The discovery of terrestrial troglobites living in caves on young oceanic islands with close epigean relatives living in nearby surface habitats offers unique opportunities to develop and test hypotheses concerning their evolution. Studies comparing the physiological ecology of troglobites with their epigean relatives suggest that troglobites are highly specialized to exploit resources within the system of interconnected medium-sized voids (mesocaverns) and only colonize cave passages (macrocaverns) with a stable, water vapor-saturated atmosphere. Few other animals can live in the mesocaverns. Rather than being relicts isolated in caves by the extinction of their epigean ancestral population, troglobites appear to evolve by a process called adaptive shift from species that are frequent accidentals in the mesocaverns.