The Surprising Reason that Some Cave Fish are Blind

The Mexican blind cave fish (click for credit)

The Mexican tetra (Astyanax mexicanus) can be found living in fresh water above or below ground. The ones living in water above ground are “normal,” but the ones living in water below ground (where light is scarce or nonexistent) are blind. As you can see from the picture above, they don’t even have noticeable eyes. Why? The standard view has always been that seeing things takes a lot of energy, so if you can’t see anything because it is always dark, it doesn’t do any good to waste energy on a visual system. However, if you can see things, it is worth the energy, because a visual system allows you to more easily find food, avoid predators, find a mate, etc.

So, if you are a Mexican tetra living in the waters on the surface of the earth and disabling mutations occur in the genes of your visual system, your fellow Mexican tetras will out-compete you, and you will probably die without passing on those mutations to the next generation. However, if you are a Mexican tetra living in the dark and get the same disabling mutation, it will not affect your ability to survive and pass that mutation on to the next generation. As a result, mutations in the visual systems of underground Mexican tetras accumulated over time, leading to blind cave fish.

A couple of years ago, a study confirmed the first part of this story. By comparing the blind version of the species to the version that can see, they showed that the energy “cost” of having a visual system was 5%-15% of the fish’s total metabolism, depending on the size of the fish. As a result, it makes sense that fish who do not use their visual system (like cave fish) would be better off not having one. However, the second part of the story (mutations building up to disable the system) seems to have been falsified, at least for this particular species.

The authors of the study looked at the genes that are known to be responsible for producing eyes in normal Mexican tetras and found that they are not significantly different from those in blind Mexican tetras. In other words, the genes for eyes in blind Mexican tetras have not been disabled by mutations. Instead, the authors found that those unbroken genes have been “turned off” through a process called methylation.

What is methylation? One of the simplest chemical components in organic chemistry is called the “methyl group.” It is a carbon atom attached to three hydrogen atoms. Methylation is a process that adds this group to a molecule, changing its chemistry. With the proper chemical machinery, a methyl group can be added to any molecule, including DNA. When a methyl group is added to DNA, it doesn’t change the genes in any way, but it affects how much they are used. If done correctly, adding a methyl group to the right places on DNA can keep the genes from being used at all. In other words, DNA methylation can “turn off” genes without changing them.

That’s what seems to have happened in these cave fish. It is called an epigenetic change, because the genes themselves don’t change. Instead, something “on top” of the genes (a methyl group) causes the change. Geneticists have known about epigenetic changes for a long time. It is probably one way an organism adjusts to changes in its life and environment. However, it was originally thought that epigenetic changes aren’t passed on from generation to generation. As time has gone on, however, there have been a lot of studies that indicate such changes can be passed on to subsequent generations. Indeed, this seems to be one of those studies. Rather than mutations disabling the genes of blind Mexican tetra, epigenetics has simply turned them off, and that “off” switch has been passed down through the generations.

If this study is confirmed (I haven’t seen a peer-reviewed version yet), it will probably be hotly contested for a while. Why? Because we know that epigenetics is influenced by an organism’s surroundings. Thus, this study seems to say that the blind cave fish’s surroundings caused its evolution from having eyes to not having eyes. This is anathema to the standard NeoDarwinian view of evolution, which sees all evolutionary change as random, because it is caused by random mutations in the DNA. It isn’t “directed” by anything, including the organism’s surroundings. Of course, once this random change happens, it is subjected to natural selection, and the surroundings play a role in that process. However, the actual change itself is supposed to be completely random.

This study seems to indicate that the surroundings play a role in the change itself, before natural selection has a chance to act. Creationists have argued that many adaptations we see in nature are caused this way. The Creator designed into His creation the ability to adapt, and He designed mechanisms that would allow that adaptation to be directed by the organism’s surroundings. Epigenetics may be one of those mechanisms.

Please note that this is not the first adaptation that has been shown to be directed by an organism’s surroundings. Lenski’s famous Long Term Evolution Experiment has demonstrated that gene changes can also be directed by the environment, at least in the case of bacteria adapting to eat a food that they don’t normally eat in presence of oxygen. Thus, the creationist idea of the surroundings influencing adaptive change seems to be getting more and more confirmation as time goes on.

16 Comments

  1. Jadan says:

    Hi Dr. Wile. Wouldn’t this story be an example of Lamarckism? And wasn’t that (i.e. acquired characteristics) disproven by August Weismen in 1889?

    1. Jay Wile says:

      Jadan, you are certainly correct that it would be a form of Lamarckism, however, Lamarck’s view was much more direct. He basically said if you use something a lot, the next generation has a stronger/longer/faster version of it. If you don’t use something a lot, the next generation has a weaker/shorter/slower version of it. That, of course, is easy to demonstrate false. This version of Lamarckism is quite different. It isn’t about use and disuse. It is about a known mechanism (epigenetics) allowing the surroundings to make a heritable change in the organism.

      Weismen claimed to test Lamarckism by cutting off the tails of mice, mating them, and observing the offspring and the offspring’s offspring. He saw that all of them had tails. However, this didn’t really disprove Lamarckism. It just showed that one form of outside-induced change (having a tail cut off) isn’t heritable.

      1. John D. says:

        Dr Wile, Do you see evidence of Lamarckism in inherited instincts / behaviors / skills ?

