Islands from a Snail's Perspective

Islands are broadly defined as inhabitable areas surrounded by a hostile environment, which makes island a relational term. The various kinds of islands from a snail’s perspective are listed, with examples of species occurring in those places. Isolation may be brought about by surrounding water, ice, or a variety of other ecological factors. Palaeoislands have existed in the geological past but ended their existence afterwards, for example by a lowering of sea level. Since there are by definition no contact zones between islands, a species or subspecies concept based on reproductive isolation under natural circumstances cannot be applied there. It is concluded still that archipelagos constitute the most important reservoir of gastropod diversity, with a high degree of endemism. Some predictions of evolutionary theory about speciation and adaptation are tested in an island setting, with also palaeoislands taken into account. Allopatry may result in differentiation and speciation, but whether, when, and how this happens differs considerably among the taxa. The adaptation to a particular island habitat, for example a cave, may follow quite different routes, probably because of the random mutations that enable the process.