In the 1960s, Sudbury's nickel smelters turned Ontario's lakes acidic, and tiny creatures evolved to survive, but when the pollution cleared, something unexpected happened

In the mid-20th century, metal-mining smelters around Sudbury, Ontario, spewed large amounts of sulfur dioxide into the air. The fallout drifted across the Canadian Shield, settling into the lakes of Killarney Provincial Park and acidifying the waters to dangerous levels within a matter of decades. The lakes, which naturally ranged from pH 4.3 to 7, were skewed towards the acidic side. The fish died out. The phytoplankton died out. The zooplankton community broke down. Entire food webs broke down.

But one tiny organism clung on and rewrote itself quietly to do so.

One species of freshwater copepod, Leptodiaptomus minutus, just a millimeter long, not only survived acidification but also evolved to thrive in it, eventually becoming the dominant organism in zooplankton communities throughout the acidified landscape. Now, according to a study published in Proceedings of the Royal Society B that tracks the genetics and physical features of the animal over about 100 years and some 200 generations, scientists say they have documented something they rarely get to see: evolution in action, in the wild, in direct response to a human-caused environmental crisis.


History sprouting out of the mud
The core of the research was an unusual method called resurrection ecology. Many aquatic organisms produce dormant resting eggs that sink to the bottom of lakes and build up in sediment layers over decades, a natural, layered time capsule. Scientists took cores from two lakes in Killarney: George Lake, which had recovered to near-neutral pH by 1994, and Lumsden Lake, which was still in chemical recovery. They divided the eggs into three time categories: pre-acidification, peak acidification and the modern recovery period, then hatched them in a lab and grew the resulting copepods in both acidic (pH 4.5) and neutral (pH 6.5) conditions.

The results were unequivocal. The copepods that hatched from eggs laid in the acidification era lived longer, reproduced more and grew faster than their ancestors from before the acid era and that was the case no matter which pH treatment they were tested under, suggesting that the entire pace of life had changed. But when lake pH began to bounce back in the 1970s, thanks to government emission controls, those acid-tolerant traits faded again over subsequent generations.

One crucial exception was the speed of development. The acidification appears to have selected a faster rate of development that did not reverse in the recovery period, despite the reversal of both survival rate and fecundity. The copepods from the recovery period kept growing rapidly. This implies that not all evolved features are reversed at the same rates or under similar drivers and that some changes that are brought about by a rescue event are more permanent than others.

Lakes have been shown to have valid historical record of aquatic organisms in their sediment seed banks. Published in Evolutionary Applications, this research on paleolimnology and resurrection ecology offers scientists a rare peek at how populations have responded to environmental change through time.

The genetic blueprint of survival
To make sure this wasn’t just random variation, the researchers sequenced the whole genomes of eggs from each time period, identifying more than 4.2 million genetic variants, called single-nucleotide polymorphisms (SNPs). Among those were hundreds of thousands that shifted greatly in frequency as the water acidified and then shifted again as the pH recovered. Notably, 19,525 SNPs were outliers across both time transitions in one lake and exhibited a reversal in the direction of allele-frequency change during recovery. The patterns pointed straight to natural selection with directionality. Some genetic variants were being systematically favored and others eliminated by the environment, generation after generation; then the process was reversed.

That is the essence of what scientists call evolutionary rescue. When conditions worsen, a population may crash, but it can bounce back as the better-adapted individuals reproduce successfully. According to a review published in Trends in Ecology & Evolution, evolutionary rescue occurs when adaptive evolutionary change restores positive population growth and prevents extinction, and is most likely to succeed when initial population sizes are large and standing genetic variation is high.

In the study, demographic modeling confirmed the classic U-shaped population curve, the signature of evolutionary rescue: a crash followed by a comeback. Historical records show that L. minutus populations in the area had already reached high densities again by 1972-1973, matching the genetic recovery signal found by the researchers. Crucially, in another lake with a different population of the same species that was experimentally exposed to acidification but lacked the same genetic diversity, it crashed to local extinction, showing that the rescue in Killarney was not a given.

Evolution isn't a one-way street
Perhaps the biggest finding was what happened after the lakes began to recover. At the phenotypic and genomic level, as the pH of the water returned to neutral, the hard-won acid tolerance began to erode. Survival rates and fecundity at acidic pH dropped. The allele frequencies reversed. Basically, the population unevolved its adaptations. Why? Because the adaptations were no longer needed and may have been costly in terms of fitness in a neutral environment, but those costs were only apparent in natural ecological circumstances such as competition and predation, which the lab setting could not fully reproduce.

This adaptive reversal has implications for how scientists think about the long-term consequences of evolutionary rescue. According to a study in Evolutionary Applications examining population demographic history and evolutionary rescue, the process of evolutionary rescue is characterized by an initial population decline due to maladaptation followed by recovery as adaptive alleles spread through the population, a pattern that can leave lasting imprints on a species’ genetic diversity. This is exactly what the Killarney copepods showed: a genetic bottleneck during maximal acidification, with reduced genetic diversity in Lumsden Lake and a reduced effective population size in George Lake. While Lumsden’s genetic diversity appeared to recover somewhat, the genetic diversity of George Lake did not, meaning the short-term survival gain came at a long-term genetic loss.

The threat isn't over
According to researchers in a study published in Proceedings of the Royal Society B, simply surviving acid rain does not mean that L. minutus is safe. The Killarney lakes have not recovered their original ecological state despite decades of chemical recovery. More pressingly, recent data suggest that L. minutus populations have been declining across multiple lake ecosystems, including both Killarney Provincial Park and Adirondack Park in New York, a direct concern for US lake ecosystems. Climate change was an active stressor during the same period as the acidification event and continues to increase. A population that evolved by evolutionary rescue with low genetic diversity may simply have less fuel to adapt to the next crisis.

That’s no small concern for a species that is found in the Great Lakes, Lake Huron, Lake Michigan, and boreal lakes of northern North America. It is really amazing what this copepod has done in over a century. The question now is whether it can do it again under faster, more complex environmental pressures.