When Pollution Drives Evolution

In humans, these pollutants can cause every kind of cancer. In fish, PCBs alone are known to cause devastating deformities and developmental issues, including impaired fertility. Yet even this toxic broth supports an ecosystem of a sort, offering a key insight into how nature responds to human impact and a glimmer of hope as to how life might adapt to a postindustrial world.

Few marine species can survive when PCBs are present in any quantity, but a small number of fish have shown unusual resilience. One is the silvery, leopard-spotted Atlantic killifish, thought to have returned to the waters of Newark Bay in the 1990s. Killifish are usually considered an indicator species, a proxy for measuring the general health of the ecosystem. They are sensitive to both dioxin and PCBs, yet here they were.

In 2016, a team of scientists at the University of California, Davis, genetically sequenced killifish from four contaminated harbors, including Newark Bay, and compared the genomes to those from uncontaminated sites. The pollution-tolerant populations had each evolved similar adaptations that rendered them up to 8,000 times more resistant to industrial pollutants, allowing them to live in water that would normally kill them.

This was evolution at a stunningly fast pace, given that the most harmful toxins were released in the 1950s and 1960s. And the killifish is not the only species to have managed this feat. The Atlantic tomcod in the nearby Hackensack River, for example, has also evolved a gene that makes it immune to the effects of PCBs.

These are examples of natural selection at work. Of the massive killifish and tomcod populations living on the eastern seaboard of the U.S., a few individuals harbor genetic mutations that make them less sensitive to toxicity. Mostly this has little impact on their prospects, but in the waters around polluted sites, they find themselves at a distinct competitive advantage. They live longer, better lives and pass on their mutations to their offspring, who do the same, and thus a new pollution-tolerant strain staggers into existence. Scientists call it “rapid evolution.”

Rapid evolution in response to human activities was first observed in Britain in the 19th century. In 1848, a Manchester lepidopterist called R.L. Edleston caught a rare variety of the peppered moth, normally a pale, delicately ornamented creature. This variant, which he had never seen before, was ebony with two white spots on its temples. It became known as the Carbonaria form.

Such dark coloration, caused by a mutant gene, was formerly a disadvantage for peppered moths, making them stand out amid the peppermint-frilled lichens shrouding tree trunks and branches. But over the previous decades, as William Blake’s “dark, satanic mills” began belching black smoke into the atmosphere, soot and acid rain were stripping those trees to their bark and blackening them. As a result, Carbonaria moths, once vanishingly rare, soon represented 98% of all peppered moths in Manchester.

In an analogous contemporary case, tawny owls in Finland are now changing their colors. Tawny owls come in two colorations, dark brown and pale gray. Records suggest that the proportion of dark owls is increasing, which researchers have linked to declines in snow cover. A 2011 paper in the journal Nature Communications called this “the first evidence that recent climate change alters natural selection in a wild population.”

Factors other than human activities can cause sudden selective pressures too—epidemics, for instance. But humans have been the greatest single evolutionary factor for centuries, maybe millennia. Overfishing and overhunting have driven the evolution of smaller fish, which are better at slipping through nets, and tuskless elephants, since tusked species are more often killed for their ivory. In one South African national park, 98% of female elephants are now born tuskless. And that’s not to mention our ongoing arms races with pesticide-resistant insects, drug-resistant viruses and antibiotic-resistant bacteria.

Man-made climate change, along with ocean acidification, will have an incalculable evolutionary impact upon the world’s flora and fauna. Not all species will be able to keep up. The winners in a rapidly changing world will likely be creatures, like the killifish, who are numerous and genetically diverse.

After the Clean Air Acts of 1956 and 1968 were passed in the U.K., air quality improved, and the peppered moth slowly reverted to its paler form. Though it will be difficult, if we successfully clean up our waterways, dredging them of their most toxic sediments, we might see mutant fish lose their advantage. The original inhabitants of Newark Bay could return, including its once-famous oyster beds.

But the effects of human industry are now impossible to reverse fully. If our species were to be wiped out tomorrow, our factories would fall silent and our generators would shudder to a halt, but the evolutionary forces we’ve set in motion would continue to act upon the genetic makeup of almost every species on Earth. They shape-shift and metamorphose, transmute and adapt, in ways that we cannot anticipate and certainly cannot control. They want to live, any way that they can.

—Ms. Flyn is an investigative journalist in the U.K. This essay is adapted from her new book “Islands of Abandonment: Nature Rebounding in the Post-Human Landscape,” which was published this week by Viking.