A drug doesn't create resistance; it selects for it. Stop the drug and resistance fades slowly, if at all.
Evolution never finishes. Drugs, immune systems, and other species keep changing the environment, so populations keep changing too. Where generations are short and selection is strong, you can watch allele frequencies shift within months.
Key terms
- Selective pressure — anything in the environment that lets some variants out-reproduce others: a drug, a predator, a host's immune system.
- Fitness — how many surviving offspring a genotype leaves in a particular environment. Change the environment and the ranking can flip.
- Resistance — a heritable trait that lets a microbe, pest, weed, or tumor cell survive a chemical that kills others of its kind.
- Cost of resistance — without the chemical, resistant organisms often leave fewer offspring than sensitive ones. Resistance has a price.
- Coevolution — two species that interact closely each act as a selective pressure on the other, so both keep changing.
- Emerging disease — an infection new to humans, or newly spreading, often after a pathogen jumps from animals or evolves new traits.
How resistance takes over
- Variation comes first. By random mutation, a few cells are already resistant before the drug arrives. The drug does not cause those mutations.
- The drug kills sensitive cells. Resistant cells survive and divide, now with far less competition.
- Generations are short. Some bacteria divide every 20 minutes, so a rare resistant type can dominate a population within months.
- No single cell changed. The population did: the resistant allele's frequency rose. That shift in allele frequency is evolution.
- Stop the drug and the advantage disappears. If resistance carries a cost, selection now favors sensitive cells, and resistance slowly declines.
Why it matters
You can see this in medicine and farming. Flu vaccines are updated yearly to match the variants now spreading. HIV is treated with several drugs at once, and farmers rotate pesticides, both to slow resistance. Species also evolve against each other: a plant's toxin and a caterpillar's detox enzyme can escalate for as long as the two interact. On the exam, expect data showing resistance climbing under a drug and falling slowly after it stops.
Exam traps
| Trap | Actually |
|---|
| The drug causes resistance mutations, or cells adapt to it during treatment. | Resistant variants already existed. The drug kills the others, so the population's makeup shifts. No individual cell changed. |
| Stop the drug and resistance vanishes. | Resistant alleles linger. They fade only as fast as their cost lets sensitive types win, often over years. |
| Flu vaccines change yearly because the virus becomes immune to them. | Viruses don't become immune. New surface-protein variants escape existing antibodies and spread, so the circulating virus changes. |
| A well-adapted species has finished evolving. | Its environment keeps changing, partly because other species evolve too. Hosts and parasites, predators and prey, keep selecting each other. |
Worked example
A patient carries about 10 billion copies of a virus. About 1 copy in 100,000 has a mutation that resists drug A. About 1 in 1,000,000 has a mutation that resists drug B. The two mutations arise independently. Which treatment is more likely to fail: drug A alone, or A and B together? Use the numbers.
- Drug A alone: 10 billion ÷ 100,000 = 100,000 copies already resist A before treatment starts.
- Both drugs: one copy must carry both mutations. Independent odds multiply: 1/100,000 × 1/1,000,000 = 1 in 100 billion.
- Expected double-resistant copies: 10 billion ÷ 100 billion = 0.1. Most likely, not one exists yet.
Drug A alone is far more likely to fail. About 100,000 A-resistant copies are already there to take over, but a copy that resists both drugs probably doesn't exist.
Drugs select from variation that already exists. A single drug meets its resistant variant on day one. Two drugs demand two rare mutations in the same genome, and multiplying the odds makes that about a million times rarer. The near-miss is thinking the second drug helps by adding dose. It helps by multiplying the odds against escape.