At least 10 different lineages of fish have adapted to live in an extreme environment using the same mechanism, according to a study led by Washington State University and Kansas State University evolutionary biologists.

The fish, which were found living in streams with highly toxic levels of hydrogen sulfide in different locations in the United States, Mexico and the island of Hispaniola, all had adapted to their harsh environments by modifying an enzyme in their mitochondria, so the toxicant could not bind to it.

The study, recently published in Proceedings of the National Academy of Sciences, challenges the idea that evolution will produce different adaptations to similar circumstances.

“Whether or not populations take the same path to adapting to novel environments is a long-standing question in evolutionary biology,” said Joanna Kelley, WSU associate professor and co-lead author on the study. “Our research shows that the same pathways have been modified in multiple different species of hydrogen sulfide adapted fishes.”

The water these fish live in is so toxic that it kills most other living things by binding to an enzyme in the mitochondria, shutting off energy production at the cellular level.

Ten small stream fish on a white background.
Researchers found 10 lineages of fish who had developed the same adaptive mechanism. Photo: KSU

All organisms have this enzyme from fungi and plants to humans. Yet this pathway is modified in these fish. The researchers also found that the fish ramped up an existing detoxification mechanism inside the mitochondria, so they can get rid of the hydrogen sulfide faster and survive in the toxic water when other non-adapted fish in the same species could not.

The multiple lineages of fish with this capability brings into question a view proposed by evolutionary biologist Stephen Gould, that if evolution repeated itself, it would lead to different outcomes every time.

“Thirty years ago, Gould said ‘if you could rewind the tape of life, you would get a different outcome every single time,’ meaning that evolution would not find the same adaptive solutions every time,” said Michi Tobler, an associate professor at Kansas State University and co-lead on the paper. “What we actually found in all these lineages – where the tape of life has been replayed as they were exposed to the same sources of selection – is that evolution actually unfolds in very similar ways. I think it tells us something very fundamental about how organisms adapt and that adaptive solutions are possibly limited.”

The researchers were able to compare the fish with the adaptation living the toxic water with ancestors that live in the normal environment because there was no barrier between habitats. Tobler said as a consequence of the fish adapting to the toxic environment, they are actually evolving into a new species.