How geographical isolatikn causes evolution?
There are many ways in which a population can be separated geographically. A change in climate, or a geological change such as an orogenic event or continental drift may, over time, split an originally single population.
More likely is the transportation of a small sample of the population to another geographical area. This sample of the population would contain only a small proportion of the total genetic variation and differentiation would therefore occur relatively quickly, as any rare alleles will be allowed to become dominant.
Once a population has been split by geographical isolation the new populations will evolve independently. This is both from adaptation to environments and non-adaptively by random genetic drift. So geographic isolation causes individuals of an original species to accumulate sufficient genetic differences to prevent them breeding with each other when they are reunited.