I’ve seen some documentaries of cave dwelling creatures that have evolved in some interesting ways, including in many instances, the loss of the eyes. The reasoning goes, that if these animals can’t see in a life of darkness, the eyes become useless and over time, the eyes are lost from the genetic code.
The other thing that seems to be lost is pigment; many of the creatures are albino, which makes no sense to me. Since blindness removes the ability for a predator to tell the color of their prey therenmust be another reason why these things lose their color.
So can anyone tell me why darkness can cause pig,met loss in creatures? Is color a trait that requires energy from the sun to appear? Or is there something else? Many of the fish in the deep ocean are well below anywhere sun can have a direct input, and yet most of them have pigment od some color,
Pigment in most cases is used as camouflage or to attract mates. Animals with pigment would have an advantage. In a dark cave with no light there is no advantage so possibly albinoism may be dominant. I believe also that albinos have a higher sensitivity to temperature and touch. This may be enough of an advantage to encourage the gene to dominate.
IAMNAEB, but these two factors are involved:
a) Pigment is an optional extra. That means it costs more. When it’s not needed, organisms that avoid that cost due to random mutations have an edge, not much of one, but enough for that trait to become dominant over time.
b) All traits are subject to genetic erosion due to random mutation. Traits that are useful are preserved in spite of this because mutated individuals are at a distinct disadvantage. Traits that aren’t useful on the other hand will become less and less common over time, even without being selected for.
I’ve asked a similar question before. The answer you’ll get here is it is biologically expensive. In a big picture way I understand that that’s true, but I have trouble understanding it on a case by case basis. Let’s say two fish are born at the same time in the same cave. One is born with a defect - no (or non-operative) eyes. What set of circumstances would occur that would cause the blind fish to have an advantage and live to procreate and not the sighted fish? Especially enough times that it becomes the dominant trait.
Two possible situations: 1)Starvation. Each organ takes energy (i.e., calories) to build and sustain. If there is limited food, the fish with fewer organs is more likely to survive. I would assume this is especially true when the fish is developing from an embryo. An individual eyeless fish might not be much more likely to survive that its sighted brother, but it might produce 1001 eyeless offspring over several generations (or even 1001 sperm without the genes for eyes) compared to 1000. Over time that creates a change in the population.
2)Chance. Maybe the fish with eyes just happens to be closer to the bigger fish that’s hungry. In an environment where the fish can see, the benefit of eyes will, over time, hugely outweigh the vagaries of chance, but where there is no advantage to having eyes, that doesn’t happen. So there is more chance for random genetic drift when a trait is under no competitive pressure.
If your eyes are not being used for seeing, then they are simply weak spots that can get scratched and infected.
I’d expect cave dwelling fish to evolve thicker skin on the eye surface and over time lose more and more soft tissue of the eye itself.
I also suspect that blind cave fish that have apparently “finished” this process would still have some remains of eye structures because the selection pressures would not be sufficient to remove them completely (well, not in mere millions of years timeframes).
Pigmentation OTOH is interesting as it’s not that clear what the significant advantage of being albino is. Sure there’s an energy cost in making pigment but the difference would have to be pretty big to account for how quickly cave-dwelling species lose their colour.
The blind fish doesn’t have an advantage is has equal fitness to the sighted fish. A mutation is like a wrench in the works, the more complicated the works, the more likely the mutation will make it work less well. In the sighted world, most mutations hamper vision and are selected against, while the rare improvement will be selected for. In the cave, without this pressure, every mutation of the eye has an equal chance to spread, most of them make it less good of an eye, and eventually, no individual is born with and uncorrupted bluebrint to build a working eye, and the species is blind.
I think it can reasonably be argued that the blind fish has distinct advantages vs the sighted one, in a completely lightless environment - eyes are a burden and a liability - The growth and maintenance of eyes has an energy cost - and they’re prone to infection and physical damage (the latter especially, in a dark place).
I don’t think it needs to be a big difference. Cave environments are extremely limited in food sources. If fish A can survive 30 days without food and fish B can survive 31 days, then not only will fish B survive some hardships that A won’t, but B is going to live just long enough to eat the dead A.
In an environment with more food, tiny difference in metabolism will still translate to slightly faster growth rates - which means more offspring sooner in many species.
Although it makes some sense to me intuitively, have there been any experiments to show that something like pigment can be a trait requiring energy (or a “nice to have”)?
