Understanding how species respond to climate change is a universal endeavor in ecology. Comprehension of climate-induced range shifts is hindered by knowledge of where populations once were. One way to retroactively investigate the status of populations in the past is to utilize natural history collections. Archived specimens document characteristics in time and space, and can offer insight into baseline conditions and microevolutionary processes that shaped today’s populations. With the northeast U.S. shelf being one of the most rapidly warming marine ecosystems in the world, natural history collections spanning this region provide a unique opportunity to track climate-induced shifts over time. Here, using double digest RAD sequencing on larval summer flounder collections (n = 428) from repeatedly sampled regions across 26 years, we sought to determine 1) if larval summer flounder exhibit regional genome-wide population structure between 1989-2012, and 2) if changes in allele frequency at particular loci, potentially associated with temperature, might explain northward distributional shifts observed in other studies of this species.
PCA suggests that genome-wide population structure of larvae from northern and southern estuaries are similar across time. Examining individual loci, the vast majority experienced little to no allele frequency change across regions and time. Yet, a small number of loci exhibited much larger allele frequency differences across time, up to 0.21. Many alleles exhibiting the greatest regional differences early in the time series also exhibited the greatest magnitude in change in northern estuaries over time, such that allele frequencies in northern estuaries converged to look more “southern” by the end of the time series. Together, these data suggest that larval summer flounder are well mixed over much of the genome on evolutionary timescales, but that individual movement has aided the species’ northward expansion with warming ocean temperatures.