Why does geological isolation cause speciation

Geological isolation causes speciation because as the species is seperated, different genes become dominant amongst the separated groups. Some recessive genes can become dominant and some previously dominant genes can become recessive as these groups are no longer in contact. This leads the groups to gain different characteristics that are not common amongst the group they separated from.

Because since the species is isolated with its partners it has to find another way to multiply so it uses another animal to make a new species to stay alive