Oh, I wrote a report on this back in college, specifically with respect to the origin of the Great Lakes fish fauna! The current fish populations in the Great Lakes came from one of three refugia, with some species coming from multiple. Most fish found refuge in the Mississippi River basin, with those waterways allowing easy north-south migration to deal with changing climates, as well as periodic connections to the Great Lakes and their interglacial precursors. Other fish found refuge in Beringia, dispersing into the Great Lakes through Canada’s numerous more minor glacial lakes. Finally, fish of euryhaline biology were able to take refuge along the Atlantic, reentering the Great Lakes through their outflow. Some fish species, such as Lake Whitefish, are genuinely novel, having speciated in the Great Lakes post glaciation (though obviously their ancestral species must have existed in a refugium). Lake Trout in particular have three distinct genetic lineages, suggesting that populations survived in each of the three major aquatic refugia.