Origin of the Laurentian Great Lakes fish fauna through upward adaptive radiation cascade prior to the Last Glacial Maximum

Abstract The evolutionary histories of adaptive radiations can be marked by dramatic demographic fluctuations. However, the demographic histories of ecologically-linked co-diversifying lineages remains understudied. The Laurentian Great Lakes provide a unique system of two lineages that are dispersed across depth gradients with a predator-prey relationship. We show that the North American Coregonus species complex radiated rapidly prior to the Last Glacial Maximum (80–90 ka), a globally warm period, followed by rapid expansion in population size. Similar patterns of demographic expansion were observed in the predator species, Salvelinus namaycush , following a brief time lag, which we hypothesize to be driven by predator prey dynamics. Diversification of prey into deepwater created ecological opportunities for the predators, facilitating their demographic expansion through an upward adaptive radiation cascade. This study provides a new timeline and environmental context for the origin of the Laurentian Great Lakes fish fauna, and firmly establishes this system as drivers of ecological diversification and rapid speciation through cyclical glaciation.