Edwards et al., 2018; Australian Systematic Botany; 31(5-6):495-503 Page 1 of 16 Species limits and cryptic biogeographic structure in a widespread complex of Australian monsoon tropics trees (broad-leaf paperbarks: Melaleuca, Myrtaceae) Robert D. Edwards A , Michael D. Crisp B and Lyn G. Cook C. A US National Herbarium, Department of Botany, National Museum of Natural History, Smithsonian Institution, 10th St. & Constitution Ave. NW, Washington, DC 20013-7012, USA. B Evolution, Ecology and Genetics, Research School of Biology, The Australian National University, Banks Building, Daley Road, Acton, ACT 2601, Australia. C The University of Queensland, School of Biological Sciences, Goddard Building, Mansfield Place, St Lucia, Brisbane, Qld 4072, Australia. D Corresponding author. Email: bortedwards@gmail.com Abstract. The Australian monsoon tropics are currently dominated by savanna and tropical woodland biomes that have arisen in response to a cooling and drying trend within the past ~3 million years. It is expected that organisms well adapted to these conditions have expanded into available habitats, leading to the differentiation of populations and species across this landscape, a process that could be magnified by the presence of several biogeographic barriers. The broad-leaved paperbark (Melaleuca leucadendra (L.) L.) complex is one such group of plants, with 14 poorly morphologically differentiated species occupying large overlapping distributions across the region, and across several recognised biogeographic barriers. Using phylogenetic and network analyses of nuclear and plastid sequences, we tested species limits among currently described species within the complex and for phylogeographic structure within species across seven of these barriers. Overall, our data suggested patterns of differentiation among species consistent with the early to middle stages of incomplete lineage sorting, and evidence for an idiosyncratic cryptic response of species to biogeographic barriers. Unexpectedly, we found a deep molecular split across all species, broadly coinciding with the northern part of the Great Dividing Range, a feature not typically considered to be a barrier to dispersal. Our study has offered one of the first insights into the dynamics within and among widespread species across the north of Australia, suggesting considerably more geographic structure than was previously recognised. SB18032 R. D. Edwards et al. Cryptic structure in broadleaf paperbarks Additional keywords: barriers, biogeography, cryptic divergence, melaleuca, reticulation, non-monophyly, the Great Dividing Range. Introduction The Melaleuca leucadendra (L.) L. (broad-leaved paperbark) complex is an important component of the tropical and subtropical Australian flora, particularly in wetlands of high conservation significance (Blake 1968, Franklin et al. 2007). As currently recognised, 14 species (Table S1, available as Supplementary material to this paper) are distributed almost continuously across seasonally wet–dry savanna habitat (Fig. 1A). Seven (M. argentea W.Fitzg., M. cajuputi Powell, M.