Hidden island endemic species and their implications for cryptic speciation within soil arthropods

Aim Specialisation to the soil environment is expected to constrain the spatial scale of diversification within animal lineages. In this context, flightless arthropod lineages, adapted to soil environments, but with broad geographical ranges, represent something of an anomaly. Here we investigate the diversification process within one such ‘anomalous’ soil specialist, an eyeless and flightless beetle species strongly adapted to the endogean environment but distributed across several oceanic islands. Location Canary Islands. Taxon Geomitopsis franzi Coiffait, 1978 (Coleoptera, Staphylinidae). Methods We performed an integrative study, including molecular phylogenetics, population genomics and morphometry. Four DNA regions (two mitochondrial and two nuclear) were amplified and sequenced for 159 specimens from 58 localities sampled across five islands for phylogenetic analyses, and a dated phylogenetic tree was obtained using a mitogenome dataset. ddRAD‐seq data were generated to evaluate mtDNA lineages in sympatry against the biological species concept. Results We found high levels of genetic differentiation (>8% COI gene divergence) among populations from different islands and among geographically coherent lineages within single islands. Lineages within Tenerife presented significant patterns of isolation by distance, with ddRAD‐seq providing evidence that lineages represent biological species. Morphometric analyses revealed limited variation. Main conclusions Geomitopsis franzi is comprised of at least seven lineages that merit consideration as biological species, and is best considered as a complex of cryptic species. The limited morphological variation across these lineages is consistent with adaptation to the endogean environment placing strong constraints on morphological change. The evolution of cryptic species should be favoured when such constraints are coupled with limited dispersal ability, a trait that broadly characterises the soil mesofauna.