They should be able to write directions for their experiment as well as demonstrate it if time allows. As they are demonstrating, they should be able to verbally explain how natural selection works. Assign a short writing exercise. Tell students to write one to two pages on a species. They can fictionalize the species, describe the species' advantages, and how it may have changed over millions of years to adapt to the environment.
Students can be as creative as they wish.
For instance, if they want to create a deer with extra long legs, the situation could be that food became higher, or that they had to run faster to get away from predators. Describe the species in the beginning. Explain how it changed from generation to generation. Papers should show how the proportion of individuals that have advantageous characteristics would increase. If you want to further the class discussion on adaptation, below are two recommended articles from the UniSci site that bring up another aspect of adaptation.