Every now and then I read an article about a new variety of fish that has been discovered living in a pool of water in a cave so far underground that it is pitch black, without a trace of light. Invariably the fish are described as blind. Why? I don’t imagine these fish evolved from the primeval ooze down there in the cave. I am sure they are the descendants of otherwise normal fish that were somehow trapped in the cave during a geological episode of some kind. In any case, these original cave-dwelling fish were almost certainly sighted. Why aren’t their descendants? What is there evolutionary desireable about being blind that resulted in evolutionary changes ending up with what we find today? Why don’t we just find fish with perfectly good eyes who just can’t see because it is so dark?
Random mutations happen all the time. If a random mutation has a large negative impact on survival it tends to result in the death of the organism so it doesn’t get passed on. However a mutation that doesn’t affect survival one way or another will be.
A random mutation that degrades the functioning of the eye has a large negative on survival rates in a lighted environment. But it doesn’t affect survival rates at all in a cave, so it tends to get passed on. It’s possible of course that a future mutation could reverse the first one, but it’s even more likely that more negative mutations will occur instead. Gradually the negative mutations pile up until the fish are completely blind or even eyeless.
It may also be that eyes are more energy costly to the body than just skin. In which case an eye that can’t see because there is no light is actually a disadvantage to teh organism.
That’s what I’ve always figured–eyes are a relatively big energy investment to create and to maintain. When they’re useless, the creatures with no eyes can spend that much more energy finding food and making babies.
I’m not buying this explaination. Admittedly, it is one step beyond my girlfriend’s Lamarckism theory of “If you don’t use it, you lose it.” theory when we discussed it during Planet Earth. There must be an evolutionary benefit to being sightless in the dark.
Let’s go with another example: why do animals tend to albinism in totally dark environments. I saw a study a few months ago (sorry, can’t think of the reference right now) that discussed how much energy the body used to make color. The point was that not only is color useless in a pitch black environment, you can save energy by NOT having color which is an evolutionary advantage in an energy-poor environment.
Nevertheless, this is quite convincing to me, although I’d state it in the opposite form – maintaining a functioning eye is useful in a lighted world, but if you’re in an unlit environment there’s no advantage at all, and the mechanisms that tend to maintain functioning eyes in the lighted world (creatures who can’t see well, or at all, don’t last long at all, and so can’t reproduce) don’t operate in the dark at all. So all the things that have to be kept “switched on” aren’t, and the things necessary to keep a functioning eye functioning aren’t selected for. Things that would go into building the eye are better used elsewhere, and perhaps the eye as a route for damage and infections eveb selects against large, useless eyes.
You’re right that “Use it or lose it” isn’t one of the rules of Darwinian evolution (I don’t think that it’s one of Lamarck’s, either), but it seems to be a consequence of the rules of natural selection, sexual selection, and random mutation.
If by “not satisfactory” you mean the additional push of other factors (better use of bodily resources, advantage of minimizing injury and disease routes), then I agree with you. But those operate all the time, anyway. Othrwise bodies would be bristling with the detriyus of unused but mutated parts. People don’t have unused gamma-ray detector parts sticking out of their bodies.
Not necessarily. Read about genetic drift.
I suspect that genetic drift plays a greater role in producing blind cave fish than the tiny selective pressure caused by the energy savings from eliminating a minor unused organ. The eye is a complicated structure and there are lots of ways that mutations can screw up its development. If they’re not selected against, those mutations should start to pile up fairly rapidly.
I think eyes are very expensive in the eyes of evolution.
When they serve a useful function, they also return a great deal on that investment.
Eyes are very vulnerable to injury and sickness. In humans, for instance, the commonest pathway leading to upper respiratory infections is hands rubbing the eyes, because they itch and seem to need a lot of maintenance. The hands pass germs into the eyes, which drain them into the nasal packages. At least, I read that somewhere a few years ago.
If you don’t believe eyes represent a big vulnerability, think - if there was an explosion somewhere near you, what would your hands do before you had time to think?
Are there any songs titled “Smoke Gets In Your ___” about something other than eyes?
Look at an infrared picture of almost any animal, and you will see a higher rate of heat loss on the eyes than anyplace else.
And eyes also get served by a big dedicated section of the brain. In fact, many say that the optic nerve and retina are more truly a part of the brain than they are sensory organs unto themselves. Brains, of course, are also very expensive to run.
Oh sure, genetic drift, random mutations, that’s just what the Stonecutters want you to believe. Fools!
As far as the eyes are concerned, it’s not only about energy. Eyes, by themselves are useless: you need neural circuitry to go with them. Neurons not occupied by processing useless input from the eyes can be put to use for other tasks, offering a definite advantage.
Indeed, cave versions of assorted animals are descended from “surface” versions. The change from sighted to sightless is a gradual one, as one might expect (thus, we see various levels of “sightlessness” in various populations).
As for why sightlessness at all? That’s a good question. Loss or reduction of traits is commonly known as “regressive evolution”, and is a topic which still sparks debate. Darwin believed that regression was not due to natural selection, but rather disuse. Other researchers argue for natural selection, others for genetic drift, and others for entropy considerations. No consensus has been reached to date as to the exact mechanism of regression (and it very likely depends on the trait being discussed anyway), so in reality, there is no “factual answer” that can be given at this time.
