You need to re-examine the results. The mutated flies were completely incapable of re-producing.
I know what I read. The flies in the individual lines reproduced quite merrily when bred to members of their own line (otherwise the lines would have died out). It was only in the case of attempted crosses between the two lines that the flies were incapable of reproducing. BOTH lines had multiple mutations. That is the point, and it proves "macro" evolution by lab experiment.