Abstract
Aim Geographical and climatic barriers to organismal dispersal and distribution play a major role in speciation. We use a sister‐pair of widespread savanna trees ( M elaleuca argentea and M . fluviatilis ) to test the influence of putative barriers on divergence within and between species across an otherwise continuous landscape. Location The A ustralian monsoon tropics ( AMT ). Methods Chloroplast and nuclear DNA sequences were used to estimate variation between and within species. Hypotheses invoking vicariance and ecological speciation as the mechanisms of divergence between species were explicitly tested using ecological niche modelling. Results We found little evidence for divergence across the C arpentaria B asin, although some chloroplast DNA haplotypes were restricted to regions to the east or west. Pilbara populations were distinct from those to the east across the Great Sandy Desert, including those from the Kimberley. There was a complex pattern of genetic divergence and niche differentiation among M . argentea and M . fluviatilis within a region of secondary range overlap coincident with currently recognized species boundaries across the Great Dividing Range. Main conclusions The two morphospecies are ecologically and genetically distinct, and maintain those differences in sympatry. Speciation might have occurred in allopatry in separate drainage basins that later came into contact. The P ilbara population appears to be distinct but requires further investigation.