Adapting to extremes: reconstructing niche evolution in Australian Acacia over time and space

Macroevolutionary analysis is increasingly being used to study biodiversity responses to climate change, for example by using phylogenetic node ages to infer periods of diversification, or phylogenetic reconstruction of traits to infer adaptation to particular stresses. Here we apply a new macroevolutionary method to investigate the responses of a diverse plant genus, Acacia, to increasing aridity and salinity in Australia from the Miocene to the present. The Niche Evolution Model (NEMo) combines environmental niche modelling with phylogenetic comparative methods in a single statistical framework, to estimate current environmental tolerances, reconstruct the history of niche evolution and infer rates of change in key aspects of environmental tolerance. Using a large database of Acacia occurrence records and presence-absence survey sites, we find that both spatial and temporal patterns in niche evolution of Acacia are consistent with the aridification history of Australia and suggest high niche lability along both axes, which has allowed Acacia to quickly exploit new niches created during the aridification of the continent, and resulting in their current dominance of many habitats across Australia. This study demonstrates that phylogenetic studies of niche evolution can move beyond application of simple trait-based models, allowing the underlying processes of speciation, adaptation and dispersal to be explicitly modelled in a macroecological and macroevolutionary context.