Speciation is the creation of a new species distinct from the original due to evolution.
I will walk through how this will work for you.
1. Animals of the same species living on either side of a stream can move from one side to another.
2. The stream changes into a river
3. Animals are now separated and must live apart
4. Environments on either side of the river change over time.
5. Animals due to natural selection, undergo change
6. After a thousands of years, two different of species form.
Hope this helps.