        I read an article about beavers which showed that they will build a dam if you simply play the sound of running water through a pair of speakers! I think I read somewhere that even a beaver that was raised in captivity and has never seen a dam will do this.

        http://mentalfloss.com/article/67662/sound-running-water-puts-beavers-mood-build

        I sort of like Lamarckism when applied to “behavioral” inheritance. I’m sure you’ve read about this but for anyone that hasnt, there’s a pretty interesting article about mice inheriting smell memory too..

        http://phenomena.nationalgeographic.com/2013/12/01/mice-inherit-specific-memories-because-epigenetics/

        1. Jay Wile says:

          There are many examples of that, John. Monarch butterflies, for example, migrate to a destination that they have never seen, and they often end up resting in the same tree as their ancestors. I am not sure that’s Lamarckism. Most scientists think of it as instincts that are hardwired into the organism. However, I suppose you could call it some sort of behavioral Lamarckism.

          The mice study you linked was one of the more convincing early studies that epigenetic changes can be inherited.

        2. John D. says:

          Wow.. that bit about the Monarchs is poetic!

          I think inherited “ideas” are extremely intriguing. I’m trying to distinguish between the idea of “genetic memory”, instincts, and why neo-darwinists insist nothing is Lamarckian – seems like a thin line. This wiki article on genetic memory is extremely vague –
          https://en.wikipedia.org/wiki/Genetic_memory_(psychology)

          Also stumbled on this study human infants pupil response to snakes and spiders. Pretty interesting.

          https://www.frontiersin.org/articles/10.3389/fpsyg.2017.01710/full

        3. Jay Wile says:

          I had not seen the one about infant reactions. Very interesting!

  2. joshua says:

    This is fascinating, thank you for sharing. This goes in line with why Christian evolutionists like Perry Marshall believe evolution was purposefully guided as animals were designed to respond to environmental changes and challenges, not just mutate randomly. In fact, as you also consider the work of YEC baraminologists, it seems there is more and more agreement that God designed animals with amazing capacities to change, and it’s simply a matter of disagreement about *how much* they were designed to change and have changed.

    I am very interested to see what impact these sorts of discoveries will have on standard neo-Darwinianism in the coming years (FYI, if you want to get more of this kind of stuff, Perry Marshall is a recent champion on his blog and his book Evolution 2.0, which has value even if you disagree with his position – it includes a positive blurb from a YEC who appreciated his contributions on the wonderful complexities of cell’s adaptive abilities and their exceptional challenges to Darwinian evolution.)

    1. Jay Wile says:

      I do think that if you believe in evolution, you have to admit that it is guided in some way. Random mutations producing new genetic information has never been a plausible scenario for macroevolution.

  3. John D. says:

    Awesome post. I’ve read about these fish here and there but I have yet to hear the term methylation before today. It’s pretty cool.

    What I find interesting is that (and correct me if I’m wrong) these fish can grow their eyes back in just a few generations when brought back into sunlit waters. To me that’s incredible. It also speaks some volume about the type of mutation. Disabling or damaging the gene which controls the eyes might be a permanent or VERY long term change. But placing a blocker around the gene is a great way to “save it for later”. More evidence of foresight in Creation!

    1. Jay Wile says:

      I am not sure they can grow back eyes on their own, John. I am aware of only two findings. The first says if you breed individuals from different cave populations, some of their offspring will see. Also, if you transplant a lens from a seeing Mexican tetra into a blind one, it will grow eyes. I am not aware of any study that indicates they will regain their sight without some outside intervention.

      1. John D. says:

        Ahhh. you’re right. I was going off memory. Still pretty cool how quickly sight gets restored when brought back into the fold. Do you think the ability for the hybridized fish to produce seeing offspring so quickly has anything to do with the fact that the mutation exists in the methyl group as opposed to a change in the gene?

  4. Phil says:

    Blind cave species all over the world really tax the idea of random DNA replication errors saving the day. This article:

    https://phys.org/news/2013-12-cavefish-evidence-alternative-mechanism-evolutionary.html

    acknowledges that evolutionists recognize the problem:

    “In the classical view of evolution, species experience spontaneous genetic mutations that produce various novel traits—some helpful, some detrimental. Nature then selects for those most beneficial, passing them along to subsequent generations.

    It’s an elegant model. It’s also an extremely time-consuming process likely to fail organisms needing to cope with sudden, potentially life-threatening changes in their environments. Surely some other mechanism could enable more rapid adaptive response.”

    They offer an interesting proposal:

    “Eye loss in these fish is considered to be a demonstration of an evolutionary concept known as “standing genetic variation,” which argues that pools of genetic mutations—some potentially helpful—exist in a given population but are normally kept silent.”

    In other words, saving up mutations for a rainy day.

  5. Chris Rohde says:

    Once turned off by epigenetic switches the genes for eyesight would then be free to accumulate deleterious mutations unhindered by natural selection until eventually they become non functional. However this is of no support to the theory of evolution since it would only show how genes can be destroyed, not created.

    1. Jay Wile says:

      But that hasn’t happened. The genes have been turned off, but they haven’t mutated at all, according to this study. You might say that there hasn’t been enough time yet, but old-earthers claim the fish have lived in that environment for 2-3 million years. According to one source, that’s 5 million generations. Those genes might be free to mutate, but they haven’t.

      1. Chris Rohde says:

        However that lack of mutation is consistent with young earth where these blind cave fish are only a few hundreds or thousands of years old.

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