As islands are usually separated from a larger landmass or continent, life on larger islands tend to diversify without little interference from it's main landmass or other continents. Insular environments are known to have unique and "odd" life compared to the mainland, making them famous in evolution.
Largely, the first animals to reach islands are flying animals, such as birds and bats, if a mammal is not already established on the island, mammals such as dolphins or pinnipeds are likely to be the largest mammals in that island region. Animals evolve differently on an island depending on habitat, size and species.
For non-flying or swimming animals, rafting or drifting across oceans, being swept by currents, is generally the method in which they happen upon archipelagos and islands. This has the implication of a given species that arrives on the island having only a handful of individuals. Lemurs, for example, are thought to have descended from a little over a dozen individuals.
The implication that rafting is the best way to get to an island means many insular habitats have few or no terrestrials mammals aside from bats; reptiles, particularly tortoises, have been noted to be far better at getting to islands then mammals of the same size in a given habitat; and flightless birds have been known to also get to close offshore islands, thought are also not good at crossing large distances over water, perhaps indicating metabolism plays a part in rafting success. Indeed, at least some mammals that have gotten to islands have been noted to be capable of hibernation, at least when they would've arrived on the island.
Insular dwarfism is an evolutionary process that conditions the reduction in size of large animals over a number of generations when their population's range in a limited amount of land, usually islands. The reasons for this are that there is not enough room or energy in the island ecosystems to support the creatures in their original size.
This phenomenon is well documented, and generally happens with species from generally large groups, such as carnivores and ungulates. For example, dwarf elephants used to exist on several mediterranean islands, some as small as 2 meters. Similarly, mammoths existed as recently as 1700 B.C. on the Russian Wrangel Island. Going further back in time, Hateg Island of the Late Cretaceous is noted for it's dwarf dinosaurs and a small azdarchid.
Many insular dwarfs, such as Malagasy dwarf hippos, island emus, and offshore jaguars, as well as much Indonesian fauna, had survived into modern history but are now extinct, because of extermination or overhunting by mankind. Several examples of insular dwarves, living and extinct, live and lived in Indonesia and the Phillipines; such as the Bali Tiger and Tamaraw, among others.
It has also been discovered in recent times that there was also a insular dwarf example of humans, in the form of Homo floresiensis (or "hobbits") from Pleistocene Flores. Described in 2004, it has been contraversial whether the meter tall skeletons were healthy or adult. However, most research points to it being a valid dwarf. They have a long ghost lineage, being fairly primitive, and use simple tools. In addition, with remains being known from as recently as thirteen thousand B.C., it is the most recent human species other then us. It's been suggested that they are perhaps the explanation for the cryptid the Orang Pendek, and the Ebu Gogo of folklore, thought this is disputed.
Island gigantism is an evolutionary process which usually involves a smaller animal increasing in size over an amount of generations to fill an ecological role otherwise occupied by another species. Although reasons for it's occurence may vary, in some cases small animals fill roles that are vacant on islands when such niches are taken on the mainland.
This is docuemented to happen with rodents, insects, and lagomorphs in particular, with several dozen examples known. Nuralagus, the largest known lagomorphs ever by a large margin, lived on mediterranean islands.
This had also been known to happen with birds that become flightless, such as the extinct dodo, moa, and elephant birds. Furthermore, Cygnus falconari was a giant swan that lived alongside the dwarf island elephants of the Pleistocene.In modern examples, the flightless cormorant is the largest known of the cormorants, being a flightless island species. In Hawaii, a species of goose, the nēnē, is only partially flightless; some individuals being born without flight, and can be seen as a transitional form. An extinct relative was also an insular giant. 
In addition, adaptive radiations are a common result of an organism entering an entirely new set of niches on an island which has these spots vacant. Adaptive radiation is where a single species diverges to take up different ecological roles or to simply survive in an island. Take Darwin's finches, multiple species of finch in the Galapagos that all diverged from an common ancestor based on what area of the island they inhabited.
Sometimes islands are the only way for living fossils to become as diverse as they may have been before a dominant family or group took over the niches, as seen with lemurs. Originally lemurs diverged form a single species that started to split off and fill different roles in different habitats; ancestral African lemurs went extinction several tens of millions of years ago. Similarly, but on a much larger scale, marsupials diversified in Australia, taking up many different types of niches, such as herbivores, insectivores and carnivores very convergent with their placental counterparts in some cases; whilst the placentals took control elsewhere.
