Abstract
Natural selection is a major driver for the origins of adaptations and new species. Whether or not the processes driving adaptation and speciation share a molecular basis remains largely unknown. Here, we show that divergence in hormone signalling contributed to the evolution of complex adaptations and intrinsic reproductive isolation in the Australian wildflower Senecio lautus. We provide evidence that differences in the auxin pathway, a hormone required for plant growth and development, has led to the repeated evolution of erect and prostrate forms along the Australian coast. Using multiple hybrid and natural populations, we show that adjacent erect and prostrate populations repeatedly diverged in auxin-related genes and auxin-dependent phenotypes, such as gravitropism. Analysis of a multi-year field selection experiment revealed that variation in fitness of an F10 hybrid population explained variation in gravitropism of their offspring. Genotyping of F11 hybrid individuals with extreme values of gravitropism revealed that variation in some of the most divergent genes explained both 65% of the variation in gravitropism and their probability of producing seed. Together, our results suggest that divergence in hormonal pathways can create a genetic link between rapid adaptation to new environments and the evolution of intrinsic reproductive isolation.