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how does antibiotic resistance show natural selection
Bacteria don't learn to resist antibiotics. A population evolves. It starts with variation: in billions of cells, a few already resist the drug by random mutation.
Now add the antibiotic. It kills the susceptible cells, nearly all of them. The resistant one survives, with no competition left.
The survivor divides, and divides, and passes the mutation to its daughters. Soon the whole population resists. That is natural selection.
Track the resistant share over time. Without the drug it stays tiny. With the drug, selection lifts it until nearly everyone resists.
It's an old story. Penicillin reached patients in nineteen forty-one, and resistant staph appeared by forty-two. Methicillin came in fifty-nine, and M-R-S-A within about two years.
So antibiotics don't create resistance, they select it: variation, a drug that sorts, and survivors that multiply. That's why we save them for bacterial infections.
Resistance is natural selection on fast forward: a drug does the sorting, and survivors do the rest.