Independent evolution of intermediate bill widths in a seabird clade

Masello J, Ryan PG, Shepherd LD, et al. Independent evolution of intermediate bill widths in a seabird clade. <em>Molecular Genetics and Genomics</em>. 2022;297(1):183-198.

Interspecific introgression can occur between species that evolve rapidly within an adaptive radiation.<em>Pachyptila</em>petrels differ in bill size and are characterised by incomplete reproductive isolation, leading to interspecific gene flow. Salvin’s prion (<em>Pachyptila salvini</em>), whose bill width is intermediate between broad-billed (<em>P. vittata</em>) and Antarctic (<em>P. desolata</em>) prions, evolved through homoploid hybrid speciation. MacGillivray’s prion (<em>P. macgillivrayi</em>), known from a single population on St Paul (Indian Ocean), has a bill width intermediate between<em>salvini</em>and<em>vittata</em>and could also be the product of interspecies introgression or hybrid speciation. Recently, another prion population phenotypically similar to<em>macgillivrayi</em>was discovered on Gough (Atlantic Ocean), where it breeds 3 months later than<em>vittata</em>. The similarity in bill width between the medium-billed birds on Gough and<em>macgillivrayi</em>suggest that they could be closely related. In this study, we used genetic and morphological data to infer the phylogenetic position and evolutionary history of<em>P. macgillivrayi</em>and the Gough medium-billed prion relative other<em>Pachyptila</em>taxa, to determine whether species with medium bill widths evolved through common ancestry or convergence. We found that Gough medium-billed prions belong to the same evolutionary lineage as<em>macgillivrayi</em>, representing a new population of MacGillivray’s prion that originated through a colonisation event from St Paul. We show that<em>macgillivrayi</em>’s medium bill width evolved through divergence (genetic drift) and independently from that of<em>salvini</em>, which evolved through hybridisation (gene flow). This represents the independent convergence towards a similarly medium-billed phenotype. The newly discovered MacGillivray’s prion population on Gough is of utmost conservation relevance, as the relict<em>macgillivrayi</em>population in the Indian Ocean is very small.