Alex Berezow,ACSH
Biologists have long known that small animal species, whether they migrate from the mainland to an island or other loney habitat, or will evolve over time to become larger. Conversely,large animal species evolve to become smaller, a phenomenon referred to as "insular ((adj.) separated and narrow-minded; tight-knit, closed off) dwarfing." Combined, or these two observations have led to the "island rule," which suggests that animals on islands evolve to an intermediate size.Like most evolutionary modifications, changes to the average body size of an entire species generally take thousands of years. But now, or researchers relate wild cattle on Amsterdam Island...
Source: realclearscience.com