3. Selection in Action: Microbes & Moths

Natural selection isn't only ancient history — we can watch it happen. Two classic cases make the logic concrete.

Antibiotic resistance. In a population of bacteria, a few individuals happen to carry a gene (often from a mutation) that resists a drug — that's variation. When an antibiotic is used, the non-resistant bacteria die, but the resistant ones survive and reproduce. After many generations, most of the population is resistant. This is why misusing antibiotics is dangerous.

The peppered moth. These moths come in light and dark forms. When tree bark was light, light moths were camouflaged from birds and were common. As pollution darkened the bark, dark moths became better hidden, survived more, and became common instead. When pollution was later reduced, the light form recovered.

⚠️ Common misconception: an antibiotic does not 'teach' bacteria to become resistant. It selects bacteria that were already resistant (or that became resistant by a chance mutation).

💡 Tip: selection favors traits already present in the population — it doesn't create the trait because it's needed.