Someone explained it to me like this. Its sort of a weird way of looking at it, but just follow along. Say a new dictator (the antibiodic) comes to power. He orders everyone's left hand to be chopped off. So as hes chopping off everyone's left hand, he comes across 2 people who were born without a left hand. While everyone else bleeds to death, the 2 people without left hands survive and reproduce having more people without left hands. These people are naturally weaker though because they only have one hand.
Amplifying on my earlier post:
Natural selection is concerned with the here-and-now. It selects the organisms that are best adapted to the environment
today. "Weaker in some previous environment" doesn't matter. "Fitter in today's environment, right now" matters. And by that standard the people with no left hand are fitter.
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