Antibiotic resistance in bacteria can be explained through Darwinian selection theory based on natural selection.

The essence of Darwinian theory is that nature selects for fitness. A colony of bacteria has built-in variation in terms of ability to utilize feed components and survive under different conditions. These variations are heritable and have a genetic basis.

When antibiotics are introduced, they act as a selection pressure. In the bacterial population, some variants possess pre-existing resistance due to chance mutations. Only these resistant bacteria can survive under the new conditions created by the antibiotic. The non-resistant bacteria die, while the resistant ones survive.

Since microbes divide very fast and have a short life span, they can multiply and become millions of individuals within hours. The resistant variants outgrow the others and appear as new forms within days or months, not centuries. This happens much faster than in animals like fish or fowl which would take millions of years for similar changes.

This is an example of evolution by anthropogenic action. It demonstrates that evolution is not a directed or deterministic process but a stochastic process based on chance events in nature and chance mutations in organisms. The fitter variants are selected by nature and leave more progeny over generations.