Evolution means change.over generations. So biologists would describe this as the species evolving to be less fit. From the perspective of our thread, the key thing is that the arrow always goes forward, and change as we follow things forward is evolution.
Putting this yet another way, a set of mutations is incredibly (astronomically) unlikely to be reverted back to the ancestral state. Reversions do happen, but it would be basically impossible for a group of more than two or three mutations to spontaneously revert. If this weren't the case maybe devolve would be a technically useful idea. But thermodynamics being as it is, reversion is super rare. So species are always going forwards, with each new generation basing itself off the last, and no real opportunity to go backwards.
I hope this is at least interesting because I am tired and it is coming out messily.