Abstract
Geographically separated populations tend to be less connected by gene flow, as a result of physical or nonphysical barriers preventing dispersal, and this can lead to genetic structure. In this context, highly mobile organisms such as seabirds are interesting because the small effect of physical barriers means nonphysical ones may be relatively more important. Here, we use microsatellite and mitochondrial data to explore the genetic structure and phylogeography of Atlantic and Mediterranean populations of a European endemic seabird, the European shag, Phalacrocorax aristotelis, and identify the primary drivers of their diversification. Analyses of mitochondrial markers revealed three phylogenetic lineages grouping the North Atlantic, Spanish/Corsican and eastern Mediterranean populations, apparently arising from fragmentation during the Pleistocene followed by range expansion. These traces of historical fragmentation were also evident in the genetic structure estimated by microsatellite markers, despite significant contemporary gene flow among adjacent populations. Stronger genetic structure, probably promoted by landscape, philopatry and local adaptation, was found among distant populations and those separated by physical and ecological barriers. This study highlights the enduring effect of Pleistocene climatic changes on shag populations, especially within the Mediterranean Basin, and suggests a role for cryptic northern refugia, as well as known southern refugia, on the genetic structure of European seabirds. Finally, it outlines how contemporary ecological barriers and behavioural traits may maintain population divergence, despite long-distance dispersal triggered by extreme environmental conditions (e.g. population crashes).
Highlights
Studies of population genetics evaluate how genetic variation is partitioned across a species’ range
Many studies suggest that closely related species and subspecies of seabirds began to diverge in the Pleistocene, when distributional changes associated with glacial cycles may have induced population fragmentation and subsequent differentiation as a result of allopatric divergence in multiple glacial refugia (e.g. Avise et al 2000; Moum & A rnason 2001; Morris-Pocock et al 2008, 2010)
In the absence of an obvious physical barrier, other, nonphysical barriers must be responsible for the existence of genetic structure between populations of seabirds (Friesen 2015)
Summary
Studies of population genetics evaluate how genetic variation is partitioned across a species’ range. Many studies suggest that closely related species and subspecies of seabirds began to diverge in the Pleistocene, when distributional changes associated with glacial cycles may have induced population fragmentation and subsequent differentiation as a result of allopatric divergence in multiple glacial refugia (e.g. Avise et al 2000; Moum & A rnason 2001; Morris-Pocock et al 2008, 2010). Continental land masses can isolate populations in different ocean basins; for example, the presence of the narrow Isthmus of Panama may serve as an effective physical barrier to gene flow for several tropical seabirds (Avise et al 2000; Steeves et al 2005). Strong philopatry is expected to lead to genetic differentiation among breeding populations through drift and⁄or selection (e.g. Friesen et al 2007; Morris-Pocock et al 2008)
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