Answer: Evolution and Natral selection both make symbiotic relationships between differnent species for instance the Egyptian plover also known as the crocodile bird, and a crocodile have a symbiotic relationship due to the bird needing food and the crocodile always catching food the bird feeds from the teeth of the gator eating left over food and the crocodile ends up having its mouth and teeth cleaned. This happend due to the lack of food sources for the bird.
Note: Hope this helps. Not sure if this is what they're looking for but its my best shot and view of it. Good luck :)