Penguins are birds that can't fly, and who love Arctic water, while cockatiels are tropical birds that can fly.
Sharks have teeth for tearing meat, and breathe in water.
Cats and dogs are carnivores who are supposed to eat meat and live on land. All of this is built into their respective genetic codes . While there is an ideal behavior, as outlined above, for each of these beings, the biochemical changes which occur in all living beings are based on how well they live up to these ideal behaviors. For instance, although we could walk on our hands and our knees, and do for part of our lives, we are probably supposed to walk standing up reasonably straight. Moreover, we are designed to breathe clean air, drink clean water, and eat some kind of diet.