Organisms evolve for an array of reasons: to better exploit resources, to improve breeding odds, to optimize efficiency, or to overcome an onslaught from predation or poison. The desire to live drives life and evolution. While adaptations may at times involve many changes, sometimes they are quite specific.
Killifish are a hardy freshwater fish that often live in ephemeral waters: estuaries, wetlands, and vernal ponds that may mostly evaporate for a spell. Killifish eggs may survive weeks without water.
There are over 1,270 species of this once-abundant fish, found throughout the Americas, and to a lesser extent in much of the world. Alas, the pollution that people produce can do in even the stoutest swimmers. Many millions of Atlantic killifish on the east coast of the United States lost their lives to a continuing chemical onslaught by human industry.
Yet some of these slippery slivers of silver survived the pollutants that plague American waterways. Genetic analysis revealed that several populations managed mutations which allowed them to withstand 8,000 times the levels of toxicity that might murder a lesser fish.
Though the genetic changes among surviving killifish were generally convergent, each population rapidly adapted to mounting toxicity in their own way. There was no other significant alteration in these killifish; only the ability to live in what would otherwise be toxic waters.
“Atlantic killifish populations have rapidly adapted to normally lethal levels of pollution in urban estuaries. Distinct molecular variants contribute to adaptive pathway modification among tolerant populations.” ~ American evolutionary geneticist Noah Reid et al