The Hawaiian Islands provide the venue of one of nature's grand experiments in evolution. Here, we present morphological, behavioral, genetic, and geologic data from a young subterranean insect lineage in lava tube caves on Hawai'i Island. The Oliarus polyphemus species complex has the potential to become a model for studying rapid speciation by stochastic events. All species in this lineage live in extremely similar environments but show strong differentiation in behavioral and morphometric characters, which are random with respect to cave age and geographic distribution. Our observation that phenotypic variability within populations decreases with increasing cave age challenges traditional views on founder effects. Furthermore, these cave populations are natural replicates that can be used to test the contradictory hypotheses. Moreover, Hawaiian cave planthoppers exhibit one of the highest speciation rates among animals and, thus, radically shift our perception on the evolutionary potential of obligate cavernicoles.