Contrasting Palaeogeographic Histories but Similar Microevolutionary Responses in Low‐Dispersal Island Grasshoppers From Two Mediterranean Archipelagos

Aim Island archipelagos serve as natural laboratories for understanding how geological history, climate change and dispersal limitation interact to shape diversification. Here, we examine how these forces influence patterns of genomic divergence, demographic dynamics and phenotypic variation in island‐endemic taxa, with a particular focus on evaluating whether contrasting connectivity regimes among archipelagos are associated with distinct microevolutionary outcomes. Location Ionian and Aegean archipelagos, Greece. Taxon Brachypterous grasshoppers including the Ionian‐endemic Chorthippus sangiorgii and the Aegean‐endemic Chorthippus biroi (Orthoptera: Acrididae). Methods We generated genome‐wide ddRAD‐seq data to estimate divergence times, infer genetic structure, assess relative levels of genetic connectivity and reconstruct the demographic trajectories of populations. In addition, geometric morphometric analyses of forewing and pronotum‐head shape were conducted to evaluate morphological differentiation among island populations. Genomic and phenotypic patterns were interpreted in the context of palaeogeographic reconstructions of island connectivity associated with Quaternary sea‐level oscillations. Results Both species show strong genetic structuring within and among islands, consistent with extremely limited dispersal. Whereas C. sangiorgii exhibits relatively shallow divergence compatible with recent and recurrent connectivity among the Ionian Islands, C. biroi displays deeper genetic structure consistent with the long‐term isolation of Crete and the Cyclades. Demographic reconstructions for both species indicate population expansions during the Last Glacial Maximum followed by contractions throughout the Holocene, suggesting broadly similar responses to Late Quaternary climatic oscillations. Geometric morphometric analyses detected modest but statistically significant phenotypic differentiation among some island pairs in both species. Main Conclusions Our findings indicate that the microevolutionary and demographic dynamics in Mediterranean island grasshoppers are shaped by the interplay between limited dispersal, historical isolation and palaeogeographic connectivity driven by Quaternary sea‐level oscillations. Phenotypic differentiation appears more limited and heterogeneous than genomic divergence, suggesting a partial decoupling between patterns of genetic and phenotypic variation. This study highlights the value of integrating genomic and phenotypic data to better characterize the microevolutionary trajectories in low‐dispersal taxa inhabiting complex island systems.