COS 194-4 - Evolutionary rescue followed by adaptive reversal allowed the persistence of copepods during historical lake acidification and recovery

Background/Question/Methods

The persistence of species facing environmental disturbance can be enabled by rapid evolution, and in some cases, evolutionary rescue. Convincing examples of evolutionary rescue have been documented in wild populations, but few studies have investigated the combination of factors (demographic, genetic and environmental) facilitating the rescue of populations in complex natural settings. Additionally, the beneficial outcome of evolutionary rescue could potentially be mitigated during environmental recovery due to a loss of genetic diversity from strong selection and reduced fitness in the recovered environment. Here, we used copepod resting eggs of Leptodiaptomus minutus from three time periods (pre-acidification, acidified and recovered) for whole genome sequencing in combination with resurrection ecology methods to retrace the evolutionary trajectory of two populations impacted by historical acidification and pH recovery in Killarney provincial park (ON, Canada).

Results/Conclusions

Resurrected copepods from the pre-acidification period showed sensitivity to acidity while individuals from the acidified period are adapted to acidic pH and showed high fitness overall. This tolerance was subsequently lost during pH recovery, indicating an adaptive reversal. We found evidence of rapid directional selection in our genome scans, confirming the results of the phenotypic assays. Important increase and decrease in allele frequencies of outlier loci during the acidification are followed by a reversal in the recovered population. Sensitivity of pre-industrial copepod to acidic pH and decrease in genetic diversity in the acidified population suggest a demographic decline during the acidification process followed by population recovery, illustrating an evolutionary rescue. Alleles lost and fixed during the acidification had variable allele frequencies during the recovery period, and partial recovery of genetic diversity following environmental recovery highlight the positive effects of migration from nearby populations and/or input from resting egg bank. Our project fills a critical knowledge gap about evolutionary rescue in the wild by employing temporal genomic data and resurrection ecology experiments spanning 100 generations in whole lake ecosystems that have been impacted by historical human disturbance.