Fission, fusion and the generation of novelty in Geospiza species on Daphne Major Island, Galápagos

Early stages in the adaptive radiation of Darwin’s finches are characterized by divergence (fission) and convergence (fusion) caused by natural selection and introgressive hybridization respectively. The causes of hybridization are rare misimprinting of nestlings on a heterospecific neighbor’s song or extra-pair mating. Resulting hybrid offspring are at no detectable fitness disadvantage in terms of survival or reproduction. We studied the long-term consequences of hybridization in three species of ground finches on the small Galápagos island of Daphne Major. Over a 30-year period, heterospecific ancestry increased in Geospiza fortis and G. scandens, linearly and without slowing or reversing. Increases imply greater genetic gains through introgression than losses through selection. The increases were due to continued interbreeding and accumulation of heterotypic alleles in backcrosses and later generations, and not due to increases in the rate of hybridization. To explain the increase in G. fortis ancestry in G. scandens we proposed a hypothesis of heterosis based on previous evidence of inbreeding depression in G. scandens. The hypothesis is supported by higher average heterozygosity at 14 microsatellite loci of the survivors of a strong environmental stress, and by an increase in the proportion of G. scandens in the finch community. The study provides an example of the erosion of differences between species, leading towards fusion into a single, variable, admixed population with an enhanced potential for evolutionary change in a new direction.