I think Darwin used natural selection as a possible reason for evolution. Natural selection says that the effects of the environment influence the survival of certain members of a species. Depending on the environment certain traits survive while others die off. In an area where most of the food is in hard to reach places, birds with longer beeks will tend to survive while those with shorter beeks will die off. The same species living in an area where food is plentiful and easy to get to the opposite might be true. In a few generations we have two sub species due to natural selection. Is it the only means of explanation? Not at all. Other un-natural causes can result in a species evolving. Acid rain, radiation, polluted water all are messing with our species and they are anything but natural.