Discrete Isolation and Continuous Area Drive Genetic Divergence Across Mediterranean Islands

Aim Comparison between islands and equivalent mainland areas to dissect the effect of area, isolation and species traits in determining island genetic endemicity and genetic differentiation. Location The Western Mediterranean region. Time Period Current. Major Taxa Studied Butterflies (Lepidoptera, Papilionoidea). Methods We analysed 10,367 COI sequences from 105 species, 34 islands and 47 sea straits, along with four functional species traits. We compared determinants of genetic diversity (nucleotide diversity, NucDiv ), endemic mutations (EM) and the Dst fixation index between island populations and similarly sized mainland populations (Continental Area Equivalents, CAEs). Generalised linear mixed models tested fixed effects and interactions of island characteristics and species traits. We also evaluated whether population‐level effects (individual species increasing differentiation with island size and isolation) or community‐level effects (larger, less isolated islands hosting more genetically divergent species) better explain observed island patterns. Results CAEs and islands exhibited highly distinct genetic signatures. NucDiv was higher in CAEs while Dst was higher across sea straits than between land areas separating CAEs, particularly for larger islands and non‐migratory species. EM increased with island area, especially in non‐migratory and small species. Island communities were significantly nested. Widespread species also showed lower genetic variation, thus producing a strong community‐level effect. The population‐level effects showed a weaker effect. Counterintuitively, increasing isolation did not increase divergence, as both processes tended to reduce genetic differentiation in more isolated islands. Main Conclusions Comparing islands with CAEs reveals that even narrow sea barriers of a few kilometres greatly reduce gene flow in non‐migratory butterflies, increasing genetic differentiation to a greater extent than similar mainland distances. After controlling for the presence of sea straits, island size predicts genetic divergence while increasing isolation does not. Overall, endemicity remains low, indicating that mainland‐driven genetic turnover, rather than in situ evolution, dominates insular genetic patterns.