At what point in evolutionary development does a group of individuals become two distinct species? This case addresses that fundamental question by asking students to decide whether apple maggot flies are distinct as a species from hawthorn maggot flies. In making their decision, students examine the different models of speciation and consider the primary forces that effect evolutionary change. Developed for an advanced undergraduate course in evolutionary biology, it would be appropriate for any biology course in which students are taught about the models of speciation. It could also be used in a general ecology course in which students consider the distribution and use of resources leading to niche specialization or in a genetics course if restrictions in gene flow are taught in the context of speciation.