Yes, but if you have a sighted arthropod in a cave that has antennae and/or legs which are longer than it’s non-cavedwelling brethern, that’s going to require a bit more energy than a blind arthropod with the longer appendages. The difference would be slight, to be sure, but given enough time, the blind ones would win out, I’d think.
I’d don’t know if it would be considered part of Lamarckism or not, but I remember reading about some experiments with bacteria which showed that the bacteria could “choose” the type of mutation which occured. They tested this by exposing the bacteria to a chemical which would kill 90% or more of them. Afterwards, they counted the number of bacteria and discovered that there were more than expected. Apparently, they were able to mutate in such manner as to allow them to survive. No idea if this was ever replicated.
I don’t find this very convincing, either. The cells in an eye do not in themselves require more energy than the cells of other organs. Thus, I find it difficult to believe that a net energy savings results from increased growth in some organs while decreasing growth of eyes (which are relatively small to begin with).
And, again, in order to argue for energy savings, one must explain why other avenues for energy savings were not taken. A common (perhaps the most common) response to nutrient-poor environments is dwarfism. Yet dwarfism is not a tendancy among cave-dwellers. So how exactly does the energy requirement to produce a single organ override the energy requirements of the entire organism? If energy savings were the key factor then I would expect even sighted dwarfs to win out over sightless non-dwarfs. Furthermore, if energy savings were all-important in evolution, why would anything evolve to any significant size? And even furthermore, in environments where nutrition is plentiful (e.g., in pools under bat colonies; cave fish are detritus feeders), the same loss of eyes is found. No, sez I, there is more to the problem than simply energy considerations.
Indeed, the development of eyes in sightless cavefish mirrors the development of eyes in the surface varieties for some duration. Divergence occurs during the so-called “growth” phase of development, wherein all the necessary cell types have been laid down, and development continues by simply adding to what’s already there (that is, development by cell division rather than cell differentiation). The eye disappears at this point by simply being “left behind” during this phase of development as the body grows around the nascent eye sturctures. This would argue against a shutting down of eyeball genes due to energy savings. The genes are still there, and they still function as they do in the surface varieties. Furthermore, what eye there is then undergoes a process of apoptosis, wherein the cells undergo a programmed die-off, which would seem to be a significant waste of energy (why bother developing the structue in the first place, only to then use more energy to eliminate what’s been laid down already?). During this stage, eye cells continue to be laid down; eye loss only results because the rate of apoptosis exceeds the rate of cell proliferation. The materials are then recycled for further development. Again, it’s difficult to see the claimed energy savings in such a process.
Now, in my readings since the OP asked his question, I have found that perhaps the most compelling explanation for the morphological and behavioral differences of cave fish relative to surface fish has to do with pleiotropy, or the influence of a single (or few) gene(s) on a variety of phenotypic traits. The focus in this thread has been on just two traits: sight and to a lesser extent, skin pigmentation. However, other traits are also affected in cave fish relative to their surface brethren: loss of schooling behavior, reduced optic tecta (the region of the brain which interprets visual information), alteration of feeding behavior, enlargement of the jaws, increased number of tastebuds, increased number of teeth, increased neuromasts (the “lateral line” structures) in the head region, alteration of craniofacial bones (especially around the eye socket), fewer ribs, more compressed body shape, and compressed body scales. Thus, we find regressive changes to eyesight and constructive adaptations for swimming and feeding.
As it turns out, analysis of gene expression in these fish has found that there are no non-functioning genes, which pretty much shoots down the genetic drift hypothesis. Some eye-development-gene expression is reduced, but the expression of other eye-related genes is increased. But, even more importantly, gene signalling from outside the eye itself has been shown to affect the developing eye and result in degeneration. That same gene signalling controlls many of the traits found in the suite of constructive adaptations having to do with feeding and/or swimming! Thus, it seems to me that the best explanation for eye loss in cavefish (note that troglomorphic traits in other animals may well have different causes) is that of pleiotropic adaptation; eye loss is a consequence of other features being selected for, rather than an adaptation unto itself.
Ah, but dwarfism wouldn’t necessarily confer the same kind of advantages that larger appendages would. For example, the ability to consume larger prey, or being less volunerable to predators. The longer appendages also mean that an animal is able to explore more of its surroundings (and thus find food or a mate) in less time, than one without.
How common is the ability to use echo location in blind cave animals? Bats have it, of course, but what about fish and other denizens of the dark depths?
