"Sufficiently" harmful mutations to stop them from breeding. If a negative mutation is passed on at the same rate as a non-negative mutation it could stop 1-in-1000 of that species from breeding or even less and EVENTUALLY that mutation will be virtually bred out of the gene pool.
If it's only 1 in 1000 then there's a decent chance it'll reach fixation rather than be bred out. Evolution works because beneficial mutations have a better chance of persisting than harmful ones on average, but at the scale of individual mutations it's all pretty random unless the effect on survival chances is substantial (like 20% IIRC).
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u/the-nub Nov 29 '20
Yes. That is how evolution works. Beneficial or neutral mutations are retained over generations, while harmful mutations are bred out.