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 mammoths evolved from normal mammoths on small islands. Similar evolutionary paths have been observed in elephants, hippopotamuses, boas, sloths (such as Pygmy three-toed sloth), deer (such as Key deer) and humans. 
The rule was first stated by the
mammologist J. Bristol Foster in 1964.  In it, he compared 116 island species to their mainland varieties. He proposed that certain island creatures evolved into  larger versions of themselves (insular gigantism) while others became smaller versions of themselves (insular dwarfism). He 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.
The idea was expanded upon in
, by The Theory of Island Biogeography Robert MacArthur and Edward O. Wilson. In 1978, Ted J. Case published a longer paper on the topic in the journal . Ecology
Further, recent literature has applied the Island Rule to plants
^ Juan Luis Arsuaga, Andy Klatt,
The Neanderthal's Necklace: In Search of the First Thinkers, Thunder's Mouth Press, 2004, ISBN 1-56858-303-6, ISBN 978-1-56858-303-7, p. 199.
^ Jean-Baptiste de Panafieu, Patrick Gries,
Evolution, Seven Stories Press, 2007, ISBN 1-58322-784-9, ISBN 978-1-58322-784-8, p 42.
Foster, J.B. (1964). "The evolution of mammals on islands". Nature. 202 (4929): 234–235. Bibcode: 1964Natur.202..234F. doi: 10.1038/202234a0.
^ Foster, J. B. (1965) The evolution of the mammals of the Queen Charlotte Islands, British Columbia.
Occasional Papers of the British Columbia Provincial Museum, 14, 1–130.
Whittaker, R.J. (1998). Island biogeography: ecology, evolution, and conservation. Oxford University Press, UK. pp. 73–75. ISBN . 978-0-19-850020-9
Case, T.J. (1978). "A general explanation for insular body size trends in terrestrial vertebrates". Ecology. 59 (1): 1–18. doi: 10.2307/1936628.
Biddick, M.; Hendriks, A.; Burns, K. C. (2019-08-19). "Plants obey (and disobey) the island rule". Proceedings of the National Academy of Sciences: 201907424. doi: 10.1073/pnas.1907424116. ISSN 0027-8424.