Evolutionary rescue of freshwater copepods during historical lake acidification

The persistence of populations facing severe environmental disturbance can be enabled by natural selection on heritable phenotypic variation - a process known as evolutionary rescue. Few studies have documented this process in complex natural settings and the long-term outcome of evolutionary rescue. Here, we used copepod resting eggs of Leptodiaptomus minutus from three time periods of lake ecological history, spanning ≈ 200 generations (100 years) in two populations impacted by historical acidification. Whole genome sequencing of the resting eggs revealed significant allele frequency shifts associated with the acidification followed by pH recovery. We used a resurrection ecology approach to retrace adaptive shifts concomitant with environmental transitions. Copepods from the pre-acidification period showed sensitivity to acidity, while individuals from the acidification period were adapted to acidic pH. This tolerance was subsequently lost during pH recovery, implying an adaptive reversal. Demographic models indicated a decline during the acidification process followed by population recovery based on historical data, suggesting that selection led to evolutionary rescue. This study fills a critical knowledge gap about the long-term implications of evolutionary rescue in the wild.