Also known as the Island rule, this refers to a hypothesis regarding how the body size of species may be influenced by various environmental factors such as the availability of resources. It states that small mammals like rats may evolve and turn bigger in size over time when they are isolated on an island with more resources and fewer predators. By the same token, larger mammals like elephants begin to develop smaller bodies if they happen to become isolated in an island where the availability of resources is scarce. The rule is named after the biologist J. Bristol Foster who proposed it in his 1964 paper ‘Evolution of mammals on islands’.
