The comparative study of range-wide genetic structure across related, co-distributed rainforest trees reveals contrasting evolutionary histories

Australia’s rainforests exhibit high taxonomic diversity and endemism, yet relatively little is known about patterns of genetic diversity across the flora. Habitat contractions caused by the aridification of the continent and the recent glacial cycles have left discrete genetic signatures on modern-day populations, with the nature of between-population differentiation likely to be influenced by a range of ecological and environmental factors. We used microsatellites to examine range-wide population genetic structure in two congeneric rainforest trees, Elaeocarpus angustifolius and E. largiflorens (Elaeocarpaceae), with similar habitat preference and dispersal potential. The aim was to investigate the relationships between genetic structure, geographic disjunction and morphological differentiation and attempt to clarify the likely evolutionary processes responsible for the observed patterns. We found substantial differences in the amount and type of genetic differentiation within the two co-distributed species. While Elaeocarpus largiflorens revealed an abrupt genetic disjunction front between two subspecies separated by a recognised biogeographic barrier (the Black Mountain Corridor), E. angustifolius showed lower genetic differentiation across a much wider geographic area. Our findings suggest that biogeographic features have different impacts on related species, and that generalisations on evolutionary patterns can be untenable without considering a range of factors. Also, on the basis of the available molecular data, a likely hypothesis is of pre-Pleistocene differentiation followed by reinforcement of differentiation patterns during recent glacial cycles (further studies are needed to conclusively date divergence).