Foster's rule (also known as the
island rule) is a principle 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).