Have small groups of students each construct a dichotomous key using a group of readily available objects (office supplies or bolts and nails). Select ten different objects (i.e., thumbtack, small finishing nail, roofing nail, bolt, screw, Phillips screw, large nut, small nut, washer, and locking washer) and give one of each type to each student group. Have them construct a key for these objects and then trade keys with another group to test how well it identifies the objects. Students generally enjoy this activity, and it helps them appreciate the time and effort involved in constructing identification keys.
If you work on a fairly large campus, a good diversity of types of trees or shrubs should be available for constructing a dichotomous identification key. Before class, construct one using 15-20 different types of specimens. Then, during class time, take students on a brief tour of the trees on campus, having them use the dichotomous key.
Alternatively, tree branch specimens or other plant specimens may be brought into the classroom.
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.