Systematics, phylogeography and evolutionary history of the Australo-Papuan butcherbirds (Artamidae: Cracticus)

The progressive aridification of the Australian landmass over the past 25 Myr has long been hypothesised to be a major driver of speciation, diversification and extinction in the Australo-Papuan region. My dissertation uses the Australo-Papuan butcherbirds and magpies (Corvoidea: Artamidae: Cracticinae: Cracticus) to test the relative impact of palaeoenvironmental changes on the population history of closely related species with different habitat preferences. I particularly focus on the biogeographic and demographic consequences of major changes in climate and landscape during the Pliocene and Pleistocene (5.3 Ma - 11, 700 ya) on the (1) widespread open-habitat generalist Pied Butcherbird (Cracticus nigrogularis), (2) rainforest and mangrove restricted Black Butcherbird (C. quoyi), and (3) the white-throated butcherbird species-group (subgenus Bulestes) that has three allopatric species distributed in monsoonal savanna (Black-backed Butcherbird, C. mentalis and Silver-backed Butcherbird, C. argenteus), and arid, semi-arid and temperate forests and woodlands (Grey Butcherbird, C. torquatus). Phylogeographic analyses indicate that palaeoenvironmental changes during the Plio-Pleistocene have had a profound, but variable, impact on the population history of these species. Range-wide phylogeographic structuring suggests that arid habitats associated with the 'Carpentarian Barrier' region are unlikely to have represented a significant barrier to dispersal for the more arid-adapted C. nigrogularis and C. torquatus, whereas this region has likely caused long-term isolation of north-eastern and north-western Australian populations of the mesic-adapted C. quoyi, and between savanna-adapted C. argenteus and C. mentalis. This result accords with ecological niche model reconstructions of suitable Pleistocene habitats and multilocus coalescent divergence with gene flow analyses for the white-throated butcherbird species-group. Integration of palaeodistribution models and spatially explicit genealogical data support a scenario where a climate-induced range shift resulted in secondary contact between north-western Australian C. argenteus and southern Australian C. torquatus allowing introgressive hybridisation and mitochondrial (mtDNA) capture. Phylogeographic analyses also suggest that increased dune activity and aridity in central-western Australia during the Pleistocene caused range contractions and population differentiation in the more arid-adapted C. nigrogularis and C. torquatus. Multilocus phylogeographic structuring of Australian and New Guinean populations of the savanna-adapted C. mentalis and the mesic-adapted C. quoyi were consistent with the former species having dispersed more recently across the exposed Arafura Shelf that intermittently connected Australia and New Guinea during the Pleistocene than the latter. This result accords with palaeoenvironmental reconstructions that suggest these intermittent land connections were dominated by open grassland and woodlands habitats. In the final research chapter I investigate the impact of earlier phases of aridity and changes in landscape on the speciation history of the Australo-Papuan cracticids (Cracticinae: Cracticus, Strepera and Peltops) using multiple nuclear introns, mtDNA and multispecies coalescent models. This approach supports: (1) the recent taxonomic decision to place the morphologically divergent Australian Magpie (C. tibicen) in the butcherbird genus Cracticus rather than retaining it in the monotypic genus Gymnorhina, (2) the hypothesis that aridification over the past 25 Myr allowed the diversification of the open-woodland-adapted lineages such as Strepera and Cracticus, and (3) the hypothesis that rainforest-adapted lineages, such as the New Guinean endemic genus Peltops, persisted on the proto-New Guinean island archipelago prior to the recent formation of the New Guinean landmass ~ 2 - 5 Myr. This thesis presents the first complete and well-supported species-level phylogeny of the Australo-Papuan cracticids, as well as the first molecular assessment of species boundaries and evolutionary history of butcherbird species in the genus Cracticus. Key taxonomic implications from my dissertation are that the (1) Australian and New Guinean lineages of C. quoyi should be raised to species-level, (2) recently diverged members of the white-throated butcherbird species-group warrant species-level recognition despite widespread sharing of nuclear alleles and mtDNA paraphyly, (3) morphologically differentiated Australian Magpie C. tibicen is a member of Cracticus, and (4) species-boundaries within Peltops warrant a comprehensive molecular and morphological revision