Multispecies Genomics Uncovers a Common Legacy of Postglacial Isolation on a Neotropical Land‐Bridge Island

Aim To evaluate whether Pleistocene sea‐level changes can explain the timing and demographic processes of population divergence between continental and insular plant populations on a land‐bridge island. Location Alcatrazes Island and adjacent mainland areas along the southeastern coast of Brazil. Taxon Three bird‐dispersed plant species ( Chiococca alba , Eugenia sulcata and Schinus terebinthifolia ). Methods We used multispecies population genomic data (RAD‐Seq) to test alternative demographic scenarios of population divergence. Approximate Bayesian Computation and Supervised Machine Learning were applied to evaluate divergence timing relative to the Last Glacial Maximum (LGM) and to assess the presence or absence of gene flow between mainland and insular populations. Genetic structure was inferred to identify population clustering within each species. Results Genetic structure analyses consistently identified three populations per species, corresponding to one insular and two mainland populations. For all species, demographic models supporting post‐LGM isolation were strongly favoured, and no detectable genomic signal of recent gene flow was recovered under the selected models. These results are consistent with postglacial sea‐level rise contributing to population isolation and divergence. Main Conclusions Our comparative genomic analyses across multiple species support the hypothesis that Quaternary sea‐level rise contributed to population divergence on land‐bridge islands. Recurrent cycles of connectivity and isolation between islands and the mainland may have left a strong imprint on the demographic history of bird‐dispersed forest plants.