A population of green lizard’s lives on one side of a white river, and a population of blue lizards lives on the other side of the same river. A scientific test reveals that individuals from the different groups can mate with each other and produce viable offspring. Which of the following is a valid conclusion about the lizard populations?

A.) The distinct populations came to look like each other due to similar environmental conditions.

B.) The lizards have been crossing the river to mate.

C.) The two lizard populations are beginning to merge into one species.

D.) The two lizard populations were once a single species.