Research on wild fish populations is adding to a growing body of evidence that human activities—particularly polluting the environment—can spur rapid evolution in complex life-forms. In the past biologists assumed that the genetic makeup of such organisms changes slowly, over thousands if not millions of years.
In the last decade, though, the Atlantic tomcod (Microgadus tomcod) and killifish (Fundulus heteroclitus) in New England have been shown to have developed resistance to toxic PCBs (polychlorinated biphenyls) dumped in lakes, rivers and coastal waters during the 20th century. Similarly, populations of yellow perch (Perca flavescens) in several Canadian lakes have managed to adapt to more than 80 years of heavy-metal emissions from smelters.
The same seems true of brown trout (Salmo trutta) in the rivers of southwestern England. Now a new study reveals not only that the local populations of these trout have changed rapidly in response to pollution, it also ties distinct genetic changes to precise events in human industrial history.
The region has a long legacy of mining, dating back to the bronze age, and zinc, copper, tin, arsenic and other heavy metals continue to get washed into watercourses. Indeed, certain river catchments are now so toxic that only local populations of trout can survive there; fishes entering from elsewhere would die.
To test for pollution-induced evolution and assess the timing of the changes, scientists from the University of Exeter compared DNA samples from 15 populations of wild trout (700 fish in total), nine inhabiting polluted catchments and six from clean sites. Not only did the genetic composition of the populations differ significantly, the divergences mapped remarkably well to key moments in recent human history.
Molecular dating suggests that the metal-tolerating fish split from their clean-river counterparts in medieval times, about 960 years ago—the period when mining in the region was starting in earnest. A second split was also discerned among the metal-river fish, with populations from the most contaminated catchments diverging approximately 150 years ago, this time coinciding with the height of mining in the area and Britain’s industrial revolution. “Dating techniques can be quite imprecise but the fact that both splits fit events in history so well was compelling and a big surprise,” says Jamie Stevens, the research team leader. “The adaptation appears to be metal-specific, with trout in each river adapted to a unique cocktail of metals. A fish in one river might tolerate arsenic but would die in a catchment high in tin or zinc.” The scientists, whose findings were published in the July 2015 issue of Evolutionary Applications, consider the trout from these waters to be unique variants of the species and should be protected, as they might one day allow contaminated rivers to be restocked.
But there is a downside for the fish. The trout variants best adapted to metal pollution also suffer from a reduced genetic repertoire—probably a result of initial population crashes when the metals first started flooding the waterways. The fish surviving these bottlenecks, as they are known, had less genetic diversity to pass on to their descendants. “While they have adapted to their unique environments,” Stevens says, “the metal-tolerating trout may not be well suited to future change as they lack the broader genetic repertoire needed to cope with new and unexpected challenges.”
- Dan Eatherley is an environmental consultant and the author of Bushmaster: Raymond Ditmars and the Hunt for the World’s Largest Viper. He lives in southwestern England.