This exercise examines when adaptive mutations arise in a population. Students prepare cultures of baker's yeast on normal agar media and then transfer the exact spatial pattern of these colonies to agar plates containing copper using the replica-plating technique. They compare the pattern of mutant colonies on the two replicate plates to test when copper-tolerant mutants arose. If the colony positions match then the mutants pre-existed in the original population (spontaneous mutation). If colonies do not match then this supports a hypothesis that mutants were induced by exposure to the copper. Students learn skills such as preparing agar plates, the sterile technique, replica-plating, and the use of pipets and dilutions.