Ecological speciation, the origin of new species via divergent natural selection, is one of the most fundamental and unresolved processes in evolution. Although the evidence for adaptation of organisms to their environment is abundant, the role of ecological selection in mediating species formation remains controversial. This knowledgegap arises in part from a scarcity of experimental evidence linking environmental selection to the creation of reproductive barriers that ultimately lead to species formation. An experimental framework to investigate ecological speciation consists in studying the genetic architecture of adaptive traits and reproductive isolation in interbreeding populations adapted to contrasting environments. In my thesis I explored the genomics of ecological speciation in plants using the Senecio lautus species complex, a diverse group of plants that have adapted to a broad array of environment across Australia.It has been suggested that local adaptation to different environments will lead to genetic divergence and speciation only if genomic regions controlling adaptive traits are not exchanged between organisms adapted to different niches. This will happen if adaptive genes also mediate reproductive isolation, thus making migration between environments difficult, or leading to poor survival of recombinants. This model of ecological speciation creates testable predictions on the genomic architecture of adaptation: Firstly, genomic divergence between incipient species is expected to be heterogeneous, where a few genomic regions display outlier differentiation. Secondly one expects divergent regions to contain genes affecting fitness in natural environments. Thirdly, these "genomic islands of speciation" will also contain loci controlling adaptive traits and reproduction.In my thesis I tested these predictions using divergent populations of plants from the S. lautus species complex. I used a combination of genomic and ecological approaches to: (i) Describe patterns of genomic differentiation between natural populations across Australia and made inferences about the forces that generated these patterns. Specifically, I tested the repeated and independent evolution of forms to coastal environments and analyzed whether genomic divergence was more heterogeneous between parapatric than allopatric populations. (ii) Demonstrate experimentally that divergent genomic regions contain genes controlling differential survivorship between environments. (iii) Detect QTLs associated to environmentally selected traits and associate their location to genomic regions of high differentiation between parapatric populations. In combination, these experiments were used to test the role of ecology in creating and maintaining the reproductive barriers that ultimately lead to plant speciation. ! ! 3!A phylogenomic study of a continental collection of S. lautus populations showed that these plants have a monophyletic origin and evolved rapidly colonizing a broad array of environments. Importantly, populations adapted...