Speciation models in subterranean taxa. Schematic diagram of (a) the 'climatic relict' and (b) the 'adaptive shift' hypotheses. In the former, a broadly distributed surface species that has exaptations (pre-adaptations) to the underground environment invades the caves. The underground population remains in contact with the surface population, limiting genetic divergence of the two. Climatic oscillations cause local extinction of surface populations, whereas surviving populations remain in the underground. The predictions from this for geographic distribution are that either only relict cave-dwelling lineages survive, or surface populations are strictly allopatric and geographically remote with respect to the underground. Over time, cave populations differentiate, developing troglomorphic characters and become reciprocally monophyletic. In the adaptive shift hypothesis caves are invaded by surface populations, exploiting new resources with the establishment of differential selection pressures in the epigean and underground environments. Speciation is driven by divergent selection accompanied by a reduction of gene flow. In this case, surface and cave species are expected to have parapatric distributions, at least during the initial phases of the process. Ellipses represent geographic distributions of populations. Troglobite is the term given to animals that have become adapted to dwell in cave environments and that cannot survive outside such environments. Diagram modified from Figures 1 and 2 in [2].

Cave-beetles and phylogenies. (a) Photograph of the cave-beetle species Cytodromus dapsoides (Leptoridini, Leiodidae) from the Vercors National Park in Southeast France. The tribe Leptodirini includes about 235 genera and around 900 species, most of them exclusively subterranean. The highest diversity is found in the north and east of the Iberian Peninsula, Corsica and Sardinia, the southern Alps, Balkan Peninsula, Romania and southern Russia, the Caucasus, Middle East and Iran. (b) Simplified phylogenetic tree obtained by Ribera et al. [5] using combined mitochondrial and nuclear sequences. The tree was linearized (fitted to constancy of molecular substitution rate) using Bayesian methods. Red circles indicate tree nodes used for calibration of the molecular clock using the mitochondrial gene cox1 only (considering 33 million years ago for the age of initial separation of Sardinian species from their sister lineage), and including all mitochondrial sequence information but excluding species from Sardinia (from which only cox1 sequences were available). In the latter case an estimated age of 37.8 million years ago was used for the separation of Bathysciola zariquieyi from its sister. The width of each clade is proportional to the number of species included in the study. The basal Speonomidius lineage includes the muscicolus genus Notidocharis. A geological timeline with the relevant epochs is provided below the tree. Figure 1a courtesy of Christian Vanderbergh.