Foster's rule

From Biocrawler's_rule

For the rule involving recovery times in long-distance running, see Jack Foster's rule

Foster's rule (also known as the "island rule") is a principle in evolutionary biology stating that members of a species will get smaller or bigger depending on the resources available in the environment. This is the core of the study of island biogeography. 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 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 into smaller versions of themselves. For this, he proposed the simple explanation that smaller creatures got larger in the absence of so many predators that they had been used to on the mainland and larger creatures get smaller with the absence of food sources.

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 of why all this happened was not completely true and was oversimplified.


  • Foster, J. B. (1964) The evolution of mammals on islands. Nature 202, 234 235.
  • Case, T. J. (1978) A general explanation for insular body size trends in terrestrial vertebrates. Ecology, 59, 1-18.

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