Continuing Evolution

Evolution is happening right now, fast enough to watch inside a human lifetime. Topic 7.8 covers two real-world cases: antibiotic resistance in bacteria and pesticide resistance in insects are natural selection running in fast-forward, driven by pressures we apply ourselves. Because bacteria divide in minutes and produce enormous numbers of offspring, a population can shift from mostly susceptible to mostly resistant in a matter of days. The same logic that took millions of years in the fossil record plays out on a clinic timescale.

The move that trips almost everyone is direction of causation. The drug does not cause resistance — it selects for it. Before any antibiotic is applied, a bacterial population already carries a spread of variation, and a few cells happen to carry a resistance allele. The antibiotic kills the susceptible majority and leaves the resistant few, which then reproduce and rebuild the population from resistant stock. It reveals and multiplies variation that was already there, rather than reaching into a cell and installing a new trait on demand. And as always, it is the population that evolves, not the individual — a single bacterium does not sense the antibiotic and remake its own genes to survive.

Seed a bacterial population that already holds a few rare resistant cells, apply the antibiotic, and run it generation by generation. Watch the susceptible majority die off and the resistant survivors reproduce until they dominate — the drug never creates resistance, it only selects the variation that was there first, and it is the population, not any single cell, that changes.

The mistakes here cluster around a few failure modes. Students imagine evolution as goal-directed or progressive — bacteria "trying" to beat the drug — when it has no plan (U7-BIO1); they say an individual adapts on purpose, as if a single microbe toughens up in response to exposure, when it is populations that evolve (U7-BIO2); they claim organisms develop traits because they need them, when the resistance variation exists first and selection only filters it (U7-BIO4); they believe the antibiotic itself causes or creates the resistance rather than selecting pre-existing resistant cells (U7-BIO16); and they assume evolution is always too slow to watch happen, missing that resistance can spread across observable generations (U7-BIO15). Every scenario in this topic asks you to keep the causation straight — variation first, drug as filter second — and populations, not individuals, as the thing that evolves.