High-stress subterranean habitats and evolutionary change in cave-inhabiting arthropods.

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.