If they had had to wait for the resistant traits to "evolve", then mortality in the exposed population would have been nearer 3/3.
They had the particular mutations because evolution is going on constantly. In the absence of selective pressures, the mutations tend to remain at fairly constant levels throughout the population. (That is, if 5% have the mutation in an initial population, about 5% of each subsequent generation will also have the mutation.) When selective pressures are added, giving those with the mutation a survival advantage, the incidence of the mutation throughout the population increases.
In this case, there were already selective pressures on the population to spread this particular mutation (there were lots of infectious diseases in Europe), which is why 2/3 of the population already had it when the Black Plague hit.
In other words, what is described in the article is exactly what we expect to see happen in an evolutionary scenario.
Grr, I should have pointed out that mutations don’t happen in response to evolutionary pressures. They happen randomly as a result of the chemistry of DNA, for the most part. Technically speaking, evolution occurs when the mutation confers a survival advantage to an environmental challenge.
Uh-huh; mmkay.
In the future all the homosexuals will be blue eyed blonds (if that's also an inheritable trait).