Allozyme and morphological variation in two subspecies of Dryas octopetala (Rosaceae)in Alaska

The Alaskan endemic shrub Dryas octopetala ssp. alaskensis and its circumpolar conspecific ssp. octopetala are adapted to closely adjacent habitats in alpine areas of Alaska. These alpine areas form geographically disjunct "islands" among which there are limited opportunities for gene flow. Allozyme electrophoresis and a common garden experiment were used to examine genetic variation between subspecies and among disjunct populations of each subspecies. Overall, allozyme variation in D. octopetala is low with little differentiation among populations or between subspecies. Morphological differences, however, are greater between subspecies than among populations within subspecies. Divergence for a few morphological and life-history characters has apparently occurred in response to strong selection, but without divergence at allozyme loci. The ancestors of both subspecies of D. octopetala in Alaska were isolated during the Pleistocene in the glacial refugia of Alaska and Yukon, which may explain low overall variation. Dryas. o. alaskensis is thought to be a Pleistocene derivative of ssp. octopetala, which may account for the low allozyme divergence between subspecies. Recent restriction to alpine areas may explain the low differentiation among disjunct populations.