Isolation can affect the evolution of populations and the number of endemic species that are found in an ecosystem. Isolated areas, such as remote islands, mountaintops, or even habitat patches far removed from similar areas may have more endemic species than those areas where individuals freely move from population to population.
In this example, Island A is close enough to the mainland to allow individuals to move back and forth. This maintains the connections between populations that allow the sharing of genes. As long as this mixing occurs, the populations will not diverge to become genetically distinct.
Island A would probably have very few species that are found only there – very few endemic species.
Island B is further from the mainland than Island A. Infrequent, random events can deliver a few individuals of a given species from the mainland to distant islands but it is unlikely that additional individuals of that species will arrive later on. This isolates the founding population and as it adapts to its new island home, it may eventually evolve into a new species.
Because of this greater isolation, Island B would likely have more endemic species than Island A.
The larger the island, the more niches it is likely to have. Island B and C are equally isolated from the mainland, and other islands, but since Island C is larger, it will generally have more species and more of these will be endemic.