I’m midway through The Origin of Species and currently reading the Laws of Variation chapter. In it there’s a section where Darwin writes about different cases of animal that live in completely or mostly dark environments and that have become blind.
He works his way through a couple of different cases and argues that:
For some, the blindness is a product of natural selection
For others, he suspects that the blindness is a product of dissuse (i.e. that either the individual animal was born with the potential of sight but got blind through dissuse of the eyes during the lifetime of the animal. Or due to the theory that environmental adaption by an individual animal during it’s lifetime (Lamarckian inheritance, right?) was inherited by it’s offspring which i’ve understood Darwin still held as a possibility, but which we rule out today with our knowledge of dna etc).
As an example of blindness introduced by the process of natural selection (1) Darwin offers the case of the tuco-tuco or Ctenomys, a south american relative to the mole. He argues that frequent inflamation to the moles (unused) eyes constitutes a competitive disadvantage in the struggle for life and therefore eyes have turned smaller and covered up with skin and fur, rendering the animal blind.
He then talks of case (2); I will quote the paragraph from Origin:
It is apparant from the quoted text that Darwin failed to imagine a competetive disadvantage in this case of unused and uneeded sight and therefore were left to conclude that some other process than natural selection was responsible for the loss of sight (and indeed of eyes) in the crabs. Elsewhere in the book, it is argued that traits that is neither an advantage nor a disadvantage can be expected to fluctuate randomly in the population, and not to lead to the elimination of the trait (i.e. of sight in this case).
Now my question: Lamarckian inheritance has since then been ruled out, and it seems to me somehow intuitively farfetched that the crabs are actually born with full sight and all lose their eyes and sight during their lifetime (though i admit i know nothing about the crabs in question).
That is why I now turn to you, my dear millions. What is the current state of our knowledge of
The loss of sight of these crabs in particular
Competetive disadvantage of unused eyes in general
Off the top of my head, building useless eyes costs energy, so losing the eyes would give a competitive advantage. It might also leave more brainpower to process other, more useful, sensory input.
I know nothing about the particular crabs in question, but a couple comments on the general situation.
First, you’re right in that Lamarckian inheritance is generally ruled out. However, during the lifetime of any particular organism, unused parts can certainly atrophy (if you’ve ever had a cast or brace that immobilized a joint, you’ve seen how fast human muscles can atrophy if they’re not used). More commonly, some parts may never develop if conditions aren’t right. Think about many seedlings – they only develop chlorophyll if they’re exposed to light, so seedlings grown in the dark will be pale and white (but if you could somehow get offspring from those white seedlings, the offspring would still be capable of turning green in sunlight). [On preview, Harmonious raises a similar possibility]
Secondly, you may be slightly misunderstanding drift in traits that aren’t advantageous or disadvantageous. In that case, any given gene is going to randomly drift around. But for something complex like an eye, once a gene drifts far enough to break it, random drift is very unlikely to fix it (things will probably just drift in a different direction, and another gene will break in the meantime, so the eye is still non functional). That means pure random genetic drift is enough to explain the loss of a functioning eye.
Finally, it’s recognized that an organism needs to spend energy and resources to grow and maintain an eye. So a mutation that eliminates whole chunks of the eye might be advantageous merely because the organism can spend its resources on growing a little faster, because it’s not wasting them on a useless eye. [As said maybe more clearly by Alky]
So, to sum up, there are three possibilities for eyeless cave crabs:
They’re genetically similar to sighted crabs, but growing up in the dark they never develop eyes.
They’ve lost the genes for functioning eyes due to random drift.
They’ve lost the genes for functioning eyes because there’s a competitive advantage to being eyeless.
I’m not an expert, but I think there’s still debate over the relative importance of drift and selection in many blind populations.
I am not a biologist, but as far as I can tell, eyes are fairly easy to damage and they’re pretty close to the brain which may make damage and infections especially dangerous. If there isn’t anything to see, the best way to keep your eyes safe is to shut your eyelids. Having them closed permanently would be even better. Best yet, is to have no eyes to damage - especially if you can fill up the holes with bone or some other hard-to-damage material.
It’s not actually all that difficult to imagine. There may be pointy or abrasive things in caves - anything that is both unnecessary and is vulnerable to being poked or scraped, could be disadvantageous.
While not crabs, the issue of eyeless fishes was dealt with in this thread.
