Abstract
Patagonia is one of the few areas in the Southern Hemisphere to have been directly influenced by Quaternary glaciers. In this study, we evaluate the influence that Quaternary glacial ice had on the genetic diversity of two congeneric fish species, the diadromous Galaxias maculatus and the nondiadromous Galaxias platei , using multilocus estimates of effective population size through time. Mid‐Quaternary glaciations had far‐reaching consequences for both species. Galaxias maculatus and G. platei each experienced severe genetic bottlenecks during the period when Patagonia ice sheet advance reached its maximum positions c. 1.1–0.6 Ma. Concordant drops in effective size during this time suggest that range sizes were under similar constraints. It is therefore unlikely that coastal (brackish/marine) environments served as a significant refuge for G. maculatus during glacial periods. An earlier onset of population declines for G. platei suggests that this species was vulnerable to modest glacial advances. Declines in effective sizes were continuous for both species and lasted into the late‐Pleistocene. However, G. maculatus exhibited a strong population recovery during the late‐Quaternary ( c. 400 000 bp ). Unusually long and warm interglacials associated with the late‐Quaternary may have helped to facilitate a strong population rebound in this primarily coastal species.