Linking environmental stability with genetic diversity and population structure in two Atlantic Forest palm trees

Aim Spatial patterns of biodiversity in the Atlantic Forest are well characterized. However, there is no consensus on the biological processes underlying these patterns, and multiple competing hypotheses have been proposed, several of which centre on climatic stability. Here, we ask if Late Quaternary climatic stability predicts contemporary population structure and genomic-level diversity in two palm species. Location Atlantic Forest of Eastern Brazil. Taxa Syagrus botryophora and S. pseudococos (Arecaceae). Methods We first model the distribution of suitable environments in 62 time-slice climate projections over the last 120 thousand years, and summarize stability over that period. We then use >25,000 RADseq-generated SNPs to (i) describe the spatial patterns of genomic variation in both species, (ii) test how well genomic variation is explained by isolation by distance and by the environmental resistance imposed by historical instability (isolation by resistance) and (iii) test for a correlation between genetic diversity and historical stability. Results The contemporary range of S. botryophora has been relatively stable over the last 30 thousand years and there are two isolated regions of high stability for S. pseudococos. The genomic data recover a clear pattern of isolation by distance in S. botryophora and two structured populations in S. pseudococos. Consequently, the contribution of isolation by resistance to overall genetic structure is much higher in S. pseudococos. Genetic diversity is not significantly correlated with historical stability in either species. Main Conclusions Based on the concordance between historical stability and genetic structure, Late Quaternary climate stability may have maintained population connectivity within S. botryophora and promoted intraspecific divergence in S. pseudococos. Conversely, historical stability does not seem to be driving spatial patterns of genetic diversity. This study supports the primary role of climatic stability in determining spatial population structure, but not genetic diversity, in the Atlantic Forest.