Speciation, the set of processes by which two populations of one species become distinct species, is an important topic in evolutionary biology. It is usually impractical to conduct experiments on how new species form, but occasionally the natural history of a species places it in a context that may be thought of as a “natural experiment” with regard to speciation. One such natural experiment involves the periodical cicadas of eastern North America, a group in which populations have become isolated in time and space. Some of these isolated populations appear to have evolved into distinct genetic lineages. A rare life-cycle switching event brought two such lineages into contact in the relatively recent past, and the two lineages are now behaving as distinct species. This natural experiment provides important insights into species differences and the processes that underlie species formation.