Genetic Introgression results in reproduction between two different, but related species (a.k.a. Hybridization). The advantage to introgression is that it has the potential to introduce novel genes into a population, and these genes may be adaptive. Likewise, introgression may be a source of speciation.
Mallard superspecies (Anas) and eight species of gulls (Larus genus) both form complex introgression amongst themselves respectively.
Rheindt, F.E. and S.V. Edwards. 2011. Genetic introgression: an integral but neglected component of speciation in birds.