Model system and rationale
We selected the Canary Islands as a study system for two reasons: first, we have a nearly complete knowledge of their terrestrial cave-adapted diversity (Oromí et al. 2021); second, they cover a time span of 21 Ma (Gillespie & Clague 2009), being the only studied volcanic oceanic archipelago in the world with such a broad range of ages (Borregaard et al. 2017). The eastern islands (Lanzarote and Fuerteventura) are very close to each other and about 100 km from the continent; the remaining five islands span towards the West over ca. 400 km. Except for Lanzarote and Fuerteventura, each island represents an independent volcanic building separated by oceanic depths, so they were never superficially connected to one another or to Africa (Troll & Carracedo 2016). Therefore, the Canary Islands can be regarded as oceanic islands that have been biologically colonised across the ocean, mainly through the marine channel separating the islands from North Africa (Florencio et al. 2021). Most subterranean species in the Canary Islands are single-island endemics, with speciation trajectories that happenedin situ from surface ancestors (Arnedo et al. 2007). All islands harbour extensive subterranean habitats, including caves, lava tubes, and MSS (Oromí et al. 2021). Regarding age, Lanzarote and Fuerteventura are the oldest islands, having undergone a long erosive period only interrupted by two historical volcanic episodes in Lanzarote. They are here considered in their senescent ontogenetic stage. The central islands of Gran Canaria, Tenerife, and La Gomera represent the islands with the highest structural complexity, although La Gomera and Gran Canaria have fewer caves than Tenerife, which might imply a smaller carrying capacity than expected by their altitude (Oromí et al. 2021). Lava tubes usually become destroyed within a few hundred thousand years only persisting in relatively young parts of the islands, thus being very few on Gran Canaria and La Gomera where volcanism has been scarce or even absent in the last 2 Ma (Oromí, 2004). These three islands are considered as geologically mature. La Palma and El Hierro are the youngest islands, and those that have concentrated most of the volcanic episodes in historical times (Longpré & Felpeto, 2021). A summary of the most relevant features of each island is in Table S1.