Historical Fragmentation of Islands and Genetic Drift in Populations of Galapagos Lava Lizards (Microlophus albemarlensis complex)

摘要

The formation of islands following a rise in sea level at the end of Pleistocene is expected to disrupt the equilibrium between genetic drift and gene flow in species with limited ability to disperse. Here we test the hypothesis that genetic drift in isolation has caused the differentiation of Galápagos lava lizards (Microlophus albemarlensis complex) found on ten islets that are likely to have been connected to a larger island, Isla Santa Cruz, during the late Pleistocene. Using 11 microsatellite loci, screened on 524 individuals from 17 localities distributed among and within 14 islands, we found marked differences in allelic richness and heterozygosity. Genetic differentiation was strong (global FST = 0.44), with pairwise differences found among populations on islets larger than differences among three localities sampled within Isla Santa Cruz. As expected under a scenario of drift in isolation, there was a positive correlation of genetic diversity with island size, no relationship between genetic and geographic distance, and a strong negative correlation between heterozygosity and measures of genetic differentiation. We conclude that sea water is a significant barrier to gene flow in lava lizards on this timescale. Our results suggest that the shallow diversification of the M. albemarlensis complex is not due to recent gene flow and that genetic drift may have played a substantial role in observed patterns of phenotypic variation among islands.