=Buckle |first=Alan |date=March 2013 |title=Anticoagulant resistance in the United Kingdom and a new guideline for the management of resistant infestations of Norway rats ( Rattus norvegicus Berk.) |url=https://onlinelibrary.wiley.com/doi/10.1002/ps.3309 |journal=Pest Management Science |language=en |volume=69 |issue=3 |pages=334–341 |doi=10.1002/ps.3309 |pmid=22730379 |issn=1526-498X}}</ref> Behavioral adaptations have also made effective rodenticide more difficult to provide; fear of new stimuli in wild populations has been linked to the widespread presence of rodenticide.<ref name="Combs" /> This fear is markedly reduced in domesticated populations.<ref name=":10" />