New genetic analyses of fish reveal how some have managed to evolve and adapt to live in polluted water. The results suggest that the high genetic diversity exhibited by these fish was key - a palette for natural selection to act on to facilitate the population's rapid adaptation to extreme pressures. In salt marshes along North America's Atlantic coast, killifish have increasingly been exposed to industrial pollutants that have reached lethal levels in recent decades. Some subpopulations have developed tolerance to the pollutants, however. To better understand this adaptation, Noah M. Reid et al. analyzed the genomes of 384 killifish, some of which have developed tolerance to toxins and some of which remained sensitive, across four regions. Tolerant fish were found to have reduced genetic diversity compared to sensitive fish - a sign of reduction in population size in polluted sites, where exchange of genetic information became limited. The researchers also identified in these fish a number of genes that were associated with increased survival, particularly genes involved in the aryl hydrocarbon receptor (AHR) signaling pathway. By examining this pathway in developing embryos exposed to toxins, the team found that many genes associated with the AHR pathway were deleted in tolerant killifish, likely impacting AHR signaling in a manner not observed in sensitive killifish. But as the AHR pathway is associated with estrogen and hypoxia signaling, regulation of cell cycle, and immune system function, what about the consequences of it being stifled? The authors identified several compensatory mutations that may help mitigate the loss of function in AHR signaling in killifish. These results are featured in a Perspective by Michael Tobler and Zachary W. Culumber.