Evolution

Antibiotic resistance in bacteria is explained by Darwinian natural selection. In a bacterial population, there is variation: some bacteria may already have mutations that make them less affected by an antibiotic. When the antibiotic is used, the non-resistant bacteria are killed, but the resistant ones survive because they are better fit in that environment. These survivors then reproduce rapidly and pass the resistance to their offspring. Over time, the population becomes mostly resistant. This shows that heritable variations that improve survival are selected by nature.

A species is a group of organisms that are similar to one another and can interbreed among themselves to produce fertile offspring. This reproductive isolation from other such groups is an important feature used to identify a species.