Intraspecific Niche Evolution in a Drought Deciduous Shrub With Implications for Climate Resiliency

Whitebrush (<i>Aloysia gratissima</i>) is a drought deciduous shrub species with two geographically distinct populations occurring in North and South America where they experience different climatic conditions. This scenario of isolated populations existing under different climatic conditions provides an opportunity to explore the use of species distribution models (SDMs) to identify and classify climatic niche evolution. Our goal was to identify intraspecific patterns of niche evolution by exploring the variation in environmental variables that underlie and constrain distributions and then apply this information to provide insight on variation in species response to changing conditions at the population level. We conducted a principal component analysis to compare climatic conditions present throughout each population in addition to identifying geographical areas characterized by climate novelty. We built SDMs with MaxEnt to characterize the climatic niche at species and population levels. Reciprocal spatial transfers between population models were conducted to evaluate niche equivalency and categorize evolutionary trends as conservative or divergent. Lastly, we projected models onto future climatic conditions to evaluate the consequences of niche evolution on climate resiliency at the population level. Comparison of abiotic conditions between populations indicated evidence of non-analogous climates and low niche overlap in both environmental and geographic spaces, suggesting that niche divergence occurs between populations. The differences in realized climatic niche of the populations are likely a result of exposure to novel environmental conditions within the dispersal vicariant population. Comparison of climate conditions and identifying the driving evolutionary forces shaping a species or population's niche will allow for adaptive management strategies in current and future climate scenarios by accounting for the spatial and temporal variation of species' responses to these pressures.