Gene transfer between species or sub-species via hybridization is increasingly recognized as an important source of diversification and adaptation. However, only rarely are systems sufficiently well understood to predict the degree and direction of hybridization, which makes current studies largely exploratory and reliant on post-hoc explanations. To circumvent this problem we have experimentally studied the extent, direction, and phenotypic determinants of hybridization upon secondary contact between two sub-species of the common wall lizard, Podarcis muralis. These lineages are now in contact in three contexts: (a) in a native hybrid zone; (b) following recent introduction of one lineage into the native range of the other, and (c) in non-native populations founded by animals of both origins. Using a large set of genome-wide nucleotide markers mapped onto the draft genome of the species, this project aims to test the following predictions: (1) introgression should be consistently asymmetric as a result of lineage-biased male reproductive success with hetero-specific females; (2) male traits that increase inter-specific reproductive success should show high levels of introgression and the resulting genomic islands of introgression should be associated with those traits and contribute to exaggeration of sexual dimorphism in hybrid populations; and (3) genotypes associated with low inter-lineage viability should show low frequency in hybrid populations.