That’s rather the point I was making, in an oblique sort of way: energy considerations alone cannot be the end-all of evolutionary cause. Elongated appendages are a constructive adaptation in lightless environments. But they also require a fair amount of energy to form. As such, the energy trade-off between elongated appendages and loss of eyes is minimal to nil. To argue that eye loss is a direct consequence of energy conservation in the face of a potentially increased energy budget doesn’t make a whole lot of sense. If energy were the primary consideration (and, again, my argument is that it is not), then we ought to see dwarfism prevalent among cave-dwellers, where nutrition is typically much more limited relative to the surface environment.
Or, to put it another way, energy savings as an adaptation unto itself is not sufficient to explain degeneration in general, nor does it explain much at all in the specific case of cave fish.
Well, aside from bats not being blind, they also produce their own sounds (as do most echolocators, for that matter). Fish aren’t known for being terribly vocal, and the typical “bugs & fish” environment isn’t going to be a terribly noisy one to begin with. The presence of bats in the environment would, of course, add some ambient noise to bounce around, but fish and bugs aren’t well known for their hearing, either.
But energy conservation would explain why the eyes disappeared, rather than simply sticking around while the front appendages grew longer. I presume that the visual cortex in cave dwellers has also shrunk while those sections of the brain related to touch have grown larger, or that the visual cortex is now activated when parts of the longer appendages touch something. In either case, this would be an example of an organism working on a conservation of energy, otherwise, one would expect to find the visual cortex unchanged, while that associated with touch much larger.
IIRC, some fish are known to use their swim bladders to make noises, and presumably crickets, at the very least, can hear the noise they make.
Except, as I pointed out in post #22, it is not simply the case that eyes and associated brain structures are lost (at least in the case of cave fish). There is a whole suite of changes involving both regression and construction. I would suspect this is just as true in cave bugs as in cave fish. And, as noted in cave fish, the developmental appartus for growing eyes remains in place, thus eliminating any potential energy savings. I haven’t seen any studies on development in cave bugs, but given the combination of regression and construction found in nearly all cave dwellers, I would not be surprised if what is true for fish is true for bugs, in a general sense. “Energy savings” cannot be considered an adaptive trait unto itself, and is therefore unsatisfactory as an explanation for degenerative traits.
The optic nerve and retina are extended central nervous system tissue, and perform a considerable amount of preprocessing before nerve signals are transmitted into the visual cortex, which is oddly enough, at the back of the brain.
I think Darwin’s Finch’s explanation is the most clear; it’s not that sightlessness (and any attendant marginal benefits in not having to support vision) is selected for, but that other characteristics which have play in the cave environment predominate. With vision offering no benefit, a fish with impeded vision from a genetic “flaw” will be at no disadvantage, whereas any fish which has some other characteristic that makes it reproductively well-adapted to the cave environment will succeed regardless of its visual capability. The genetic capacity for vision then tends to atrophy over time because there is no impetus to select for it, not because there is any real advantage to losing it, and as Darwin’s Finch points out, the essential aparatus for vision still (like other atavistic traits, such as gills on fetal mammals) remains during development.
Evolution is the process of lumping one new adaptation on top of another, sort of like building a high performance sport car starting with a Ford Model A. Eventually, you get to the point that you’ve replaced virtually all of the original parts with new or scabbed on components, but it still starts out with the same heritage. Anybody who wants to argue that this is any way an “intelligent” or teleological process needs to review basic anatomy.
[QUOTEStranger on a Train]
Anybody who wants to argue that this is any way an “intelligent” or teleological process needs to review basic anatomy.
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Fortunately, I don’t beleive that anybody in this post has actually said that, so it is not an issue here.
As the OP I want to thank all those who took the time to respond to my question. I am glad that a question that has always bothered me turns out not to be a trivial question. I think what it shows is that although the underlying truth of evolution is really beyond doubt, there is a lot to it we do not undertstand.
Can I just point out that Charlie Darwin adressed this point satisfactorily 150 years ago. Quite simply eyes are very fragile organs because they are the only organ that needs to be directly exposed to the environment with no skin covering. As such eyes are easily damaged and they are very, very prone to infection. Any organsim thart can get by without eyes is at a massive advantage if it actually sheds them since it removes a major physical weak point and a major entry point for infection.
No other explanation is required. Having eyes opens up an organism to physcial attack and to renders it much more prone to infetcions. In a lightless environment any organism that can reduce or remove the eyes has a massive evolutionary advantge. No energetic benefit is required.
He did, however, get close to the most likely explanation (and the one that has been backed up by experiemental evidence), as I gave in post #22:
As I (and Darwin) noted, selection does actively create adaptations for a lightless environment. However, where Darwin got it wrong is that it’s not disuse that results in eye loss, but rather a pleiotropic effect resulting from those very adaptations. Had Darwin known more about the finer points of development and gene expression, he very likely would have nailed it in one.
