Bird speciation in the Gulf of Guinea

The Gulf of Guinea island system, West Africa, constitutes a spectacular centre of bird endemism, with 33 species unique to the region. It comprises three oceanic islands (Annobón, São Tomé, Príncipe), one land-bridge island (Bioko) and one ecological island (Mount Cameroon), all part of the Cameroon line of volcanoes. A long-term project with the objective of inferring the causes behind such high endemism levels started in 2002. This project works as an umbrella for several independent projects on well-defined questions or that focus on specific groups and has already resulted in a PhD thesis [PhD-Martim.pdf].
We have been using molecular data to infer the evolutionary history of all endemic groups. The genetic data is then combined with morphological, behavioural and field experiments (mate choice) to investigate: i) the importance of isolation for the speciation process; ii) the applicability of the current 'ecological model' of speciation, which was developed in parapatric and sympatric situations (presence of gene flow), to allopatric situations (no gene flow), and iii) the link between character divergence at the population level and the evolution of reproductive isolation.

Molecular phylogenies revealed that previous systematic assessments based on phenotypic characters were often incorrect. High levels of phenotypic differentiation of island taxa are not related to time since origin and can evolve within very short periods. Species that diverged the most in phenotype were those that speciated after establishing sympatry with related populations, providing strong evidence for the importance of secondary contacts in promoting phenotypic diversification and speciation.

Although most species originated by diverging in isolation from their mainland source populations (allospeciation), two cases of archipelago radiation have been described for five white-eye taxa (Zosteropidae) and for two seedeater species (Fringillidae). Remarkably, molecular evidence strongly suggests that the two seedeater species (Príncipe Seedeater Serinus rufobrunneus and the São Tomé Grosbeak Neospiza concolor ) may have speciated in full sympatry. This case is now being analysed from a genomics viewpoint