Understanding the formation of new species, termed ‘speciation’, is one of the central issues in evolutionary biology. The role of divergent selection in initiating and promoting the speciation process, termed ‘ecological speciation’, and maintaining species differences has been under intense study in recent decades. Despite accumulating individual case studies documenting individual reproductive barriers evolving between populations due to divergent selection, a synthetic view of the ecological speciation process requires: (1.) identifying the source(s) of divergent selection, (2.) identifying the trait(s) under divergent selection, and (3.) estimating the strength of multiple reproductive barriers as a result of divergent selection. When these issues can be linked in a single system, one can take a holistic approach to understanding the evolution of species. Moreover, despite emerging research linking these micro-evolutionary processes to macro-evolutionary patterns, there are still outstanding questions regarding the role of divergent selection as a common mechanism generating biodiversity across taxonomic orders. This is a challenging question, as many other factors can vary together and obscure clear patterns. To answer this question, we need to combine both bottom up and top down approaches in a broad but controlled system where many taxa can be queried simultaneously. If this challenge can be met, we can improve our predictability of the evolutionary outcomes under divergent selection, particularly, whether populations under divergent selection can complete the speciation process, and we can test the repeatability of ecological speciation across different species and taxonomic groups at different stages in the speciation process to bridge the micro-level evolution to macro-level evolution of biodiversity. In this dissertation, we address the above challenges by, first, using a magnifiying glass approach to study how divergent host-use generates and promotes RI between populations of a host-specific gall-forming insect, Belonocnema treatae (Chapter 1, 2, 3), and, second, testing whether the same divergent host environments drive ecological and genetic divergence among host-associated populations repeatably across five additional species of gall-forming insects (Chapter 4, 5). Chapter 1 of this thesis examines two reproductive barriers that are the direct results of divergent selection: reduced immigrant viability and fecundity, the latter is a potentially important, but often overlooked reproductive barrier. We found asymmetric, but opposing reproductive barriers that combined together to reduce gene flow in both directions. In chapter 2, we also identified one important source of selection related to divergent host use: the plant immune response that affects survival. In Chapter 2, we found that intra-specific variation in host quality among individual trees within each divergent host species, rather the tree species per se, is the major driver of hybrid fitness. Hybrids vary widely in both the direction and strength of hybrid fitness relative to pure crosses across individual trees. Thus, potentially inhibiting the speciation process on some trees, while promoting reproductive isolation on others. In Chapter 3, we tested whether a critical process occurs in the evolution of strong RI in host-associated populations of B. treatae: adaptive coupling, defined as selection favors the arise of another reproductive barrier as a response towards an existing reproductive barrier. We detected signatures of asymmetric adaptive coupling: in the presence of the cost of migration- reduced immigrant fitness and hybrid viability, stronger habitat, and sexual isolation are found among sympatric populations in the direction where migration rate is higher. In Chapters 4, we expand the scope of this work to include many species of gall-forming insects (and their predators) distributed across the same two host plant environments to test general principles of speciation. We demonstrate that temporal isolation between host-associated populations is present in six gall-forming insects and three natural enemies associated with gall-forming insects, suggesting phenological differences between different host plant species cascades across multiple species at multiple trophic levels. In sum, by combining field survey, manipulation experiment and behavior observation, we found that both divergent selection and migration are critical in the evolution of RI. Our findings call for more studies on assessing factors that affect the progression of ecological speciation, such as the role of heterogeneity within each divergent environment in promoting or inhibiting RI, population demography and multiple traits under divergent selection. Only by knowing the conditions that promote or inhibit the speciation process, can we better predict the evolutionary outcomes under divergent selection.