High genetic diversity and no detectable founder effects following a rapid range expansion of a long-distance migrant ( Acrocephalus scirpaceus )

Many species are currently experiencing range shifts in response to climate change. The colonisation of new regions can have serious genetic consequences for the shifting population, given that repeated founder events and strong genetic drift are expected to erode genetic variation along the expansion axis. Such a loss of genetic diversity could reduce adaptive potential, slowing or even halting the expansion. These genetic consequences are known to be particularly acute in species with low dispersal capabilities and therefore low levels of gene flow. Whether highly mobile species experience similar genetic bottlenecks as they undergo environment-driven range shifts is, however, less clear. Here we combined ecological and genomic evidence to (1) infer the origin and (2) quantify the genetic consequences of a recent and rapid northward range expansion of the common reed warbler ( Acrocephalus scirpaceus ), a philopatric long-distance migratory passerine capable of high dispersal. Together, historical observations, ringing data, and genomic RAD-seq data covering the species’ European breeding range support an expansion origin from southwest. There were no reductions in levels of nucleotide diversity or allelic richness along the colonisation route, and no detectable allele frequency clines persisting from potential past founder effects. The genomic dataset confirmed the occurrence of low but significant levels of geographically continuous population structure across the European range. These results suggest that high dispersal capabilities can help even strongly philopatric species to escape the genetic costs of rapid range expansions. The lack of genetic patterns also highlights the value of combining multiple lines of evidence to reveal range shifts as genomic data alone was not sufficient to detect the occurrence of this large-scale range expansion.