It's the theory of the selfish replicator. Darwinism is a much more general idea than the particular version of Darwinism which happens to explain life on this planet. Darwinism in this more general universal sense refers to the differential survival of any kind of self-replicating coded information which has some sort of power or influence over its probability of being replicated. DNA is the main kind of replicating entity that we know on this planet that has that property. When we look at living things on this planet, overwhelmingly the kind of explanation we should be seeking, if we ask what the functional significance is an explanation in terms of the good of the genes.
Any adaptation is for the good of the genes which made that adaptation.