Mutations don't show up because they're needed
- The idea
- Predict
- See it
- Why
- Check
The idea
“Faced with a threat like an antibiotic, an organism develops the mutation it needs.”
Resistance shows up after a drug comes into use, so the drug looks like the cause. Saying bacteria "adapt" makes it sound like a choice.
The short version
The mistake: Faced with a threat like an antibiotic, an organism develops the mutation it needs.
What's actually true: Mutations happen at random, needed or not. The environment then selects the ones that help.
- Mutations come mostly from DNA copying errors or damage. They're random with respect to need, so a cell can't aim one at a helpful gene.
- In a big population, rare mutations keep turning up, so a few resistant cells often exist before any drug.
- The drug is a selective pressure. Resistant cells survive and pass on the resistance allele, so it spreads. That's natural selection.
- Stress can raise the overall mutation rate, but it can't target the helpful gene.
On the exam: The environment picks among mutations that already exist. It doesn't write the one that's needed.