Demonstrate what a fossil is by making an imprint of a leaf in sand or soil. Describe how scientists study imprints of soft structures. Then show students a number of examples of fossils, describing what they are and, if possible, their age. Discuss how the age of fossils is determined, and describe other types of organisms that were alive at the time of the fossils the students are viewing.
Divide students into groups and pass out 10 red and 10 black jelly beans to each group (any pair of color combinations will do). With a population of 20, they can easily see that the dominant gene has a frequency of 0.5, the same as the frequency of the recessive phenotype. Devise a scenario to change gene frequencies in the succeeding generations. For example, a certain type of predator (jelly bean jackal) prefers red jelly beans to black and, as long as they are fairly abundant in the population, the jackal will take the time to seek out, chase down, and consume the reds. Remove 5 red jelly beans and replace them with 5 black ones (keep the total population at 20, assuming that is the carrying capacity for the environment). Show how gene frequencies are changed.
In the next generation, remove 3 more red jelly beans, and replace those with black ones, again illustrating the change.
Now, with few red jelly beans to be found, the jelly bean jackal must alter his eating habits a bit, and select equally from blacks and reds. Have students remove 10 jelly beans at random, and then calculate gene frequencies of the remaining group of ten. You should see differences among groups as to gene frequencies, illustrating both what can happen by random chance as well as what happens through time as gene frequencies are affected by natural selection.