What is the first step in allopatric speciation?

The word Allopatric is derived from the Greek words allos = other and Patris = fatherland. Allopatric specification happens when the biological population of the same specie got separated from one another such that it stopped their gene flow or gene migration. It happens due to the geographical changes such as the movement of continents, changes in glaciers, mountains or oceans and it could be happened because of human activities like agriculture and industrial development etc. The first step of Allopatric specification is Geographical isolation. The species got separated and the long distances makes them unable to interbreed.