We investigate speciation mechanisms in plants. To this end, we analysed reproductive barriers between two naturally hybridizing plant species, Silene dioica and S. latifolia and study range wide genomic divergence. In this project, we will make use of a newly available draft genome sequence for Silene latifolia, v4 is available since September 2023 by our collaborators (Gabriel Marais, University of Porto). Heterogeneity in genomic divergence is currently a much-debated issue. Initially, all highly divergent regions were attributed due to adaptation or even to "speciation islands". The field has since come to recognize that such heterogeneity even arises in the absence of selection and that many different processes, most of all variation in recombination rate, but also selection against universally deleterious genes (background selection), gene richness and GC content affect divergence landscapes throughout the genome. An additional, fairly recently proposed mechanism, is that incompatibility among alleles in highly variable species arise more often in certain regions of the genome and thereby generate divergence peaks. This idea is theoretically supported and may play a large role for divergence and speciation in general, however, empirical evidence is very scarce. We have generated data to test for the relative roles of incompatibilities and adaptation for genomic divergence between S. dioica and S. latifolia. To detect loci associated with environmentally independent incompatibilities we generated ddRADseq data from a series of reciprocal back-crosses. In these crosses, we can detect allelic incompatibilities in the form of transmission biases. Such incompatibilities can arise at various stages, for example, as early as pollen or ovule production, alternatively, during gamete interactions or even shortly after zygote formation. Two datasets, form an earlier transplant experiment and from the crosses, will be informative on their own while the comparisons of the number and genomic location of the loci associated will either adaptation or transmission bias will allow us to assess the relative contribution of incompatibilities or adaptation to genomic divergence. We will assess genomic divergence using further pre-existing ddRADseq data (range-wide population samples) and plan to add more data on the specific populations from which crossing parents were sampled, as well as samples on an outgroup to be able to analyze whether loci of interest have evolved recently.