Search

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.

Recent discoveries of obligate cave species in the tropics and in medium-sized subterranean voids provide opportunities to test hypotheses developed during pioneering work in temperate limestone caves. Most obligate cave species share similar morphological, physiological, and behavioral features, which indicates the presence of similar strong selection forces in their highly stressful subterranean environment. Major stresses include perpetual darkness and humidity, lack of important environmental cues, complex mazelike living space, stressful or even lethal gas mixtures, patchy food resources, barren rocky substrates, wet and slippery vertical surfaces, and occasional flooding. In cavernous regions the boundaries of surface species are vertical as well as linear, with abundant food resources sinking across the vertical boundary into the high-stress subterranean biome. Parsons's stress-determined species boundary model on the role of stresses in increasing phenotypic and genotypic variability at boundaries is expanded to explain how an adaptive shift could occur at such food-rich boundaries, which would allow a new population to diverge from its parent population and exploit resources in a novel environment.

Cave-adapted arthropods have evolved in lava tubes in Hawaii. This is the first report of cavernicoles from the Hawaiian islands. The specialization of the cavernicolous insects and the recent origin of the lava tubes suggest that subterranean connections between lava tubes regularly occur and provide dispersal routes. The discovery that lava tubes were colonized by representatives of the adaptively radiating native fauna offers significant potential for evolutionary studies.