An animal that is healthier will have a better chance of survival due to better strength, and also be better suited to winning a mate. Animals that are weak or sick are most likely to die off or be killed by a predator or natural reasons, if this animal survives, it will most likely not get a mate because the mate will not be attracted to the sickened animal, considering how most animals search for a healthy mate to reproduce with. (Usually there is some type of court ship dance or song of some sort, and it is usually the more healthy animal that is able to "perform" the best dance/song for the mate it is trying to win over) Hope that this helped!
find a article that talks about those. start you sentence off by saying the things the our making our enverment bad and try finding quotes from the article you choose
