Explaining the extent, causes, and consequences of biotic distributions in space is fundamental to our understanding of how species evolve and cope with particular environments. Yet, identifying extrinsic barriers to migration imposed by landscape structure and predicting their impacts on intraspecific genetic diversity remains a major challenge in population biology. In this study, 30 populations (771 individuals) of brook charr (Salvelinus fontinalis, Salmonidae) representing six major river drainages from Maine, USA, were characterized at six microsatellite loci to quantify the role of landscape features, such as habitat size, altitude, contemporary and historical connectivity, in shaping genetic diversity at three spatial scales: within lakes, within river drainages, and among river drainages. Within-population expected heterozygosity was negatively correlated with altitude, whereas no significant correlation was observed with lake size. Conversely, the extent of heterozygote deficiency within lakes was negatively associated with habitat size. The hierarchical analysis of genetic variance revealed that the extent of among-drainage differentiation was unexpectedly low relative to the pronounced population structuring within drainage. Geographically proximate St. John and Penobscot River drainages were characterized by opposite effects of altitude and geographic distance in shaping the pattern of population differentiation within drainages. The geographic pattern of differentiation among drainages could not be accounted for either by an isolation by distance or by a stepwise range expansion model. Overall, this study provided evidence for the role of contemporary landscape features in shaping the observed pattern of genetic diversity at smaller geographic scales (within and among populations within river drainage). On a broader geographic scale, contemporary landscape structure appeared to be only a minor factor determining the observed pattern of genetic structuring among drainages. These results add to the increasing evidence for nonequilibrium conditions between drift and migration in a wide array of animal taxa. The development of more realistic theoretical descriptions of nonequilibrium population structure thus appears to be important to better understand the relative influence of historical and ecological factors in shaping genetic variation in young habitats, such as recently deglaciated areas.
Corresponding Editor: B. Bowen