It's a lousy title, but the deal is: if a mutation is rare but then everyone without it dies, it will become the norm, and so for the group, the result is a shi

It's a lousy title, but the deal is: if a mutation is rare but then everyone without it dies, it will become the norm, and so for the group, the result is a shift to the newly-common resistance, which is what the poorly-worded title was trying to say.