Recent years have seen a resurgence of interest in the process of speciation but few studies have elucidated the mechanisms driving the evolution of reproductive isolation. In theory, the indirect effects of adaptation to different environments, the direct effects of reinforcing selection for increased mating discrimination where interbreeding produces hybrid offspring with low fitness and random genetic drift can each promote speciation. We tested the role of these three processes in causing the evolution of reproductive isolation between populations of Timema cristinae walking-stick insects. Pairs of populations adapted to living on different host-plant species exhibit greater levels of reproductive isolation than pairs of populations using the same host. Second, mating discrimination against individuals from the alternate host plant is greater in populations that exchange migrants with a neighbouring population using the alternate host and where there is the opportunity for maladaptive hybridization than in populations that are geographically-isolated from all other populations. Finally, levels of reproductive isolation are unrelated to the degree of neutral genetic divergence between populations indicating that increased exposure to genetic drift has not facilitated the evolution of reproductive isolation. Thus adaptation to different host plants and reinforcing selection has driven the evolution of reproductive isolation and the greatest progress towards speciation occurs when both processes operate.