Divergence with gene flow as facilitated by ecological differences: within-island variation in Darwin's finches

摘要

Divergence and speciation can sometimes proceed in the face of, and even be enhanced by, ongoinggene flow. We here study divergence with gene flow in Darwin's finches, focusing on the role of eco-logical/adaptive differences in maintaining/promoting divergence and reproductive isolation. To thisend, we survey allelic variation at 10 microsatellite loci for 989 medium ground finches (Geospizafortis) on Santa Cruz Island, Galapagos. We find only small genetic differences among G. fortisfrom different sites. We instead find noteworthy genetic differences associated with beak. Moreover,G. fortis at the site with the greatest divergence in beak size also showed the greatest divergence atneutral markers; i.e. the lowest gene flow. Finally, morphological and genetic differentiation betweenthe G. fortis beak-size morphs was intermediate to that between G. fortis and its smaller (Geospizafuliginosa) and larger (Geospiza magnirostris) congeners. We conclude that ecological differencesassociated with beak size (i.e. foraging) influence patterns of gene flow within G. fortis on a singleisland, providing additional support for ecological speciation in the face of gene flow. Patterns ofgenetic similarity within and between species also suggest that interspecific hybridization mightcontribute to the formation of beak-size morphs within G. fortis.