| Summary: | 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 Tome, Principe), one land-bridge island (Bioko) and one ecological island (Mount Cameroon), all part of the Cameroon line of volcanoes. This thesis used genetic, morphological and behavioural data from finches, whiteeyes, thrushes and kingfishers 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, to allopatric situations, 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. For example, Speirops (Zosteropidae) and Neospiza (Fringillidae) are well within the genera Zosterops and Serinus respectively. 'Aberrant' characters did not constitute evidence for shared ancestry, e.g. the "genus" Speirops proved not to be monophyletic. The two Gulf of Guinea endemic kingfishers Alcedo spp. are island populations of the mainland Malachite kingfisher A. cristata, rather than being distinct island species. In contrast, molecular evidence in combination with phenotypic data strongly supported the elevation of the thrush population from Principe Turdus olivaceofuscus xanthorhynchus to species status. Molecular data also revealed a possible case of cryptic speciation within the Principe white-eye Zosterops ficedulinus. The high number of endemic bird species in the Gulf of Guinea islands was the result of recent speciation events rather than the accumulation of relict species extinct on the mainland. Therefore, the Gulf of Guinea constitutes a very important centre of bird speciation in Africa. Because the oceanic islands are surrounded by one of the richest centres of biodiversity in the world they were colonised by several bird groups which, by occupying different niches, reduced the possibilities of radiations within the archipelago. Therefore, most species originated by diverging in isolation from their source populations (allospeciation). Data from the Principe seedeater Serinus rufobrunneus showed that selection rather than drift was the main driver of divergence in allopatry, thereby supporting the applicability of the ecological model of speciation to allopatric situations. At the same time, the most divergent species 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. Molecular evidence suggested that the São Tome grosbeak Neospiza concolor may have speciated in full sympatry - which if confirmed would make it unique among birds. In S. rufobrunneus, mate recognition traits were the first to diverge and may therefore be implicated in the first stages of the speciation process. This was further supported by playback experiments showing that populations no longer recognise the songs of foreign populations, suggesting that reproductive isolation may evolve as a by-product of independent divergence of mate recognition systems in allopatry. Overall, this study supported the view of speciation as a selection-driven process more likely to be completed in sympatry after an initial period of isolation. This is likely to constitute the most general model of speciation.
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