How important are environmental barriers between species and populations now and in the future? Currently, environmental barriers to movement across habitats that have persisted since the last ice age are breaking down, resulting in gene flow among previously isolated populations and even hybridization between species. What are the consequences of this gene flow? Local genetic adaptations to the specific conditions of a habitat are though to be threatened when gene flow occurs, but we know little about the long-term evolutionary effects such events have on species. Recent ancient DNA work on polar and brown bears even suggests that gene flow may be beneficial, rather than detrimental for the adaptation and survival of species during times of rapid climate change.This project aims to investigate the extent of gene flow among and its effect upon the survival, adaptation and evolutionary history of temporarily isolated populations of animal species during periods of rapid climate change. This goal will be achieved by looking back into the late Pleistocene, when our world experienced repeated and rapid periods of massive climatic change to which species had to adapt.The project will target the evolutionary history of four species (mammoth, spotted hyena, cave bear, and grey wolf) by sequencing large parts of the nuclear genome of each species across both time and space. In each species conflicting evolutionary histories are provided by morphological and mitochondrial DNA analyses, suggesting that (so far undetected) gene flow of nuclear DNA must have occurred. Undetected gene flow may explain aspects of their evolutionary history, and also the way these species adapted to the rapidly changing environmental conditions of the late Pleistocene.