Uplift-driven diversification, dispersal, and niche evolution in New Zealand Veronica

Anne Thomas, Javier Igea and Andrew J. Tanentzap, Department of Plant Sciences, University of Cambridge, Cambridge, United Kingdom, Heidi M. Meudt, Te Papa Tongarewa Museum of New Zealand, Wellington, New Zealand, Dirk C. Albach, University of Oldenburg, Oldenburg, Germany, William G. Lee, Manaaki Whenua Landcare Research, Dunedin, New Zealand

Background/Question/Methods
Understanding the mechanisms of lineage diversification is a fundamental question of evolutionary biology. Mountains are often hotspots of species diversity, with topographical heterogeneity promoting novel niches and range fragmentation. High species diversity in mountain lineages may be driven by both repeated colonization from other habitats and in situ diversification. Which of these processes is more constrained by niche filling in mountain habitats, and which are more likely to contribute to high mountain diversity? Colonization may allow species with diverse traits to occupy new niche space, while in situ cladogenesis may allow adaptive exploration of niche space. Shifts in trait disparity may shed light on which process provides the most access to niche space in mountain lineages.
Here, we estimated the dispersal, contraction, and in situ cladogenesis of a New Zealand angiosperm radiation, the genus Veronica sect. Hebe (Plantaginaceae), in response to mountain uplift over the last 4 million years. We estimated the most complete phylogeny to date of New Zealand Veronica via target capture sequencing of 353 nuclear genes for 115 species. The phylogeny was time-calibrated with StarBEAST2 and species elevational distributions were estimated from herbarium records and flora descriptions. Using these data, we modeled the clade’s historical biogeography constrained by the timing of mountain uplift with BioGeoBEARS. We modeled climatic niche traits with the TTR species distribution model.
Results/Conclusions
The emergence of montane environments with uplift was a major driver of diversification in Veronica sect. Hebe. Between 2 and 3 mya, per capita rates of in situ diversification were around 8 times greater in montane habitats than in lowland environments. Dispersal into the alpine zone became the dominant dynamic as this habitat emerged during the last one million years. Climate niche disparity remained high throughout the clade’s history, suggesting that both colonization and cladogenesis were able to access novel niche space and contribute to diversity. These findings support the importance of mountain uplift as both a cradle and recipient of past and future biodiversity generation.