How can natural selection change a population over generations?

  Students in Teacher Harvey’s EP-M.6 Natural Selection Lab explored how natural selection can be observed using yeast as a model organism. Natural selection is difficult to observe directly in the wild because it usually occurs gradually over many generations, but yeast has a short generation time of around 90 minutes to 2 hours under optimal conditions, allowing students to observe many generations within just a few days. Students exposed yeast populations to different salt concentrations as a selection pressure and observed how the population changed over successive generations. Through this investigation, students learned that selection pressure can result in a higher proportion of resistant or tolerant individuals in a population, demonstrating the basic mechanism of natural selection and evolution.