Understanding the appearance of reproductive isolation is essential for understanding mechanisms underlying the origin of species. This project aims at understanding the relative importance of different isolating barriers during adaptive speciation by looking at patterns of genomic divergence. I will use numerical modelling, analysis of RNAseq data from several biotypes (incipient species) of a model organism (pea aphid), and will perform mating experiments to address four research questions:(1) Which patterns of genomic divergence arise when different isolation barriers apply? (2) Which patterns of genomic divergence do pea aphids show? (3) Do pea aphids biotypes have behavioural mate choice?(4) Which reproductive barriers are needed for the completion of speciation in hybridizing biotypes?Successful implementation of this project will improve my career perspectives, because it's training objectives were chosen to guarantee addressing skills gaps, diversifying experience and strengthening existing skills. European Research and Society will also benefit from results of this project, both though academic and economic pathways.