Allopatric Speciation happens when species arise as a result of geographic isolation.
When the special rain came, the chemicals in it turned some of the lions blue.
When some of the lions start to reproduce, more and more turn blue. With more and more turning blue, they start to mate with the same color. The more and more they reproduce, genetic drift takes place.
The blue lions end up on one side of the river and mate with other blue lions. Over time, the blue lions will develop into a new species.
Allopatric speciation is more likely to occur in small populations because of a smaller gene pool.