Artificial Island EvolutionEdit
Although not studied to be the case in many or any living taxa extensively, the Maastrichtian tyrannosaurine Nanuqsaurus, discovered in 2014, was a dwarf by comparison to other tyrannosaurs, however it lived on the mainland; although other small tyrannosaurs are know, it is hypothesized that the mountains separating Alaska, where it lived, at the time, from the rest of North America, caused a sort of artificial insular dwarfism, as it was separated from others of it's kind.
Although insular evolution is commonly considered weird, and it creates "odd" creatures, there are several factors which influence how a creature will evolve to adapt to the environment, as in any environment; the size of the island, life on it, and the geography are major factors in this.
The reason that the creatures that evolve to island life are so unique compared to their mainland counterparts is likely focused mostly on ecologic factors. On the mainland, other creatures in the environment, and the niches taken by them, restrict a given group so as to not evolve into some directions. On a island with few or no mammals, or any large vertebrates, there is much more freedom into what a group can evolve to do.
For example, the fossa and kin on madagascar only evolved because there were no other carnivores in the niche; islands let evolution do it's fullest. In this sense, it is likely that the forms on islands can represent the extremes a group could evolve into; Myotragus, a buck toothed, low metabolism caprine with forward facing eyes, and helpless young, less developed then those of any other artiodactyl, is perhaps an example of how far an ungulate could go in the amount of time given, if there was no competition. The extremes in insular habitats are important to understanding how evolution works.
All of these factors generally apply to the anatomy of the creatures in question, such as; whether or not an adaptive radiation is possible for a given group would depend on these factors, and a lot of what that radiation causes would depend on these factors; however, insular dwarfism and gigantism (as is pictured by the extinct fauna of Malta, below) are only partly affected by some of these factors (being almost entirely based off size factors; see below and in the respective section earlier in the article). Other weird features, like those of Myotragus, or several features of Balaur, a Cretaceous maniraptor from Romania, have little to do with the island's size, and are impacted by the ecology of the ecosystem.
Introduction & Feral SpeciesEdit
Islands that have species introduced by humans can cause there existing ecosystems to become unstable. Though it may be evolutionarily possible for an animal to be established in the ecosystem or evolve to suit that ecosystem. In New Zealand, a large amount of mammals have been established that are pushing many bird species out of their ecological roles. It is safe to assume in the future, mammals will start to take up more ecological roles on the archipelago than they did pre-human colonization.
Goats are among the most common of these feral species. Cats, pigs, dogs, rats, chickens, ferrets, weasels, and several other species closely allied to man are also common examples. In many cases, the insular fauna isn't used to predators (cats, mongoose, weasels, ferrets, dogs, etc), are succeptiple to brought illnesses (rats). In many examples, the ecosystem has never experienced large or even mid-sized herbivorous mammals; often no mammals at all.
ExtinctionEditThe arrival of man and feral animals has caused the majority of island species across the globe to become endangered or even extinct, and commonly entire ecosystems have been destroyed by these "assaults" by man. There are several ways in which this can happen, commonly man will eat the local creatures, and their animals will have a much larger effect on the ecosystem. It is commonly noted across several historical accounts, covering several different islands and species, that these creatures on predator-less environments are not at all scared of man, and indeed are completely docile and calm natured, having no fear of humans.
- ↑ Mouse Lemur Factsheet
- ↑ Extinction of Bali Tiger
- ↑ Tamaraw (Bubalus mindorensis); ARKive
- ↑ Article on the Orang Pendek; Cryptid
- ↑ Homo floresiensis and the Orang Pendek Related?; The Paranormalist
- ↑ Article on Homo floresiensis; Smithsonian Museum of Natural History
- ↑ You have your giant fossil rabbit neck all wrong; Tetrapod Zoology
- ↑ Nuralagus; Prehistoric Wildlife
- ↑ Quintana et al. (2011): Nuralagus rex, gen. et sp. nov., an endemic insular giant rabbit from the Neogene of Minorca (Balearic Islands, Spain); Journal of Vertebrate Palaeontology
- ↑ Northcote (1982): Size, form, and habit of the extinct Maltese Swan Cygnus falconeri; Wiley Online Library
- ↑ Flightless cormorant (Phalacrocorax harrisi); ARKive