The process of mutation is the only known source of the raw materials of genetic variability, and hence of evolution. Mutation does produce hereditary changes, but the mass of evidence shows that all, or almost all, known mutations are unmistakably pathological and the few remaining ones are highly suspect. I doubt if among the many thousands of known mutant types one can be found which is superior to the wild type in its normal environment, only very few can be named which are superior to the wild type in a strange environment. Ibid., p. 100. In general, they are useless, detrimental, or lethal.
Visible mutations are easily detectable genetic changes such as albinism, dwarfism, and hemophilia.
Winchester quantifies the relative frequency of several types of mutations. Lethal mutations outnumber visibles by about 20 to 1. Mutations that have small harmful effects, the detrimental mutations, are even more frequent than the lethal ones. Winchester, p. 356. I took a little trouble to find whether a single amino acid change in a hemoglobin mutation is known that doesn't affect seriously the function of that hemoglobin.