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.
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