There are many species of cave fish, salamandars and crayfish. It seems after a few generations these species lose their eyes (Convergent evolution). What this tells me is that there is an evolutionary advantage over having no eyes in a dark environment. Now, I understand that, of course, eyes are not a useful adaptation in such an environment, but that alone doesn’t explain why they natural selection would favor eyeless mutants. Why are eyed cave animals at an evolutionary disadvantage?
It’s not just that eyes are energetically expensive; they’re also entropically expensive. There are a lot of things that can happen to a working eye to turn it into a nonworking eye, but very few things that can happen to a nonworking eye to turn it into a working eye. So if there’s no advantage to eyes, creatures with eyes will evolve into creatures without eyes a lot more often than creatures without eyes will evolve into creatures with eyes.
Another explanation is that no-eyes is a more stable state; since the genes that are responsible for building an organ act in a set of more-or-less sequential stages, a random mutation that prevents the eye from forming properly in the first place will overrule any already-existing mutations that made the eye bigger, for example (technically, this effect is called epistasis). Since the odds of a mutation that exactly reverses the no-eye mutation is extremely unlikely, if there is no selection to preserve the eye it will eventually be lost, just through the accumulation of random drift. (The same argument goes for pretty much any non-selected trait.)
Put a different way, animals that mutated to lose, or have vestigial eyes, have an ecological advantage in cave environments, since they do not have to invest resources in building and maintaining functional eyes. So there’s a small but real selection pressure for losing eyes.
Non working is not the same as non existent. Eyes are disadvantageous in a lightless environment. Imagine living inside a pitch black cave but constantly getting dust or sand into your eyes.
But how does that work in a real world application? Will an identical, yet sight-challenged fish, get more food and reproduce better? What is the day to day advantage for being blind?
There are species of moths out there without a mouth. No mouth! They cannot eat anything. They spend all this time frolicking amongst the trees as a caterpillar, go into a cocoon, and emerge knowing that they’ve got about a week (at best) before they kick it. To me, that just seems strange.
And yet, I really think that knowing you’ve got about a week to live and your sole driving force is to go have sex as much as possible in that time would be a pretty good incentive. Plus, you wouldn’t waste all that humpin’ time on consumption. No “may I take you to dinner?” or even that post coitus glass of OJ. Just wham bam thank you moth.
That doesn’t answer my question at all. “Expensive” how on a day to day basis. Two fish are born, one is normal and the other has an eye defect. As they go about their day what disadvantage in getting food and reproducing does the sighted fish bear?
Let’s say it costs you 10 calories per day to maintain your eyes. That’s 3600 calories per year you must eat just to maintain your eyes, yet they provide no benefit in a cave. If food is scarce (and it always is in caves) that’s enough to be the difference between a healthy fish and a starving one… or between a starving fish and a dead one.
That’s ignoring the possibility of injury, infection, irritation, etc.
And to follow that up, now imagine the cave goes through a period of starvation - a flood comes through and washes out all the nutrients, or there’s a drought, or whatever. You’d expect the eyeless fish to survive the starvation period much better than fish burning calories to maintain those useless lumps.
I am sure they live a long and gourmandish life as larvae, and don’t see the imago portion of their lives as anything other than a way to generate eggs that hatch into more larvae.
This. That cost might come about as an aggregate of lots of factors like added brain power to process visual stimulus, scarcer vitamins and nutrients to prevent eye disease*, higher metabolism, parasitism, etc. Either way there is always a price to pay.
*obviously an animal with a sick organ, even if it’s an useless one, is at a disadvantage compared to a healthy one.
And if you don’t like the idea of the cost of running the day-to-day operation of the eye, think of a bunch of baby fish, still developing - some with eyes and some without - and go from there.
Of course, “few” is relative. But for many salamanders and crayfish, there are still related species that live nearby outside the caves. If it took “many” generations, then the relatives outside the cave would be more distantly related.
A vague memory here, but I recall reading that the genetic changes in a particular eyeless fish species resulted in developmental changes that did more than just eliminate the eyes, they resulted in increased development of nearby structures. Imagine a cat’s face, with eyes above whiskers. Instead of just removing the eyes, the mutation results in a cat with a band of whiskers twice as high as usual. This overwhiskered blind cat may have a survival advantage over ‘merely’ eyeless cats that goes beyond the caloric burden of maintaining eyes/whiskers because of the sensory advantages of extra whiskers in that environment.