I’m not sure about the current state of research into eye loss in crabs, but for certain fishes, the eyes are lost not because of injuries or energy concerns, but because of pleiotropic effects; certain traits that are linked to eye development are actively selected for, with the eyelessness being a byproduct of this selection rather than an actual adaptation unto itself.
There are two possibilities: the “eyes” trait is under negative selection (i.e. it’s a disadvantage to have eyes) or neutral selection (eyed vs eyeless both perform equally well). Either one could easily lead to it being eliminated, though obviously negative selection would do the job quicker. Neutral selection would be an entirely random process - if you have a small population and the eyeless mutation pops up at a high enough rate, it could happen that way. Several mechanisms for negative selection have already been proposed, and they’re all more or less plausible.
You can also think of it this way: outside of caves, eyes will be under positive selection - that is, if you’re born without eyes, you’re going to be at a huge disadvantage. This enables eyes to exist as a product as a complicated series of genetic interactions. Mess up any one of them, and you’re dead, so only those with all the parts survive. Once that selective pressure is gone, though, you’re free to start accumulating mutations and lose eyes with no problems.
From this quote by Darwins Finch in the thread he linked to I get the impression that Darwins proposed explanations (i.e. environmental triggers during the life of the individual animal; Lamarckian inheritance already being ruled out) has been disproven, at least for certain kinds of fish:
I also personally find Darwins Finch argument for pleiotropy being the cause as the most plausible suggestion.
Wouldn’t genetic drift produce in a species a diversity of different kinds of unusable visual systems as they degenerate in different directions with passing generations, under no selective pressure?
I also thought this suggestion by Tuckerfan was interesting as a possible disadvantage and cause for negative selection, though no-one followed up on it in that thread.
If you would rephrase that as this: eyesight, as all senses, constitutes an evolutionary advantage only as input to a system (i.e. in animals usually the nervous system) that controls the individuals behaviour in a way that increases the chance of survival and to procreate. Maybe feeding the control system with inputs that it was not originally adapted for (i.e. lack of sensory input) in some way degrades the proper functioning of the control system and therefore constitutes a disadvantage to the individual?
I would also like to add that in his linked thread Darwins Finch argues quite persuasively against “energy conservation” as a plausible selective pressure for loss of eyesight.
Keep in mind, however, that that’s only been demonstrated to be the case for at least some species of cavefish. I honestly have no idea what might be going on with cave crabs; it’s entirely possible that their eye loss was the result of an entirely different mechanism (or mechanisms).
Yes but I found it at least one plausible mechanism for the general cases that darwin could not explain or explained by “disuse” (2).
For the crabs (1) I think we still need someone with pretty targeted expert knowledge on the subject of the cave crabs Anyone out there?
Apart from that, do you have any thoughts on Tuckerfans suggestion? I must admit I had a somewhat similar idea before starting the thread and would like to hear your thoughts on the subject…
I think a lot would depend on the nature and physiology of said “tricks”. It could well be that an imagination of sorts is required for “trick of the eye” to even be meaningful. Or it could be a byproduct of how visual information is interpreted by the brain…but then, maybe there’s a certain threshold for brainpower before visual hallucinations manifest themselves.
In short, I’d want to determine if the eyes of species closely related to cave-dwellers can be tricked before invoking it as a possible explanation for eye loss in the cave-dwellers themselves.
Let’s forget about Tuckerfans specific framing, since it was slightly antropocentric. I’d like to reframe the argument in more general terms.
I have a intuitive feeling that variations in individuals behaviour would have as least as much impact on chances of survival and ability to procreate as physical disposition.
Example: would the giraffes long neck be a competetive advantage if giraffes had the instinct to act like a lion?
So what i’m really getting at is that there may a disadvantagous impact of the robustness of the the neurological system that produces behaviour when fed with inadequate input (i.e. no sensory stimulation).
I’ve read that in humans eyes are a component of the process by which most colds are caught. Eyes are itchy and sensitive and otherwise frequently ask for attention, so people often touch them. But they are fairly good at harvesting viruses from the fingers and sending them down the tear duct to the back of the nose, where the viruses plant themselves and multiply.
Crabs don’t have noses or catch colds that I know of, but it is easy to imagine that there are various other weaknesses eyes introduce.
The general proposition that some neurological systems can malfunction when they’re not getting certain input they ‘expect’ is certainly true.
But in the case of eyes specifically, I tend to doubt it. Most animals, after all, are already dealing with darkness for half their life, so it seems fairly likely that natural selection has already removed visual/neurological systems that seriously malfunction in the dark.
Anyone out there?