Abstract
Many forest tree species are recently evolved, poorly differentiated at the molecular level, and form geographic and morphological continua, which can make identification of management units difficult. This is seen clearly in Eucalyptus globulus, a species complex consisting of four taxa variously described as species or subspecies (bicostata, globulus, maidenii and pseudoglobulus). These taxa are morphologically and geographically distinct, but are linked by intergrade populations that are intermediate in morphology. The origins of the intergrade populations are unknown; some could be the result of primary differentiation (divergence within a continuous series of populations resulting from varying selection pressures) while others may result from secondary intergradation (hybridization and introgression between previously isolated taxa). The intergrade populations represent a significant proportion of the distribution of the E. globulus species complex, however their diverse and intermediate morphology confounds taxonomic classification. Correct classification is important for conservation planning, as well as for seed collections for native forest regeneration, reforestation and breeding programs. To assess themolecular affinities within the complex, we used nine microsatellite DNA markers to genotype 1239 trees representing 33 morphological core and intergrade populations from across the natural range of the species complex in southeastern Australia. This analysis provided insights into the evolutionary processes that have shaped the patterns of genetic variation in the E. globulus species complex, and a framework for identifying the potential impact of gene flow from E. globulus planted within the natural range of the complex.