Evolutionary origins of a latitudinal plant height gradient

Latitudinal gradients are among the most pervasive patterns in biodiversity, yet their evolutionary origins remain poorly understood, especially in plants. Here we identify a latitudinal gradient in plant height within cacti, an iconic and rapidly evolving plant family spanning the Americas, with taller species inhabiting warmer, lower latitudes. To uncover how this gradient arose, we reconstructed repeated directional changes towards taller and shorter descendants across the cactus phylogeny, distinguishing these from shifts in evolutionary variance and growth-form differences. We combined continuous-space reconstructions of ancestral geographical dispersal with palaeoclimate to recover the warming and cooling experienced by species throughout their diversification. We find that species that dispersed through warmer places increased in height, whereas those that dispersed through colder places shrank. These opposing associations reveal how historical climate change was linked to directional height evolution, providing an evolutionary explanation for the emergence of the latitudinal height gradient. By reconstructing complex phenotypic, geographical and climatic histories together in high resolution, we trace the origins of an extant latitudinal gradient through the evolutionary changes from which it emerged. Our framework provides a method to reveal the evolutionary origins of geographical gradients and trait-environment relationships across the Tree of Life.