A TEST OF MECHANISMS OF POPULATION DIFFERENTIATION IN GANNETS (MORUS SPP.) USING COMPARATIVE PHYLOGEOGRAPHY AND MORPHOMETRICS

The relative importance of gene flow, genetic drift, and natural selection in driving genetic divergence of local populations, as well as the factors that disrupt gene flow in natural populations, are uncertain. Comparative analyses enable us to test explicit hypotheses regarding the roles of these factors. Gannets (Morus spp.) include three morphologically and ecologically similar seabird species: the Cape Gannet M. capensis, which breeds in southern Africa; the Australasian Gannet M. serrator, which breeds in southeastern Australia and New Zealand; and the Northern Gannet M. bassanus, which breeds in the North Atlantic. We assessed the extent of differentiation in mitochondrial control region sequences and morphometrics among colonies of each species to test mechanisms of divergence. No evidence was found for introgression of mitochondrial DNA between Cape and Australasian gannets, despite the existence of mixed-species pairs, consistent with their taxonomic status as biological species. Significant population differentiation, both in control region sequences and in morphology, was found within both Australasian and Northern gannets. Analyses based on coalescent theory indicated that little to no female-mediated gene flow occurs either between Australasian Gannets in Australia versus New Zealand, or between Northern Gannets in Canada versus Europe. Divergence of these regional populations, as well as Cape versus Australasian gannets, appears to pre-date the Pleistocene glaciations and corresponds to differences in non-breeding distributions. Northern and Australasian gannets may each represent at least two management units for conservation, but additional data are needed on nuclear DNA variation in individuals sampled from more colonies to confirm this result.