Digest: Postzygotic isolation barriers stabilize a hybrid zone between two grosbeak species

Speciation involves the build-up of reproductive isolation through prezygotic or postzygotic barriers. Research on avian speciation has emphasized the role of prezygotic barriers, whereas postzygotic barriers have remained underappreciated. Revisiting the hybrid zone between two grosbeak species with genomic data, DeRaad et al. (2025) detected ongoing hybridization but strong selection against later-generation hybrids, consistent with mitonuclear incompatibilities. These results demonstrate that postzygotic isolation, whether intrinsic or extrinsic, can be essential in maintaining a hybrid zone.