Island dwarfism
This process may occur not only on traditional islands, but also in other situations where an ecosystem is isolated from external resources and breeding. This may include caves, desert oases, isolated valleys and isolated mountains ("sky islands"). Insular dwarfism is one aspect of the more general "island rule": when mainland animals colonize islands, small species tend to evolve larger bodies, perhaps because the island may not have key predators. On the other hand large species may  evolve smaller bodies because food supplies are limited.