Evolutionary rescue and reintroduction of resistant frogs allows recovery in the presence of a lethal fungal disease

Vast alteration of the biosphere by humans is causing a sixth mass extinction. Adaptation to modified environments often is the only means for species to persist, but the extent to which such “evolutionary rescue” can prevent or reverse biodiversity loss is largely unknown. Us- ing results from genomic analyses, a 15-year reintroduction effort, and population modeling, we provide a compelling example of evolution reversing the declines of an imperiled amphibian and allowing the reestablishment of extirpated populations. The once-common mountain yellow-legged (MYL) frog is threatened with extinction by the human-facilitated emergence of a lethal fungal pathogen ( Batrachochytrium dendrobatidis ; “Bd”). Although most MYL frog populations are extirpated following disease outbreaks, some persist and eventually begin to recover. Using an exome capture approach, we show that MYL frogs have undergone substantial evolutionary change following disease outbreaks. Moreover, in naturally recovering populations observed changes in immune function loci are associated with increased resis- tance/tolerance to Bd infection. Large-scale reintroduction of frogs from rescued populations resulted in the establishment of reproducing populations despite ongoing disease. In addition, results from viability modeling suggest that established populations have a low prob- ability of extinction over 50 years. Collectively, these results provide a rare example of how evolutionary rescue and the reintroduction of resistant/tolerant individuals can allow the landscape-scale recovery of disease-impacted species. This example has potentially broad implications for the many taxa worldwide that are threatened with extinction by anthropogenic stressors. ### Competing Interest Statement The authors have declared no competing interest.