What populations are usually small and isolated from the rest of the gene pool

Island populations are a prime example - perhaps where a peninsular of land was eroded away to form a separate island, isolating a small population of animals, that no longer have access to the mainland and subsequently only have a limited gene pool.

Isolated populations can also occur due to habitat fragmentation for example dormice that live in ancient woodland cannot cross an open field (instead they will use natural wooded corridors such as hedges to travel), if the hedgerows connecting several woods together are cut down to make larger fields for agriculture then the populations of dormice become isolated from each other and the wider gene pool.