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Natural selection is the theory that accounts for the existence of adaptations among living organisms, though it may not be the driving force behind evolution. However, the neutral theory which could be misunderstood to be anti-Darwinian, says nothing about one way or the other about the importance of Darwinian natural selection. The neutral theory propounded that nothing caused adaptations.
Natural selection chooses the best hereditary characteristics which will ultimately survive. Fitness, in Darwinian theory, meant the qualities which assisted in a living organism's survival and reproduction in the future.
Sexual Selection
Sexual Selection is the theory that organisms evolving specific attributes for the purpose of attracting mates. However, the distinction between natural selection and sexual selection can be misleading at times due to the fact that qualities which are ideal for sexual selection contrast with those used for natural selection, or individual survival. For example, Darwinism highlights udders and wombs as organs which evolved by natural selection, not sexual selection because they do not assist in their winning over of mates.
Wallace, who was the co-discoverer of natural selection, abandoned partnership with Darwin over their disagreement with sexual selection. He believed that the ornamental characteristics of organisms had other functions than just to attract mates.
Later theorists accept that extravagant male ornamentation is used to help females make their choice of their mates.