Sorry but there are many observed examples of allopatric speciation. Here is an example:
The Isthmus of Panama, a tiny strip of land joining North and South America, is fairly young in geological terms: it's only three million years old. Before the isthmus existed, there was just ocean in that area, and that ocean included fifteen species of snapping shrimp.
Snapping shrimp
Today, there are fifteen species of shrimp on one side of the isthmus - and fifteen
different species on the other side, which we call their
sister species.
In a single population of shrimp (before the isthmus), a mutation that arose in one individual could eventually spread through the whole population as the shrimp mated with each other. But once there is a
barrier splitting the population in half, a new mutation can only spread through half the population. That's why a lack of interbreeding means the two populations evolve separately.
There can also be slightly different factors for survival in the different areas: maybe the temperature or currents are different on each side; maybe the food sources are different. This can also help the populations
diverge, or become different, from each other.
(thanks to education-portal.com)