I disagree. If eyes were so very fragile, and “very, very” prone to infection, I would think they’d be more likely to have never evolved at all. I doubt the likelihood of injury is as high as you are positing here. The shedding of a “major weak point” you are also positing would only be relevant if eye injuries were more common than not, particularly in a lightless environment (thus confering a selective advantage for getting rid of them). If the eyes are not injured with any statistically-significant frequency, however, then the selective pressure would likely never exert itself; an indiviual who has uninjured eyes would be at no disadvantage relative to an individual which had no eyes. So, unless there is experimental evidence out there which indicates that eye-possessing individuals in lightless environment are prone to eye injuries, then the pleiotropic explanation makes the most sense, in my opinion.
Hmmm, now that I think about it, in total darkness, eyes tend to play tricks on you. Perhaps the evolutionary advantage to going blind has to do with not running from (or after) phantoms created by bored eyeballs.
I’ll see if I can dig up his original quote concerning disease and damage leading to disuse
,
Firtsly are you seriously disputing that eyes more are prone to infection and damage than an identically sized patch of skin?
If not then what exactly is your point?
If so then perhaps you could provide evidence for such an extraordinary asserton, or at least some logic to support it.
Look, lots of organs and systems are prone to damage and infection. That doesn’t prevent them from evolving provided that the benefit they provide outweighs that disadvantage. Jiminy Cricket the human spine is extremely prone to damage due to our upright posture. That clearly didn’t prevent it from evolving now did it?
In a lightless environment of course eyes have absolutely no advantage and as such there extreme proclivity to infection and damage is overwhalming.
Straw poll: has any member of the Straight Dope not suffered an eye injury or infection at some point in their life that didn’t incpacitate them for at least 5 minutes?
I personally have lost count of how many times I have been incapacitated by being poked in the eye, getting objects in my eye, or contracting conjuctivitis. I can’t think of another oragn in the body that I hace injured with incapacitating results as often as my eyes. As fragile as the testicles are they are far less prone to incapacitaing damage because they can be tucked away from the environment to some degree. Eyes by their very nature have to be in apositionof maximal exposure.
I can only guess at what sort of shletered life you must have led not to relaise how prone the delicate membranes of the ey are to damage. A peice of sand that wiouldn’t even register as stimulus on any other externa surface will result in instantaneous incapacitiating damage to the eyes. How much more prone to damage do you want?
Eye injuries are more common than not in any environment. 2000 cases of eye damage in each 8 hour workday. As I said, I will bet that there is not a single human bing over the age of ten who hasn’t been incapacitated by an eye injury on numerous occasions. The event is so common that we hardly even note it. In a natural environment with predators any organ that incapaitates its owner for several minutes at a time numerous times in its life is hardly an evlutionary advantage, now is it?
But they are. 2000 people suffer eye injuires every work day. That is statistically significant when you consider that any eye injury is likely to incpacitate the victim temporarily.
Nonsense. My eyes are not injured at this very moment, but they have been injured in a way that incapaictaed me numerous time sin my life, as have every other person’s. Were a predator to be watching me when I was incapacitated I would now be dead. So despite not having perpertually injured eyes I would be at a huge disadvatage to someone who had been born without eyes if we lived in a lightless enviroment.
Perhaps a better starting point would be for you to explain why the heck living in lightless evnrionment could possibly reduce the incidence of sand getting into the eyes, or an individual running into sharp stick. One thing we do know is that people and animals who lose the sight in an eye usually end up losing the eye physically at a later date. thats; because without the ability to see they also lose the ability to protect the eye, which is then knocked out by later trauma. Based on that we would expect individuals in a lightless environment to suffer mor eye injuries, not less.
Once again I want to stress that I am flabbergasted that you are apparently arguing that eyes are not regularly injured in an incapacitating manner.
I agree - many kinds of injury - including eye injury must be more common, if you’re fumbling around in complete darkness. I expect that’s not the whole story, but it’s bizarre to say it isn’t a factor at all.
I don’t believe I’ve made any extraordinary claims. It was you who proposed the extreme vulnerability of eyes to injury. If eyes are thus so prone, as you propose, then the disadvantage of having them would likely outweigh any advantages. So, I guess I’ll have to ask you for a cite on your claim.
Again, I think you are overstating the vulnerability. Sure, everything is vulnerable to injury in some fashion. But you are proclaiming such an extreme vulnerability that it is evolutionary advantageous to lose the structure entirely. I’d like to see some actual evidence for that.
That proves nothing. A temporary incapacitation especailly will have no evolutionary effects, and short of death, even an eye infection which results in total eye loss isn’t going to have one, either (especially given the uselssness of eyes in a lightless environment in the first place).