I, for one, am not wholly convinced by the “energy savings, therefore natural selection” arguments. Many cave dwellers may well lose or reduce their sense of sight, but they make up for it by honing other senses, each of which require additional energy expenditures over those of their “normal”, sighted relatives. Among arthropods, for example, it is not uncommon to find elongated antennae and/or legs to better act as “feelers” in the dark. Clearly, these enlarged organs require energy above and beyond that of the non-sighted version, and the net energy savings for loss of sight vs. (e.g.) increased limb development is probably not all that great, if even present. Of course, the “enhancements” which generally arise to make up for the loss of sight are typically readily explained via natural selection.
Further, if it was all about saving energy, then overall body size reduction would seem to be the way to go. Yet dwarfism is not a typical trait of cave dwellers (though reduced metabolism as a consequence of limited food sources, is). Furthermore, the developmental precursors to, for example, eyes, typically remain in place, even among the truly sightless forms. Many cavefish, for example, begin to develop eyes, but that development is then halted and “paved over” by other tissues (the developmental mechanisms are a bit better known, but it is still not fully known whether the ultimate cause for the developmental rearrangement is due to natural selection or not).
The fact that the so-called “troglomorphic suite” of traits is consistant across so many cave dwellers, though, certainly indicates that something is at work, and it probably isn’t just genetic drift (though that may be the case for some traits, like pigmentation). We (a general “we”…I’m not actually involved in any of this research) just aren’t entirely sure what “it” is, yet.
The thing I don’t get about evolution and/or natural selection is that its mechanisms are always explained in sort of passive terms (for lack of a better word?). A creature may or may not have an utterly random mutation that may or may not benefit it, and if the mutation happens to be beneficial then the creature has a slight edge in surviving and passing down its genes.
Which seems OK as far as it goes, but goes against my sense that nature must be more dynamic than that to fill ecological niches whereever it can, including environments so hostile that logically there shouldn’t be life there at all–if it only relied on passive mechanisms for its propigation.
And in our discourse we tend to use the active voice to talk as if nature had some active role in favoring or disfavoring certain traits–but that would be just a fantasy, not even a good metaphor, if only a passive form of natural selection were at work.
I think the bolded above is crucial here - it is common sense, based on the length of our puny human lifetimes and experience of manufactured items which tells us instinctively that there must be an active will behind evolutionary development. If it was a bit easier to conceive of millions of years of trial and error and vast mutating populations, evolution would no doubt be easier to intellectually stomach.
Pls don’t put hyperlinks in my mouth. I’m not suggesting the “watchmaker analogy”, but just wondering whether there is some other mechanism at work aside from random mutations.
Nature may be pretty dynamic, but that doesn’t mean it has filled every ecological niche. One of the most famous debates in ecology essentially boils down to: Why is the world green? and the contradictory arguement many ecologists responded with: Why isn’t the world more green? (Here’s one of the papers that kicked off that debate. It should not be taken as a given that niches are efficiently filled or that we even have a good metric for determining at what efficiency unfilled niches are filled.
It is just a fantasy and not a good metaphor to use the active voice when describing evolution, because it implies intent. Therefore scientists use the passive voice.
I’m not trying to insert anything into your mouth - I was just pointing out that common sense doesn’t always tally with reality, especially if the scale of the reality is so vast as to be beyond the everyday human experience.
Well, this opens up a-whole-'nother can of worms. There is much debate as to whether niches exists independently of the organisms which fill them, or if the concept of “niche” only has meaning when when coupled with those who occupy them (see, for example, the wikipedia entry for “vacant niche”).
Random mutations is not a mechanism unto itself. Rather, random mutations (which may or may not be so random after all) provide the “pool” of available traits from which selection “chooses”. The mechanism itself is, of course, natural selection.
Evolution is cool, because it’s all about sex and death.
Sex provides the pool of genetic diversity (asexual reproduction and genetic drift do too, but not so rapidly) and death (well, death before sexually reproducing, anyway) provides the cutting edge of natural selection. Natural selection produces change by a winnowing process more remorseless than any human tyrant.
Evolutionary change requires death. Death is the engine, sex is the fuel. We are the result.
Sailboat
One thing to remember about evolution is that it doesn’t just cause things to change, it also causes things to stay the same, if they’re already well-suited to their environment. The cockroaches of today are almost identical, so far as we can tell, to the cockroaches of a hundred million years ago: They’ve spent the past 100 million years running an evolutionary Red Queen’s Race, running as hard as they can to stay exactly where they are. Roaches are extremely well-suited for their niche, so if a roach is hatched which is slightly different in some way from the rest of the roaches, it’s almost invariably inferior, and it (or its descendants) gets outcompeted.
Likewise, most random changes which happen to a creature’s eyes would probably make them worse eyes. In a sunlit world, a creature born with worse eyes will be at a disadvantage, and will therefore be outcompeted by its better-seeing colleagues. So on the surface, evolution is running to keep eyes around, because they’re so useful. But underground, when a creature is born with worse eyesight, it doesn’t matter: On average, it’ll be just as fit as any other creature of its kind, so on average, it’ll have just as many descendants as any other creature of its kind, and the poor-vision gene it has will persist in an approximately constant proportion in the gene pool. Eventually, some other creature is born blind, and adds its blind gene to the pool, too, and so on. Occasionally, some creature will be born with better sight, but there are a lot more ways to break an eye than there are to fix one, so this will be very rare. So eventually, the various blindness genes come to dominate the gene pool, and all of the critters are blind.
Nope. Evolution can happen without death. Even if no organism dies, the organisms which produce more offspring will tend to dominate the population.
And, of course, I’m sure you’re aware of the many organisms (probably most) that reproduce non-sexually.