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Precisely, they are random DNA changes, that result in change of existing traits, new traits, disappearance of traits, thus leading to either a new variety within the same species (sexually compatible) or appearance of new species (sexually incompatible). Human skin colors are result of a random mutation in MC1R gene 1.2m years ago that was later reinforced by natural selection, but we are all varieties of the same species and we can reproduce regardless of skin, hair or eye color.