Conspecific versus heterospecific gene exchange between populations of Darwin's finches

摘要

This study addresses the extent and consequences of gene exchange between populations ofDarwin's finches. Four species of ground finches (Geospiza) inhabit the small island of DaphneMajor in the centre of the Galapagos archipelago. We undertook a study of microsatellite DNAvariation at 16 loci in order to quantify gene flow within species owing to immigration and betweenspecies owing to hybridization. A combination of pedigrees of observed breeders and assignments ofindividuals to populations by the program STRUCTURE enabled us to determine the frequencyof gene exchange and the island of origin of immigrants in some cases. The relatively large popu-lations of Geospiza fortis and G. scandens receive conspecific immigrants at a rate of less than oneper generation. They exchange genes more frequently by rare but repeated hybridization. Effectsof heterospecific gene flow from hybridization are not counteracted by lower fitness of the offspring.As a result, the standing genetic variation of the two main resident populations on Daphne Major isenhanced to a greater extent by introgressive hybridization than through breeding with conspecificimmigrants. Immigrant G. fuliginosa also breeds with G. fortis. Conspecific immigration was highestin the fourth species, G. magnirostris. This species is much larger than the other three and perhapsfor this reason it has not bred with any of them. The source island of most immigrants is probablythe neighbouring island of Santa Cruz. Evolutionary change may be inhibited in G. magnirostris bycontinuing gene flow, but enhanced in G. fortis and G. scandens by introgressive hybridization.