GENETIC DIVERGENCE IN PERIPHERALLY ISOLATED POPULATIONS OF CHAFFINCHES IN THE ATLANTIC ISLANDS

Peripherally isolated populations of common chaffinches (Fringilla coelebs) in the Canaries, Madeira, and Azores were compared genetically with their putative ancestral stock in Iberia and Morocco, and with a population of blue chaffinches (F. teydea) from Tenerife, using protein electrophoresis of 42 loci. The continental populations are only weakly differentiated genetically (F = 0.092), despite distinctive subspecific differences in plumage and morphometrics between Iberia and Morocco populations. Estimated levels of gene flow among continental populations are high enough to account for their relative genetic homogeneity, and it is unlikely that homogenizing selection is operating to mimic the effects of gene flow. In contrast, the Atlantic island populations are well differentiated genetically (F = 0.321), and have diverged considerably from their continental conspecifics. The development of significant genetic differentiation within the Canaries but not the Azores likely results from smaller population sizes, very restricted gene flow, and enhanced random drift in the former populations. There is no convincing evidence in support of stronger directional selection acting on genotypes or phenotypes to reduce within-population variability in Canaries populations as proposed by Grant (1979), although other tenets of his model of island evolution are supported by our analysis. Although genetic variability is reduced in four of the Canaries populations, only the Hierro population appears to have encountered a severe bottleneck. Yet it has not differentiated markedly from the La Palma population to which it is subspecifically allied. We conclude that gradual divergence in isolated populations of small to moderate size is the most plausible explanation for the evolution of intraspecific and interspecific diversity in Atlantic island chaffinches.