A pine forest covers a valley, and one of the inhabitants is a fox species. After days of rain, a river forms and separates the valley with a river that the foxes cannot cross, thus producing two isolated populations of foxes. What change needs to occur for the two populations of foxes to become separate species?

The correct answer is: Reprodutive Isolation.

Explanation:

According to the question, the formation of a river in their habitat separates the foxes into two populations which are unable to come into contact with each other.Gradually with the passage of time,each of the populations of foxes develop mutations which make them more adaptable to the different environments they are exposed to. This eventually results in the development of a reproductive barrier in between the two populations.Reproductive isolation can be defined as the evolutionary phenomenon due to which individuals belonging to two different species are incapable of reproducing or mating and producing viable and fertile offspring.Reproductive isolation results into the formation of  new species.In the given question, the two populations of foxes gradually undergoes divergent evolution to form two different species.