From what I've read about animals, the isolated population of an island results in faster evolution due to 'close' breeding. Rare non-dominant genes are more likely to meet up and be expressed. The lack of geographic diversity also means that species need not evolve to be generalists. There is no advantage to adapting to anything but the island environment. In humans, by way of example, that might mean an island population more rapidly adapts to an endemic disease as they cannot escape it. They don't have people living elsewhere brining in non-adaptive genes and so the local adaptive genes spread more quickly than if they were on the mainland.