populations of existing species are separated by a physical or geographical barrier. As a result, over time genetic differences leading to pre- or postzygotic isolation arise between the two groups. In the second stage, the diverged populations may again come into contact. If this happens, speciation becomes complete through the action of natural selection.异地物种形成模型认为,物种可能起源于两个阶段。第一阶段,已经存在的物种群体可能由于自然或地理屏障被隔离。结果,长时间后,在两个群体间遗传差异导致前合子或后合子隔离的产生。第二阶段,这些趋异群体可能再次接触,物种发生可能通过自然选择发生。