Allopatric speciation occurs when a population of a species becomes geographically isolated, leading to the formation of new species. This isolation can happen due to physical barriers like mountains, rivers, or human activities. Over time, the separated populations adapt to their unique environments, accumulating genetic differences. As these differences increase, the populations may become so distinct that they can no longer interbreed, even if they come back into contact. This process illustrates how geographical separation can drive the evolution of new species, highlighting the role of natural selection and genetic drift in shaping biodiversity.