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, deer (for example Key deer) and humans. It was first stated by J. Bristol Foster in 1964 in the journal Nature, in an article titled "The evolution of mammals on islands". In it, he studied 116 island species and compared them to their mainland varieties. He proposed that certain island creatures evolved into larger versions of themselves while others became smaller versions of themselves. For this, he proposed the simple explanation that smaller creatures get larger when predation pressure is relaxed (due to the absence of some of the predators of the mainland) and larger creatures become smaller when food resources are limited (due to land area constraints).
Later, that idea was expanded upon by the publication of The Theory of Island Biogeography, by Robert MacArthur and Edward O. Wilson. And in 1978, Ted J. Case published a much longer and more complex paper on the topic in the journal Ecology. Case also demonstrated that Foster's original conjecture for the reason all this happened was oversimplified and not completely true.