  The impact of climate change on populations will be contingent upon their
contemporary adaptive evolution. In this study, we investigated the
contemporary evolution of four populations of the cold-water kelp Laminaria
digitata by analysing their spatial and temporal genomic variation using
ddRAD-sequencing. These populations were sampled from the center to the
southern margin of its north-eastern Atlantic distribution at two-time points,
spanning at least two generations. Through genome scans for local adaptation at
a single time point, we identified candidate loci that showed clinal variation
correlated with changes in sea surface temperature (SST) along latitudinal
gradients. This finding suggests that SST may drive the adaptive response of
these kelp populations, although factors such as species' demographic history
should also be considered. Additionally, we performed a simulation approach to
distinguish the effect of selection from genetic drift in allele frequency
changes over time. This enabled the detection of loci in the southernmost
population that exhibited temporal differentiation beyond what would be
expected from genetic drift alone: these are candidate loci which could have
evolved under selection over time. In contrast, we did not detect any outlier
locus based on temporal differentiation in the population from the North Sea,
which also displayed low and decreasing levels of genetic diversity. The
diverse evolutionary scenarios observed among populations can be attributed to
variations in the prevalence of selection relative to genetic drift across
different environments. Therefore, our study highlights the potential of
temporal genomics to offer valuable insights into the contemporary evolution of
marine foundation species facing climate change.
