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{{Short description|Ecogeographical rule in evolutionary biology}}
{{Redirect|Island rule|the concept in black hole theoretical physics|Black hole information paradox#Recent developments}}
[[File:Garganornis ballmanni (reconstruction by Stefano Maugeri).jpg|thumb|''[[Garganornis ballmanni]]'', a very large fossil goose from the Gargano and Scontrone islands of the [[Late Miocene]]]]
'''Foster's rule''', also known as the '''island rule''' or the '''island effect''', is an [[ecogeographical rule]] in [[evolutionary biology]] stating that members of a [[species]] get smaller or bigger depending on the resources available in the environment. For example, it is known that [[pygmy mammoth]]s evolved from normal [[mammoth]]s on small [[island]]s. Similar evolutionary paths have been observed in [[elephant]]s, [[hippopotamus]]es, [[Boidae|boa]]s, [[sloth]]s, [[deer]] (such as [[Key deer]]) and [[human]]s.<ref>{{Cite book |last1=Arsuaga |first1=Juan Luis de |url=https://archive.org/details/neanderthalsneck00arsu |title=The Neanderthal's necklace : in search of the first thinkers |last2=Klatt |first2=Andy |last3=Sastre |first3=Juan Carlos |date=2002 |publisher=New York : Four Walls Eight Windows |isbn=978-1-56858-187-3}}</ref><ref>{{Cite book |last1=Panafieu |first1=Jean-Baptiste de |url=https://www.worldcat.org/title/132681532 |title=Evolution |last2=Gries |first2=Patrick |date=2007 |publisher=Seven Stories Press |isbn=978-1-58322-784-8 |edition=1 |location=New York |oclc=132681532}}</ref> It is part of the more general phenomenon of [[island syndrome]] which describes the differences in [[Morphology (biology)|morphology]], [[ecology]], [[physiology]] and [[ethology|behaviour]] of insular species compared to their continental counterparts.

The rule was first formulated by [[Leigh Van Valen]] in 1973<ref name="Valen1973">{{cite journal|last1=Van Valen|first1= L.|title=Body Size and Numbers of Plants and Animals |journal= Evolution|volume= 27|issue= 1|year= 1973|pages= 27–35 |doi= 10.2307/2407116 |jstor= 2407116|pmid= 28563673}}</ref><ref name= "Lokatis2018">{{cite journal|last1= Lokatis|first1= S.|last2= Jeschke| first2= J.M.|title=The island rule: An assessment of biases and research trends |journal= Journal of Biogeography |volume= 45|issue= 2|year =2018 |pages= 289–303|doi= 10.1111/jbi.13160|doi-access= free|bibcode= 2018JBiog..45..289L}}</ref> based on the study by [[Mammalogy|mammalogist]] [[J. Bristol Foster]] in 1964.<ref>{{cite journal | last= Foster | first=J.B. | year=1964 | title=The evolution of mammals on islands | journal=Nature | volume=202 | pages=234–235| doi= 10.1038/202234a0 | bibcode=1964Natur.202..234F | issue=4929 | s2cid= 7870628 }}</ref><ref>{{Cite thesis |last=Foster |first=J. Bristol |date=2012 |title=The evolution of the native land mammals of the Queen Charlotte Islands and the problem of insularity |publisher=University of British Columbia |url=https://doi.library.ubc.ca/10.14288/1.0107150 |language=en |doi=10.14288/1.0107150}}</ref> In it, Foster compared 116 island species to their mainland varieties. Foster proposed that certain island creatures evolved larger body size ([[insular gigantism]]) while others became smaller ([[insular dwarfism]]). Foster proposed the simple explanation that smaller creatures get larger when predation pressure is relaxed because of the absence of some of the predators of the mainland, and larger creatures become smaller when food resources are limited because of land area constraints.<ref>{{cite book | first=R.J. | last=Whittaker | title=Island biogeography: ecology, evolution, and conservation | publisher= Oxford University Press, UK | year=1998 | isbn=978-0-19-850020-9 | pages= 73–75 }}</ref>

The idea was expanded upon in ''[[The Theory of Island Biogeography]]'', by [[Robert MacArthur]] and [[Edward O. Wilson]]. In 1978, [[Ted J. Case]] published a longer paper on the topic in the journal ''[[Ecology (journal)|Ecology]]''.<ref>{{cite journal | last=Case | first=T.J. | year=1978 | title=A general explanation for insular body size trends in terrestrial vertebrates | journal= Ecology | volume=59 | pages=1–18 | doi=10.2307/1936628 | issue=1 | jstor=1936628 | bibcode=1978Ecol...59....1C }}</ref>

Recent literature has also applied the island rule to plants.<ref>{{Cite journal |last1= Biddick|first1= M.|last2= Hendriks|first2= A.|last3= Burns|first3= K.C. |date= 2019-08-19|title= Plants obey (and disobey) the island rule|journal= Proceedings of the National Academy of Sciences|volume= 116|issue= 36 |pages= 17632–34|doi= 10.1073/pnas.1907424116|pmid= 31427521 |pmc= 6731657|bibcode= 2019PNAS..11617632B|doi-access= free}}</ref>

There are some cases that do not neatly fit the rule; for example, [[Even-toed ungulate|artiodactyls]] have on several islands evolved into both dwarf and giant forms.<ref name = "Mazza2015">{{cite journal |last1= Mazza|first1= P.P.A. |last2= Rossi|first2= M.A.|last3= Agostini|first3= S.|title= Hoplitomerycidæ (Late Miocene, Italy), an Example of Giantism in Insular Ruminants|journal= Journal of Mammalian Evolution|volume= 22|issue= 2 |pages= 271–77|date= 2015|doi= 10.1007/s10914-014-9277-2|s2cid= 16437411}}</ref><ref name="Geer2018">{{cite journal|last1=van der Geer |first1= A.A.E.|title= Uniformity in variety: Antler morphology and evolution in a predator-free environment|journal= Palaeontologia Electronica |year= 2018 |issue= 21.1.9A|pages= 1–31|doi= 10.26879/834|doi-access= free}}</ref>

The Island Rule is a contested topic in evolutionary biology. Some argue that, since body size is a trait that is affected by multiple factors, and not just by organisms moving to an island, genetic variations across all populations could also cause the body mass differences between mainland and island populations.<ref>{{Cite journal|doi = 10.1111/jbi.13160|title = The island rule: An assessment of biases and research trends|year = 2018|last1 = Lokatis |first1 = Sophie|last2 = Jeschke|first2 = Jonathan M.|journal = Journal of Biogeography|volume = 45|issue = 2|pages = 289–303|doi-access = free| bibcode=2018JBiog..45..289L }}</ref>

== References ==
{{reflist}}

== External links ==
* "[https://www.pbs.org/wgbh/nova/eden/giants.html Why Do Islands Breed Giants (And Sometimes Dwarfs)?]", episode of PBS's ''Nova''

{{Biological rules}}
{{modelling ecosystems}}

[[Category:Insular ecology]]
[[Category:Ecology terminology]]
[[Category:Evolutionary biology]]
[[Category:Ecogeographic rules]]
[[Category:Animal size]]