Natural selection states that the most advantageous trait in a species will become more common in the population, and can even replace other non useful trait over time. That is evolution by natural selection. In the case of anteaters, these animals became better adapted to the environment by growing a longer snout to have better access to food (ants).
There are three, since each population is a group of individuals of the same species that live in the same area and can interbreed and produce fertile offspring.