Molecular evolution and population genetics of Nesospiza buntings

Abstract

Nesospiza is a genus of buntings restricted to the Tristan da Cunha Islands in the central South Atlantic Ocean. They have undergone an adaptive radiation at the islands and currently two species are recognised based on morphology: the small-billed Tristan bunting (N. acunhae), which is a dietary generalist and the large-billed Wilkins' bunting (N. wilkinsi), a dietary specialist. Both species occur on Nightingale Island with no apparent hybridisation between them. On the neighbouring Inaccessible Island there appears to have been a breakdown of the species barrier and the two species hybridise extensively. Also two altitudinally segregated colour morphs of N. a. acunhae occur on Inaccessible Island. The morphological differentiation of Nesospiza is not reflected in either the mitochondrial DNA or the microsatellite data. Rather the data suggest that there are two island lineages and that the sympatric populations on each island are more closely related to each other than to their allopatric (presumed conspecific) island neighbours. The molecular data support sympatric speciation with parallel evolution in Nesospiza, possibly as a result of divergent selection, acting on the sympatric populations on each island, which could have resulted from a change in feeding ecology. Furthermore the molecular data differentiate between the two sympatric colour morphs of N. a. acunhae, which appear to be speciating as a result of assortative mating.