We think the diversification occurred cyclically across multiple geologic cycles, rather than the currently accepted model of "divergence post-Pleistocene deglaciation". The Great Lakes were formed iteratively over several glacial cycles, like a cheese grater, as our geology colleagues say.

At any given time, fishes in these systems have a contrasting mix of genetic variation shaped by conditions associated with glacial maxima or interglacial periods, so a sort of temporal balancing selection. Today's rare deleterious variants are tomorrow's savior.