"Processes like allele surfing and genetic bottlenecks during range expansion have genetic consequences expected to result in differentiated wave-front populations with low genetic variation and potentially introgression by a local species. The northward expansion of Peromyscus leucopus in southern Quebec provides an opportunity to test these predictions using population genomic tools. Our results show evidence of recent and post-glacial expansion. Genome-wide variation in P. leucopus indicates two putative glacial lineages are separated by the St. Lawrence River, with a more recent post-glacial divergence of populations isolated by the Richelieu River. Most regressions of genetic diversity against relative expansion distance were not significant and half contradicted expectation by showing positive trends. However, in two of three transects we documented northern populations with the lowest diversity in at least one genetic measure. Consistent with bottlenecks and allele surfing during northward expansion, we document a northern-most population having low nucleotide diversity, divergent allele frequencies, the most private alleles, as well as the lowest effective population size and heterozygosity levels that indicate outcrossing. Ancestry proportions revealed putative hybrids of P. leucopus and P. maniculatus. A formal test for gene flow confirmed secondary contact, showing that a reticulate population phylogeny between P. maniculatus and P. leucopus was a better fit to the data than a bifurcating model without gene flow. Thus, we provide the first genomic evidence of gene flow between this pair of species in natural populations. Understanding the evolutionary consequences of secondary contact is an important conservation concern as climate-induced range expansions are expected to result in new hybrid zones between closely related species, with potential consequences for emerging infectious diseases." --