The macroevolution of climatic niches and its role in ant diversification

Investigating how climatic niches change over evolutionary timescales is a necessary step to understanding the current distribution of lineages, yet few studies have addressed this issue using comprehensive datasets. In this study, the evolution of ant climatic niches is investigated at a global scale based on bioclimatic data associated with 163481 ant occurrence records. The resulting dataset was subjected to principal component analysis, and the scores obtained were used to characterise the main axes of ant climatic niche evolution. Principal component axis 1 (PC1) reflected variation in average temperature and seasonality - consistent with typical tropical/temperate gradients - whereas PC2 was associated with varying levels of aridity. Evolution along these two niche axes was markedly different: differences in the amount of explained variance between PC1 (65%) and PC2 (19%) suggest that climatic niche evolution was nearly three times more pronounced along a tropical-temperate climate axis. There was statistically significant phylogenetic signal on PC1, with genera occupying more tropical conditions diversifying at a faster rate, yet neither of these results is significant on PC2. In addition, most of the ancient ant lineages are associated with conditions of low seasonality and high temperatures. These results provide partial support for the tropical conservatism hypothesis as an explanation for geographical patterns of ant species richness.