Biogeography and Evolution in the Oceanic Islands of the Gulf of Guinea

Abstract

AbstractAs with most archipelagos, geography played a central role in the assembly and evolution of the endemic-rich biological communities of the Gulf of Guinea oceanic islands. The islands are located at moderate distances from the species-rich African continent that surrounds them to the east and north. This proximity facilitated colonization by many branches of the tree of life, but gene flow between the islands and continent was low enough that many lineages evolved in isolation once they reached the archipelago, resulting in many endemic species. Furthermore, several of the island taxa belong to groups typically considered to be “poor dispersers” across sea barriers, which strongly supports a role for natural rafts in seeding the islands. Oceanic currents, including the freshwater pathways that extend from large river drainages into the Gulf of Guinea during the rainy season, also support this hypothesis. The distances between the islands are equivalent to those between the islands and the continent such that inter-island dispersal events appear to be relatively rare and thus few taxa are shared between them. Still, the islands present multiple cases of secondary contact leading to hybridization and genetic introgression between closely related lineages—providing several models to study the role and consequences of gene flow in evolution. Most taxa for which molecular estimates of divergence time have been derived are much younger than the ages of the islands. This pattern is consistent with high species turnover, likely resulting from a combination of small island sizes, proximity to the African continent and a long history of intense volcanic activity. The Gulf of Guinea oceanic islands provide multiple examples of classical adaptations to island life (the “island syndrome”), including giants and dwarves, ornament and color loss, among others. In addition, emerging studies of birds are highlighting the importance of competition regimes in driving phenotypic change—with examples of both character release (low inter-specific competition) and character displacement (inter-specific competition upon secondary contact). Collectively, the Gulf of Guinea oceanic islands offer unique opportunities to study adaptation and speciation in a range of taxa and contexts.