福斯特法则（英语：Foster's rule），又称为岛屿法则（英语：island rule），是演化生物学中的一种规律，指物种会因为环境资源的变化而变得更大或更小，这是岛屿生物地理学研究中的核心内容。例如，从普通猛犸象演化而来的侏儒猛犸象（Mammuthus exilis）因生活在小岛上而体型变得很小。此外，大象、河马、蟒蛇、鹿以及人等动物中也发现了类似的演化路径。

这一法则最初由约翰·布里斯托尔·福斯特（John Bristol Foster）于1964年提出。当时他在《自然》上发表了一篇题为“哺乳动物在岛屿上的演化”（The evolution of mammals on islands）的文章，他对116座岛上的物种进行研究后发现，一些动物与它们在大陆上的同类相比变得体型更为庞大，另一些则变得更小。他认为，岛屿上小型动物因缺乏天敌而变得越来越大，大型动物则因为食物资源的缺乏而变得越来越小。

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).