How the Aridification of Australia Structured the Biogeography and Influenced the Diversification of a Large Lineage of Australian Cicadas
Over the last 30 million years, Australia’s landscape has undergone dramatic cooling and drying due to the establishment of the Antarctic Circumpolar Current and change in global CO2 levels. Studies have shown that many Australian organisms went extinct during these major cooling events, while others experienced adaptive radiations and increases in diversification rates as a result of exploiting new niches in the arid zone. Despite the many studies on diversification and biogeography in Australia, few have been continent-wide and none have focused on a group of organisms adapted to feeding on plants. We studied 162 species of cicadas in the Australian Pauropsalta complex, a large generic lineage within the tribe Cicadettini. We asked whether there were changes in the diversification rate of Pauropsalta over time and if so: 1) which clades were associated with the rate change? 2) did timing of rate shifts correspond to known periods of dramatic historical climate change, 3) did increases in diversification rate along select lineages correspond to adaptive radiations with movement into the arid zone? To address these questions, we estimated a molecular phylogeny of the Pauropsalta complex using ∼5300 bp of nucleotide sequence data distributed among five loci (one mtDNA locus and four nDNA loci). We found that this large group of cicadas did not diversify at a constant rate as they spread through Australia; instead the signature of decreasing diversification rate changed roughly around the time of the expansion of the east Antarctic ice sheets ∼16 Ma and the glaciation of the northern hemisphere ∼3 Ma. Unlike other Australian taxa, the Pauropsalta complex did not explosively radiate in response to an early invasion of the arid zone. Instead multiple groups invaded the arid zone and experienced rates of diversification similar to mesic-distributed taxa. We found evidence for relictual groups, located in pre-Mesozoic habitat, that have not diversified and continue to reside on mesic hosts in isolated “habitat islands”. Future work should focus on groups of similar ages with similar distribution patterns to determine whether this tempo and pattern of diversification and biogeography is consistent with evidence from other phytophagous insects.