Question: how can having no eyes be a competitive advantage over have eyes? In the case of cave fish? it can't surely offer any greater advantage than having eyes that don't work?

Andrew Beale answered on 2 Jan 2014:

Hi there,

This is going to be a bit of a long answer, but I hope you stay to the end because it’s a brilliant question. You’re also in prestigious company in thinking about that question. In Chapter 5 of Darwin’s Origin of Species he writes:

“As it is difficult to imagine that eyes, though useless, could be in any way injurious to animals living in darkness, I attribute their loss wholly to disuse.”

Cavefish offer a brilliant opportunity for scientists to answer queries like these because, almost uniquely in the animal kingdom, the cavefish’s near ancestor is still living in surrounding rivers. So, scientists like William Jeffery in the USA and Yoshiyuki Yamamoto in the UK can really look at both the process and reason behind the loss of eyes in cavefish by comparing the embryonic development of the two populations – this is a really good example of evolutionary developmental biology.

By comparing the cavefish and the river fish Jeffery and Yamamoto have found out a few things which have helped them work out what’s going on, and the broadly accepted conclusion now is that the loss of eyes doesn’t have much to do with the eyes themselves.

Firstly, they found that a gene called sonic hedgehog is important. In cavefish, sonic hedgehog is expressed over a bigger area in the middle of the embryo than the surface fish. The result is that the eyes don’t develop properly in cavefish, and if they put more of this gene into the surface fish embryos, their eyes don’t develop properly either.

That’s not the only result of the larger sonic hedgehog expression – the second thing they found out is that more sonic hedgehog in the cavefish means that they develop a wider jaw, which also has many more taste receptors that the river fish.

The third and final part of this story is linking the changes in the eyes and the jaw, finding out how this has happened during evolution. Jeffery and Yamamoto’s found out that it looks like over evolutionary time, natural selection has favoured the wider jaw and more tastebuds has been because of the benefits this gives to the fish when they are feeding in the dark. As we now know, the mechanism for this change is an increased expression of sonic hedgehog. A side effect of this mechanism? A loss of eyes.

So the loss-of-eyes story is more a gain-of-big-mouth story. In evolutionary terms this is called a pleiotropic effect. Darwin might have been right after all, that there is no disadvantage to the loss of eyes, but there is no advantage to keeping them either, so a mechanism which causes them to be lost is allowed during natural selection.

He actually almost comes to this conclusion slightly later in Chapter 5 when talking about moles that have very small eyes:
“The eyes of moles and of some burrowing rodents are rudimentary in size, and in some cases are quite covered up by skin and fur. This state of the eyes is probably due to gradual reduction from disuse, but aided perhaps by natural selection.”

Thanks for the question – I hope my answer has helped!

(Weirdly in cavefish, the eyes actually begin to develop normally, before beginning to die off at around 3 or 4 days after the egg is fertilised. Why do the eyes start to develop? That’s an unknown at the moment, though there are some good ideas out there.)