Many variations are so trivial that they could not possibly aid an organism in its struggle for existence. The theory does not explain how the gradual accumulation of trivial variations could result in the appearance of some of the more complex structures found in higher organisms. I could never accept this answer. Random shuttling of bricks will never build a castle or a Greek temple, however long the available time. Natural selection is irrelevant to, or negligible in context of, macroevolutionary change.
Natural selection is differential reproduction, organism perpetuation.
Prebiological natural selection is a contradiction in terms. Reproduction of stable patterns and stable variants of these patterns is essential for evolution by natural selection. Nobody has ever succeeded in producing a new species, not to mention the higher categories, by selection of micromutations. I venture to say that few who have made a special and practical study of evolution, and are well-acquainted with recent progress in that study, have much faith in Natural Selection. Natural selection per se does not work to create new species.