Explain the process of allopatric speciation

A population of one species is separated by a physical barrier, e.g a geographical barrier such as the movement of tectonic plates. There will then be 2 isolated groups of the same species unable to interbreed with each other. The two groups will expereince different selection pressures and therefore different alleles will have selective advantages. Overtime the offspring of the two groups will become so different that if the two groups were to meet again, they not be able to interbreed to produce fertile offspring. The two groups are now said to be different species.