Studying the effect of similar environments on diverse genetic backgrounds has long been a goal of evolutionary biologists with studies typically relying on experimental approaches. Pink salmon, a highly-abundant and widely-ranging salmonid, provide a naturally-occurring opportunity to study the effects of similar environments on divergent genetic backgrounds due to a strict two-year semelparous life-history. The species is composed of two reproductively-isolated lineages with overlapping ranges that share the same spawning and rearing environments in alternate years. We used restriction site-associated DNA (RAD) sequencing to discover and genotype approximately 8,000 SNP loci in three population pairs of even- and odd-year pink salmon along a latitudinal gradient in North America. We found greater differentiation within the odd-year than the even-year lineage and greater differentiation in the southern pair from Puget Sound than in the northern Alaskan population pairs. We identified 15 SNPs reflecting signatures of parallel selection using both a differentiation-based method (BAYESCAN) and an environmental correlation method (BAYENV). These SNPs represent genomic regions that may be particularly informative in understanding adaptive evolution in pink salmon and exploring how differing genetic backgrounds within a species respond to selection from the same natural environment.