Evolutionary mechanisms underlying divergence in bill morphology and the timing of breeding using whole genome analysis

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Animals have evolved into a huge diversity of forms exploiting a wide arrangement of trophic niches, a process that can be controlled by evolutionary mechanisms and limits on available ecological niches. The diversity of forms and their interactions ensure the functioning of the ecosystems, forming one of the most essential bases of life. To fully comprehend the origin of biodiversity, we need to understand speciation, a fundamental evolutionary process. Between taxa still sufficiently closely related to produce viable offspring, speciation may be facilitated by introgressive hybridization, which provides genes for both adaptation and reproductive isolation. Prions (Pachyptila spp.) are seabirds distinguished by bill morphology (thin to broad, with or without lamellae) and distinct breeding schedules. The genus comprises a rapid Plio-Pleistocene radiation characterised by incomplete reproductive isolation, which manifests itself in inter-specific introgression. Although speciation through homoploid hybridization (HHS) is considered extremely rare in animals because the establishment of reproductive isolation as a product of hybridization is uncommon, we have shown previously that the medium-billed Salvin’s prion (P. salvini) could be a hybrid species between the narrow-billed Antarctic prion (P. desolata) and broad-billed prion (P. vittata) (Mol.Biol.Evol.36:1671–1685). However, unlike most hybrid species, Salvin's prion possesses phenotypically intermediate traits, suggesting that additivity of divergent parental traits alone can lead directly to both increased hybrid fitness and reproductive isolation. That hybridisation could lead to increased hybrid fitness and reproductive isolation, suggests the pleiotropic action of certain genes or groups of genes among these seabirds, however on the other hand, intermediate hybrid phenotypes speak against the notion of speciation through divergence islands, a recent idea previously hypothesised for the evolution of several hybrid systems. Given the intriguing nature of the Pachyptila system, comprising speciation through both divergence and introgression, and with the latter potentially leading to additive hybrid phenotypes, we propose using this genus as a model to first test the hypothesis of homoploid hybrid speciation for P. salvini and secondly to quantify adaptive variation with this system using whole genomes.