Island faunas exhibit some of the most iconic examples where similar forms repeatedly evolve within different islands. Yet, whether these deterministic evolutionary trajectories within islands are driven by an initial, singular divergence and the subsequent exchange of individuals and adaptive genetic variation between islands remains unclear. Here, we address this issue using a gradual, repeated evolution of low-dispersive highland ecotypes from a dispersive lowland ecotype of Calosoma beetles along the island progression of the Galápagos. We show that repeated highland adaptation involved selection on multiple shared alleles within extensive chromosomal inversions that originated from an initial adaptation event on the oldest island. These highland inversions first spread through dispersal of highland individuals. Subsequent admixture with the widely distributed lowland ecotype resulted in polymorphic dispersive populations from which the highland populations evolved on the youngest islands. Our findings emphasize the significance of an ancient divergence in driving repeated evolution and highlight how a mixed contribution of inter-island colonization and within-island evolution can shape parallel species communities on islands.

Funding provided by: Belgian Federal Science Policy OfficeROR ID: https://ror.org/01fapfv42Award Number: BR/121/PI/GENESORT Funding provided by: Belgian Federal Science Policy OfficeROR ID: https://ror.org/01fapfv42Award Number: BR/175/PI/PARAWINGS Funding provided by: King Leopold III FundAward Number: Funding provided by: Belgian Federal Science Policy OfficeROR ID: https://ror.org/01fapfv42Award Number: B2/233/P2/MUTER