Islands are a hot-spot for evolution which contain many epidemic species. Being separated from other land-masses they evolve unique wildlife.
Quite often these are Volcanic island like Surtsey which rise out of the sea, in this case life has to be blown in from across the ocean. Plant seeds are blown in or carried in the droppings of birds. Insects and other invertebrates are blown in or arrive on floating rafts of vegetation, birds are common as they fly in and bring in seeds of plants which makes them an important part of the ecosystem. Flightless birds are very common an example being the Dodo. Reptiles are also common usually arriving on floating rafts of vegetation as they can survive the harsh conditions, the Giant Tortoises of Galapagos Islands. The only mammals usually on the islands are bats, but small mammals like rodents quite often arrive on floating rafts. Amphibians aren't present on these islands as they cannot survive the salt water, the only way they can get in is if they are introduced by humans. Other examples like these are reef islands and sand islands.
Islands that split from other land masses
Some islands are land masses which have broken away from other land masses, examples of this are New Zealand, New Caledonia and Madagascar. In this case the islands still contain life that was there before they split off from the mainland and sometimes that life has gone extinct elsewhere. Examples arae Madagascar's lemurs and New Zealand's Tuatara. The flora is also unique such as the Forests of New Caledonia where many of the plants have roots going back to the age of the Dinosaurs. Fresh Water ecosystems are unique because they have been separated for so long and have kept their animals such as fish and amphibians. Mammals would be expected to survive although they quite often die out for unknown reasons leaving only bats which fly in together with birds and reptiles to take their place. On Madagascar surviving mammals such as the lemurs and are unique. Birds will quite often fly in and evolve to become flightless while other are ancient remnants such as New Zealand Wrens. Reptiles can also arrive on the Islands while others are still there from before the island broke of such as Madagascar crocodiles. Invertebrates can also come in from overseas while others such as the Weta are unique.
Due to the long separation from other land masses imported species are devastating because the native animals have not evolved the capacity to compete with imported species. This together with human colonization has forced many native species to extinction.
In Speculative Evolution islands are quite often used as they are separated and evolve unique organisms, an example in popular culture is Skull Island from the King Kong movies. Another example is from the book Fragment. On Islands non-avian dinosaurs are quite often speculated to survive and continue evolving though no scientist has ever found any.