The typical cave fish does not work in industrial environments which introduce dangers above and beyond whatever natural dangers one might face…
And in how many of those instances were you prevented from reproducing? Again, I’m not disputing that eyes, or any other body part, can be injured. I’m disuting your point that it was actually evolutionarily advantageous to lose eyes because of the risk of injury, thus explaining eye loss in cave fish. I’ve already provided a perfectly reasonable hypothesis for the phenomenon; what issue do you have with pleiotropy, especially given that that explanation, unlike yours, is backed up by experimental evidence?
Look, there’s no reason to get all condescending here. You made a fantastic claim. And even if the eyes themselves are ripped from one’s skull as a result of injury, it doesn’t justify losing them in a sightless environment as an adaptation.
As a fish, one is going to swim around and bump into stuff, sure. But, unlike humans, their eyes aren’t at the front of the body. Unless they’re swinging their heads from side to side, their eyes are not going to be at significantly greater risk, even in darkness. Besides which, they’re slow-swimming bottom feeders anyway.
Show me evidence that fish suffer the same fate as humans with respect to incapacitation. Among fish, only sharks have ever evolved any traits which reduce damage to eyes (above and beyond whatever traits fish might already have in that respect), and that because their eyes get up close and personal with their prey (which often thrashes about).
Except a predator isn’t going to be watching you! They’re just as blind as you are in the situation we’re talking about! No one, save you, will ever know your eyes are injured, unless it completely incapacitates you (and not just temporarily).
Or, perhaps an even better starting point would be for you to explain why the pleiotropic explanation is lacking, and that “just so” stories are instead being concocted in this thread?
And, once again, losing the eye itself due to injury, infection, disease or what have you is not sufficient to explain their loss in an evolutionary context, even if it is a common occurrance. What must happen is that the individual is no longer able to reproduce because of the injury. If one is blind anyway because there’s no light, then having eyes, not having eyes: the same!
You are flabbergasted because that’s not what I’m arguing at all. Once again, I am arguing that the susceptability to injury cannot possibly be as great as you claim such that it is actually evolutionarily advantageous to lose them, and thus serve as the explanation (“no other explanation is required”) for why cave fish are blind.
And I’d say it’s bizarre that you would make that statement at all, given that I never made such a claim (and at the same time, I wonder why you don’t think it bizarre that such injuries are the explanation…). There is a huge difference between “more common” (which is entirely speculative at this point anyway) and “so common that there are selective pressures against the formation of eyes”.
Surely no one here believes that these fish were suddenly whisked from the surface world to their subterranean caverns, wherein they were forced to survive in an alien world of complete darkness, right? The transition from sighted to sightless was very likely just as gradual as the loss of light in their environment. By the time these animals found themselves in complete darkness, they were likely not “fumbling” at all, having gradually adapted for life in lower-and-lower light environments. Remember all those other adaptations I mentioned that these fish have? Those don’t arise by suddenly throwing sighted fish into a dark pool and letting them bump into stuff, maybe getting lucky enough to find food and/or mates as they go.
There is some work that has been done, especially on Mexican cave fish of the genus Astyanax, which has both cave-dwelling and non-cave dwelling members.
Here’s a discussion of some of lost traits and ‘reversals’ of evolution:
(Bolding mine)
It would seem that we know a lot more about how eyes are lost (a combination of many different genetic and developmental factors) than about why eyes are lost.
This is really not surprising - although offspring of eyeless fish can develop functional eyes, this says little about what pressures or sequence of events (founder effect maybe?) contributed to the loss of eyes. In order to really know whether the adaptationist explanation for eye loss made sense, we would really need to attempt to measure the selective pressure on fish in the wild (or maybe an introduction experiment comparing the survival rates of sighted fish vs. non-sighted.)
This kind of research is rarely done because of the demands on the researcher’s time and effort and the low probability of good data as a payoff, so unfortunately I think this question of why is going to be up in the air indefinitely.
Or more to the point, has anyone ever raised generations of sighted fish in a totally dark environment and monitored their development?
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Developmental studies have been done on both sighted and non-sighted versions of Astyanax. Note that development in light for sightless or the dark for sighted will not really reveal much, beyond the fact that eye loss in Astyanax is an adaptation (i.e., it is not merely the developmental result of loss of light triggers; it is “programmed” at the genetic level).
As wevets notes, the jury is still out on a definitive cause. However, the 2005 study that I linked demonstrates that midline signalling genes (specifically, pax6, pax2 and hedgehog) can act as protagonists during development, and that all act on the developing eye in some fashion. If one signal is increased, eye development is altered dramatically. It does remain to be demonstrated conclusively which of the other adaptations I mentioned (and which are mentioned in the linked study) are affected by the same triggers; as the author notes:
However, as also noted, hypoetheses based on energy conservation and neutral mutations are not supported by the current research.