Natural selection is the mechanism of evolution caused by the environmental selection of organisms most fit to reproduce, sometimes explained as "survival of the fittest." An example of evolution by natural selection is the development of bacteria that are resistant to antimicrobial agents as a result of exposure to these agents. Triclosan, which is a broad-spectrum antimicrobial agent that is effective against bacteria, and viruses, is a particular example. This article describes research on the resistance of wild clonal populations of "Escherichia coli" and "Staphylococcus aureus" to triclosan and the subsequent reversion of these resistant bacteria back to wild-type when triclosan is removed from their environment. These experiments can serve as a practical, timely, and engaging model for the study of natural selection in the biology classroom and can be performed either as long-term open inquiry or as a teacher-guided inquiry. (Contains 5 figures.)