That is because living organisms do not exist in a vacuum. They are affected by what is present in their surroundings. If their traits make them unsuitable for their environment, they will perish. For example, if a frog that lives in a pond suddenly finds itself in a desert, then it will probably die. The desert environment becomes the selective agent, and it will "select" animals and plants that can survive in that environment. Most organisms cannot change its environment and are at its mercy. Therefore it cannot select its own traits even if it wants to. Only those traits that allow it to survive and reproduce in a particular environment will persist. Of course animals also have the option of migration. But this option is often limited by geographic barriers and by the ability of the animal to migrate. Birds are much better at migrating than, say, snails.
