Solidago riddellii Frank is an outcrossing, clonal, herbaceous perennial species with a localized but widespread distribution. It occurs in fens and wet meadows with a global range from northern Arkansas through the upper Midwest with eastern extensions into Ontario and western populations known from southern Manitoba. Genetic variation within and among populations of Solidago riddellii was assessed using allozyme polymorphism to investigate the consequences of rapid migration of plant species on population genetic structure. Modern distribution is inferred as the result of colonization northward following retreat of glaciers in the upper Midwest and the fragmentation of wetland habitat by modern land use patterns. A total of 416 individuals from 30 populations collected from 10 states and 2 Canadian provinces was characterized using enzyme electrophoresis. Estimation of population level genetic variation was accomplished using standard methods. Total genetic variability is consistent with that of other species with similar life history characteristics. Mean identity was 0.93, with identity values for regions ranging from 0.97 to 0.91 (south, including Arkansas and Missouri populations). UPGMA cluster analysis using genetic identity revealed 5 clusters. Cluster 1 includes 2 eastern populations (Ohio, Illinois), Missouri and Arkansas populations. Cluster 2 includes 9 eastern populations, 1 western population (Wisconsin) and 1 Missouri population. Cluster 3 includes 3 eastern populations, 8 western populations, and 1 Arkansas population. Cluster 4 consisted of a single, anomalous population in Missouri, and Cluster 5 a single Minnesota population. No strong pattern of genetic differentiation between geographic regions, or distribution of genetic variability was seen although weak regional patterns could be developed.

Key words: genetic variation of rare taxa, Solidago