AP® Biology · Unit 7 · Topic 7.8
Continuing Evolution
Resistance evolves because random mutations already present in a population let some individuals survive a drug or pesticide, and those survivors reproduce and take over. The chemical never creates resistance; it only selects for it.
Evolution keeps happening in real time because mutation keeps supplying raw variation and selection keeps acting on it. Before a bacterium ever meets an antibiotic, random mutations in its population have already produced a few cells with altered proteins that happen to resist the drug. The antibiotic doesn't cause those mutations. It just changes who survives. Susceptible cells die off, resistant cells keep dividing, and within generations the population is mostly resistant. Because bacteria reproduce in hours, this shift can happen fast enough to watch happen within a single infection.
The same logic explains a 1950 antibiotic failing now: decades of repeated exposure, across millions of patients, kept selecting for the rare resistant survivors until resistant strains became common worldwide. Stopping a prescription early makes this worse. It kills the weakest, most susceptible bacteria first, leaving moderately resistant ones alive to multiply and pass on their genes. Finishing the full course is more likely to wipe out the infection entirely, leaving fewer bacteria of any kind to carry resistance forward.
This topic closes out the unit on natural selection by showing evolution isn't just a fossil-record story. It's an ongoing population-genetics process, the same allele-frequency shift from earlier lessons, now visible in clinics and farm fields. If an antibiotic or pesticide is withdrawn for years, resistance often declines, since maintaining resistance machinery can cost energy with no payoff when the pressure is gone, so susceptible alleles can drift back up through later generations.
Key terms
- Selective pressure
- An environmental factor, like a drug or pesticide, that causes some individuals to survive and reproduce more than others.
- Resistance allele
- A gene variant, arising from random mutation, that lets an organism survive exposure to an otherwise lethal chemical.
- Allele frequency
- The proportion of a specific gene variant among all copies of that gene in a population.
- Differential reproduction
- The pattern where individuals with certain traits leave more surviving offspring than others, driving allele frequency change.
Watch it explained
- Common mistakeWhy doesn't exposure to a pesticide cause insects to become resistant?
- How it worksWalk me through a resistant bacterium surviving a course of antibiotics and taking over.
- How it worksHow does finishing a full prescription slow resistance from spreading?
- Going furtherWhat happens to a resistant population when the antibiotic is withdrawn for years?
Check yourself
End of topic.