For decades, Britain’s peppered moth has been the textbook example of how humans can rapidly drive evolution in another species. New textbooks might want to use a New Zealand stonefly instead.
The story of the peppered moth, though a bit tattered by critiques of some of the research behind it, runs like this: Before England industrialized, the moth generally was white with black spots. But when industrial pollution stained vegetation, the white version gave way to a darker one. When environmental legislation in the 1950s started to clear the air, the white version enjoyed a resurgence.
But scientists in New Zealand have now discovered an arguably more clearcut example of human-induced evolution at work, one that has a clear genetic foundation, they announced recently in Science.
This is the story of two stoneflies. One, the black stonefly, or Austroperla cyrene, is a forest-dwelling insect that has rendered itself toxic to predators by producing cyanide in its body. The other is the long-tailed stonefly, Zelandoperla fenestrata, which isn’t toxic and lives in both forests and in open landscapes that were once forested.
There are two versions of the long-tailed stonefly, one that is brown and another that is black and looks strikingly like its poisonous cousin, all the way down to the yellow stripes on its legs.
When a team of New Zealand scientists looked at where the different versions of the long-tailed stonefly occurred, a clear pattern emerged among 1204 specimens taken from across much of the nation’s South Island. In forested places where the toxic stonefly lived, between 30 and 40% of the long-tailed stoneflies were the dark version. By contrast, in places deforested by burning or logging, where the black stonefly won’t live, far fewer of the bugs mimicked the poisonous relative. In places that had been cleared the longest – as much as 750 years ago – the change was greatest. There, fewer than 10% were mimics.
“The removal of forests since humans arrived has removed the poisonous species. As a result, in deforested regions the mimicking species has abandoned this strategy – as there is nothing to mimic,” says Jon Waters, a University of Otago researcher involved in the study.
The scientists found clear reasons why the darker mimic might disappear. Birds that prey on stoneflies apparently lose their fear of the dark mimics when they aren’t in the forest. When researchers placed decoy versions of the dark and light stoneflies in places that weren’t forested, the birds tried to eat them both at roughly similar rates. It was only in forests that they gave the darker ones a wide berth.
If birds attacked the two different colors of stonefly at similar rates in open areas, you might expect an equal number of the different colors, all other things being equal. But it turns out taking on that poisonous disguise comes at a cost. The darker versions are at a reproductive disadvantage because the females tend to produce smaller collections of eggs.
This color switching has a clear genetic underpinning, showing that the evolutionary pressure is being felt down to the DNA level. A mutation in a single gene called “ebony” is the driver of this color difference, and the occurrence of the darker ebony version of the gene was more prevalent in forests.
Scientists have been documenting many examples of how species are responding to the growing human footprint on this planet, such as the rise of new species in urban environements. But this study is a particularly clear and elegant demonstration of how humans have been shaping the natural world, one that traces it down to a particular snippet of DNA, long before the Industrial Revolution and the emergence of terms like “Anthropocene.”
The scientists also take it as a sign of the resilience of nature, and the speed with which at least some organisms can adapt.
“This study is important because it shows that, at least for some of our native species, there is the possibility of adapting to the environmental changes caused by humans, even when the change is rapid,” said Graham McCulloch, another University of Otago scientist involved in the study.
Ni, et. al. “Human-driven evolution of color in a stonefly mimic.” Science. Oct. 24, 2024.
Image: Wikimedia Commons
