Reproductive isolation occurs when

The population on the left bank is now geographically isolated from the population on the right bank. This isn't enough, by itself, for reproductive isolation to arise. If the river dries up the populations are again free to interbreed and no reproductive isolation occurs.Any of the factors that prevent potentially fertile individuals from meeting will reproductively isolate the members of distinct species. The types of barriers that can cause this isolation include: different habitats, physical barriers, and a difference in the time of sexual maturity or flowering.

But picture a river that last for 2 million years. These two populations will undergo their own individual evolution. Genetic drift and resultant natural selection will then act on these two populations. They might evolve different mating rituals, or undergo sexual selection and develop bright blue skin flaps on the males that the females find really attractive. You now have two distinct species where you had one in an example of allopatric speciation. Now the river dries up, the two populations can once again intermingle but there is no more inbreeding. The two species are behaviorally isolated and as a result they are reproductively isolated from each other.

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