To me, it could be much simpler. Like an on/off switch gene. Where the organism has a color (on) or albino (off). Outside of that decision, what does a body need to maintain the color gene? If it is grabbing its color from what it eats, like the pink flamingos and shrimp, there is no extra energy for the organism to spend. If a salamander was always blue, however, and after a long evolutionary timeframe in the dark transforms into white, there has to be some external pull on the color gene to flip it to white. (white would have a slight advantage because it required no energy to produce or maintain).
I guess I’m a bit skeptical on albinism as being a competitively advantageous trait to have in total darkness.
As far as eye loss in cave fish is concerned, in at least one case (PDF), eye loss was because a mutation in a single gene (“sonic hedgehog”) resulted in an advantage in the fish’s jaws and taste buds but also resulted in loss of the use of their eyes.
So it’s possible that loss of pigmentation is because that there is a gene that affects both pigmentation and other features of the organism, and a mutation provides some efficiency in the other features but eliminates pigmentation.
I don’t think that it’s that albinism is competively advantageous but that turning stuff into pigments to color an organism’s exterior is work. There has to be a process for that to happen, there has to be info in the DNA saying make these proteins that form these chemicals that trigger these receptors and so on. If that process gets interrupted somewhere, part of the dna that controls the protein makeup or the receptor or something gets accidentally turned off, then the rest of the process can work just fine but no color change happens. If there isn’t pressure to have that part turned back on, then the organism just keeps on trucking. Since an error in many different parts could all have the result of no skin pigmentation, the population slowly turns albino in a similar way but through various breakdowns in the process. Perhaps I’m wrong but that’s what makes the most sense to me.
Found a recent blog postabout cave fish on different continents being found to be related. This does not speak to how they evolved, but to how long ago.
The figures were taken from the PLoS ONE article,here.
I think you’re over-complicating things here. Consider that destructive mutations are far more common than beneficial ones. To give a simple analogy, there’s a lot of ways to hit your car engine with a sledge hammer and break it, there’s very few ways to hit it with one and happen to knock in a way to make it ever so slightly more efficient.
In a sighted world, all the mutations that would make the eyes less effective will have some selective pressure to prevent them from spreading and the very few beneficial mutations will have selective pressure in favor of spreading. Thus, in the sighted world, if a fish is born blind, it is far less likely to live long enough to produce offspring. Thus that mutation will likely die off.
In your scenario, there is no pressure selecting against the blind fish, so both fish will produce offspring in roughly similar numbers. Yes, there may be some other small benefits associated with loss of vision including like fewer calories or less vulnerability, but I think those play a much smaller part to simple fact that without selective pressure in favor of features, those destructive mutations by virtue of being much more common will dominate and the eyes will quickly become non-functional or even disappear. When you add in that there may be some minor pressures in favor of those losses, it should be little surprise that it happens.
It would work much the same way with pigmentation. There’s selective pressures in favor of pigmentation in a sighted world, but the selective pressures as non-existent in an unsighted world.
Is it possible that in many cases blind or albino fish and other animals simply migrated deeper into the caves as the enviroment agreed with them more? They very well may have been mutants spawned from normal animals close to cave entrances. Once inside they simply bred with each other and likely inbreeding would fix the gene for future generations.
But albinism simply means unable to produce pigment. Since that is usually a multistep process, going from raw food nutrients to finished appropriate pigment, any failure in the chain produces less, wrong, or no pigment. That can have detrimental effects in the sighted world - susceptibility to sunburn etc., daytime blindness, less camoflage against predators.
Also, very important - in most breeding, if females select the mate, they tend to select for what looks healthy; often males do too (except maybe humans). Bad colouring will tend to make them less likely to select a defective mate. When trysts happen in the dark, they are less likely to notice if the mate would otherwise look unappealing.
After that, it’s a matter that failure to eliminate the bad gene(s) means the trait becomes common, and new instances of it pop up by random mutation to add to the mix.
If a population has been isolated for hundred or thousands of years, that could mean an awful lot of generations to allow the condition to spread.
Fair to say, but I’m assuming, at least for this thread, that most species loose sight or flight or whatever gradually. I’m willing to bet that long before the advantage of having skin where you used to have an eye is apparent, they’ve already collected enough errors in the DNA to be blind.