Darwin pointed out that the number of possible descendants left by living organisms is far greater than the number we see. It is therefore evident that those which survive are the ones with a competitive advantage over their own species and others that they have to share their habitat with. It is the struggle for existence, and the selection of traits which allow greater competitive ability which is the basis for evolution. It is also a theory of individual selection. Selection for the good of the group or species has no part in Darwin's original theory.
Evolution proceeds through both natural genetic variation and chance mutations.
One of the best known living advocates of the Darwinian view is Richard Dawkins, and in his popular science best seller he continues the theme of the importance of competition as a driving force for evolution. The Darwinian emphasis on competition influences our thinking on the role of competition and co-operation in human society. If altruism is indeed merely enlightened self interest, then the existence of a categorical moral imperative is in doubt. This leads to another conclusion of Darwinism, creation of the universe wholly by natural causes. There is therefore no need for a Creator.