Abstract
Wide‐ranging mammals face significant conservation threats, and knowledge of the spatial scale of population structure and its drivers is needed to understand processes that maintain diversity in these species. We analysed DNA from 655 A laskan caribou ( R angifer tarandus granti ) from 20 herds that vary in population size, used 19 microsatellite loci to document genetic diversity and differentiation in A laskan caribou, and examined the extent to which genetic differentiation was associated with hypothesized drivers of population subdivision including landscape features, population size and ecotype. We found that A laskan caribou are subdivided into two hierarchically structured clusters: one group on the A laska P eninsula containing discrete herds and one large group on the M ainland lacking differentiation between many herds. Population size, geographic distance, migratory ecotype and the K vichak R iver at the nexus of the A laska P eninsula were associated with genetic differentiation. Contrary to previous hypotheses, small M ainland herds were often differentiated genetically from large interconnected herds nearby, and genetic drift coupled with reduced gene flow may explain this pattern. Our results raise the possibility that behaviour helps to maintain genetic differentiation between some herds of different ecotypes. Alaskan caribou show remarkably high diversity and low differentiation over a broad geographic scale. These results increase information for the conservation of caribou and other migratory mammals threatened by population reductions and landscape barriers and may be broadly applicable to understanding the spatial scale and ecological drivers of population structure in widespread species.