Did blind cavefish evolve by breaking the laws of evolution?

The discovery that a cavefish lost its sight because key eye genes were switched off via epigenetics, rather than mutation, will fuel an evolutionary debate

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We’ve found out why a Mexican cavefish has no eyes – and the surprising answer is likely to be seized upon by those who think the standard view of evolution needs revising.

Over the past few million years, blind forms of the Mexican tetra (Astyanax mexicanus) have evolved in caves. Maintaining eyes and the visual parts of the brain uses lots of energy, so the loss of eyes is a big advantage for animals living in the dark. Instead the cavefish “see” by sucking.

It was assumed that these fish became blind because mutations disabled key genes involved in eye development. This has been shown to be the case for some other underground species that have lost their eyes.

But Aniket Gore of the US’s National Institute of Child Health and Human Development and colleagues haven’t found any disabling changes in the DNA sequences of eye development genes in the cavefish.

Instead, the genes have been switched off by the addition of chemical tags called methyl groups. This is what is known as an epigenetic, rather than genetic, change.

“Although a central role for DNA methylation in development and disease has been well-documented, our results suggest that epigenetic processes can play an equally important role in adaptive evolution,” the team writes.

The researchers propose that this epigenetic mechanism allowed the cavefish to shed its eyes faster than if the change had happened via DNA mutations in eye genes.

That is a controversial suggestion, and one that will be music to the ears of those who think that evolution can occur via heritable epigenetic mechanisms, and that standard evolutionary theory needs to be broadened to include such processes.

“This is a most interesting paper,” says evolutionary biologist Douglas Futuyma of Stony Brook University in New York. But he doesn’t think it poses any challenge to standard evolutionary theory as the epigenetic change is itself most likely a result of a genetic change.

Gore’s team shows that the silencing of the eye genes is due to the increased activity of a specific gene involved in methylation, Futuyma points out. So the question then is, what is making this gene more active?

“I think it likely that there has been an alteration in DNA sequence of that gene,” he says.

“You cannot completely rule out genetic mutations,” says Eva Jablonka of Tel Aviv University, Israel, who thinks evolution can occur via epigenetic changes and that we need a new evolutionary synthesis. “Maybe there was genetic variation that contributed to the eye loss.”

However, Jablonka thinks that heritable epigenetic changes alone could explain the loss of eyes. What is more, she even thinks it possible that the epigenetic changes were somehow triggered by the cave environment in the first place. That would be a form of Lamarckian evolution: the idea that characteristics acquired during an individual’s lifetime can be passed on to descendants.

David Shuker at the University of Edinburgh, UK, is unconvinced by this or any of the other proposed examples of evolution via epigenetic mechanisms.

There is no doubt that some animals respond to the environment via epigenetic mechanisms, he says, but these mechanisms have evolved via genetic changes. “It goes through genetically built systems,” Shuker says.

So, like Futuyma, he thinks standard evolutionary processes such as mutation and natural selection still explain all we have discovered.

“We are always finding new ways in which these processes manifest themselves,” says Shuker. “We have found lots of amazing things.” But the basic principles remain valid, he says.

Shuker is suspicious of some efforts to promote the idea of an “extended evolutionary synthesis”. He thinks some people are trying sneak religious ideas back into evolutionary theory.

“They are trying to allow organisms to have agency not controlled by genes,” he says.

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