4.3 | Adherence to the stream hierarchy model
In stark contrast, the analyses of the Meramec River only dataset revealed a spatial distribution of genetic diversity that is structured in an upstream to downstream pattern that corresponds to the stream hierarchy model. There was evidence of a strong correlation of genetic differentiation with river distance (Figure 5B) that was further supported by the pattern observed in the DAPC (Figure 4C) and the pairwise FST values (Table S3B). There is greater genetic diversity (Table 1C) observed in the Meramec River than in the northern Volga River, which is best explained by differences among individuals within populations (Table 2C). There were also fewer genetic clusters observed (Figure 3C) suggesting greater connectivity among the local populations.
The adherence to the stream hierarchy model observed in the Meramec River only dataset suggests that habitat in the region remained relatively undisturbed by the climatic changes of the Quaternary period. The river valleys of the Ozark Plateau are older and entrenched, and lack sedimentation levels found within the glacial drift plains in the North (Forman et al. 1992; Galloway et al., 2011). The lotic habitat of these rivers also contains nearly continuous suitable habitat for the rainbow darter. Taking these factors together, the best explanation for the pattern observed is that life history of E. caeruleum , including high site fidelity and diet preferences of the species, shaped the spatial distribution of genetic diversity. Rainbow darters and their preferred prey have a preference for fast-moving riffle habitats and are less prevalent in deeper slow-moving pools (Hicks & Servos, 2017; Mueller et al., 2020). Therefore, the riffle-run-pool structure of streams may represent a permeable barrier to migratory movement further contributing to the observed pattern.