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When you first enter Waterton Lakes National Park, just north of the Canadian border from Montana, you may be forgiven for not immediately examining the prairie grass. The Rocky Mountains rise so abruptly and imposingly out of the ground they tend to draw the eye upward instead of down. But it is down in the grass that an easily overlooked insect flutters from blade to blade—a newly designated species of butterfly, Satyrium curiosolus, which has made its home here for millennia, surviving inbreeding and changes in climate.
“It’s not a very beautiful butterfly,” says Julian Dupuis, an assistant professor in the Department of Entomology at University of Kentucky, who studies the species, a type of hairstreak butterfly. “It’s a small, drab, kind of grayish, brownish-bluish, butterfly,” Dupuis says, “but it captures the uniqueness of this environment in a way that many other species don’t.”
It took a calamity for scientists to discover the butterfly’s curious ecological history. In the fall of 2017, a wildfire blazed through the park and scorched more than half of the butterfly’s habitat—a small alluvial fan called the Blakiston Fan, where two rivers collide. The following year, a survey revealed that only a handful of individual butterflies had survived in the area.
To help the population recover, scientists figured they would need to bring in butterflies from a related population of hairstreak butterflies which live more than 200 miles away. But first, they decided to sequence the genomes of the remaining individuals in the Blakiston Fan, to make sure the populations would be compatible. What they found startled them.
It was so distinct—it had its own evolutionary trajectory for so long—that it needed its own name.
The results from the sequences, published earlier this year, showed that the butterflies have been genetically isolated for an estimated 40,000 years with no cross-breeding with other hairstreaks. Furthermore, the genetic patterning suggested the population had always persisted in small numbers, likely 1,000 or 2,000 individuals, says Zac MacDonald, a genomics expert at UCLA’s Institute of the Environment and Sustainability and author of a new paper about the butterfly. That’s a small number to sustain an insect population over such a long time.
“It was so distinct—it had its own evolutionary trajectory for so long—that it needed its own name,” says MacDonald. “We were so surprised. Nobody knew this isolated population lives here,” he says.
The scientists settled on Satyrium curiosolus, and as a common name, the Curiously Isolated Hairstreak. The description codifies the population’s hermetic history on the alluvial fan.
Such an extensive presence in a geographically confined place (it is possible to walk the length of the fan in less than a quarter of an hour, according to MacDonald), suggested a history of inbreeding and immense resilience to variations in climate by the population. Though it’s possible the Satyrium curiosolus population started elsewhere and migrated to the Blakiston Fan, it most likely survived at least part of the last ice age here, with the Blakiston fan acting as a refuge due to its relatively mild microclimate.
“When a population that’s that small and that isolated persists, it probably underwent this thing called genetic purging,” says MacDonald. While some of the offspring in an inbred community will face high mortality rates—offspring of related parents have a higher chance of accumulating deleterious traits, which makes them more susceptible to disease and less able to survive big changes in the environment—those that survive emerge as highly adapted to their niche.
The population genetics of cheetahs reveal a similar trajectory. Most of the individuals around today are highly inbred, as the cats’ population underwent a severe bottleneck an estimated 12,000 years ago. Despite this history, today’s cheetah populations are generally well adapted to their environment, threatened more by loss of habitat and poaching than by the genetic effects of inbreeding.
After the population of the Curiously Isolated Hairstreak crashed in 2017, it gradually rebounded without intervention. It is likely this wasn’t the first time the butterfly had eschewed extinction, and hopefully it will continue to endure in its home at the rivers’ edge. 
Lead photo: Zac MacDonald et al.
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