Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

On islands, it occurs relatively frequently that large species of animals tend to get smaller and small species tend to get larger. The usual evolutionary explanation is that, on the mainland, the greater interspecies competition or pressure of carnivores forces them beyond the limits of their optimum size, i.e. all their members tend to be large, so that they can better resist carnivores, or small, so that they can hide better and manage with a small amount of available resources. When they reach an island, where their natural enemies or competitors are absent, they can return to their optimum size – i.e. get larger or smaller to the size at which their body functions best. This could be the right explanation. However, it is necessary to consider that not every change in body structure that we encounter on islands has the nature of gigantism or nanism and, in addition, a great many long-term isolated populations in areas with low intensity of inter-species competition are apparently also formed on the mainland (however, mostly by splitting off of part of a large population), without gigantic or dwarf forms occurring to the same degree as on islands.