4 | Discussion
The results of this study support the hypothesis that historical glacial cycles during the Quaternary period influenced the observed genetic signature of rainbow darter populations in drainages of central North America. They support the prediction that populations in glaciated drainages in the North do not fit expectations under the stream hierarchy model, whereas populations in the unglaciated South do. The pattern observed within the Volga River in the upper Mississippi River basin reflects recurrent extirpation and recolonization of E. caeruleum into the region caused by a disruption in the riverscape by repeated glacial advance and retreat. In contrast, the pattern observed within the Meramec River in the Ozark Plateau strongly adhered to the stream hierarchy model. This region was not directly impacted by glacial advances and the age and relative stability of the habitat and drainage patterns allowed for populations of E. caeruleum to persist uninterrupted. The observed genetic signature of the southern population was shaped by the life history and habitat preferences of the species rather than cycles of geologic and climatic disturbance.