“…Advance and retreat of glaciers during and after the Pleistocene had dramatic effects on topography, stream characteristics, and temperature regimes that likely influenced genetic diversity and structuring among North American freshwater populations, including fishes (Bernatchez & Wilson, 1998;Morrison et al, 2006;Near et al, 2001;Strange & Burr, 1997) and unionids (Elderkin et al, 2007(Elderkin et al, , 2008Hewitt et al, 2019;Inoue et al, 2013Inoue et al, , 2015Pieri et al, 2018;Scott et al, 2020;Zanatta & Harris, 2013). Although several studies of unionids have found that structuring generally follows major hydrogeologic basins (Galbraith et al, 2015;Mock et al, 2010), our STRUCTURE results for P. clava and P. oviforme do not align fully with present-day drainages.…”