And one species which evolves into another can not change back. As a matter of fact, there is a evolutionary law that an organ once lost or modified, can't return to its original state.
For instance. Snakes are believed to have evolved from a burrowing lizard type reptile which was on the way to losing its eyesight as many fossorial animals have. In the process of losing its eyesight, the ability to focus on objects at different distances was lost. When snakes later evolved back into terrestrial animals, they had lost the ability to focus their eyes as all other vertebrates do - by changing the shape of their lens. So they developed another way to do so by changed the POSTION of the lens with respect its distance from the retina. They are the only vertebrates which do this. It was impossible for them to “re-evolve” the lost method of focusing their eyes.