A population founded by a single pair of individuals: establishment, expansion, and evolution

Events occurring at the founding of a population, and in the next few generations, are potentially of great importance for the future evolution of the population. This study reports demographic, genetic, and morphological changes that took place during and after the colonization of the small Galapagos island of Daphne Major by three male and two female large ground finches, Geospiza magnirostris, at the end of 1982. Using assignment tests with microsatellite DNA data we demonstrate heterogeneity among the immigrants. Their sources included both a near island (Santa Cruz) and a far island (Marchena). However, almost all immigrants that stayed to breed were from an intermediate island (Santiago) and its satellites. Song may have been responsible for this selectivity. Mean heterozygosity stayed roughly constant over the next 15 years while allelic diversity almost doubled, after an initial decline, as the breeding population increased to a maximum of 30 pairs. Although close inbreeding occurred, with a reduction in heterozygosity, an expected net decline in heterozygosity did not occur, for two reasons: it was counteracted by continuing gene flow from immigrants at a low rate, and inbred birds (in one cohort) were at a selective disadvantage. An abrupt step-function shift in beak shape occurred after 9 years. Thus the study provides evidence of drift and selection causing morphological and genetic divergence in the establishment of a new population and in the first few generations.