Nib asked
How does antibiotic resistance show natural selection?
Antibiotics don't make bacteria resistant. In a huge population a few cells already resist by random mutation; the drug kills the rest, and the survivors multiply. That sorting is natural selection.
6 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1variation first
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.
a common mix-upYour body does not become resistant, and no bacterium learns resistance; resistant cells already exist by chance in the population.
2the drug sorts
Now add the antibiotic. It kills the susceptible cells, nearly all of them. The resistant one survives, with no competition left.
The drug does not create resistance; it removes the susceptible cells.
3survivors multiply
The survivor divides, and divides, and passes the mutation to its daughters. Soon the whole population resists. That is natural selection.
4the shift
Track the resistant share over time. Without the drug it stays tiny. With the drug, selection lifts it until nearly everyone resists.
5every new drug
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.
6the whole idea
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.