The entire developmental apparatus for eye development remains in place. It’s not a matter of “imperfect efficiency”, it’s a matter of no energy savings whatsoever. If energy is not conserved, then it’s rather pointless to argue that eye loss or degeneration is a direct function of their being selected against as a result of energy concerns.
That’s not what it says though. It’s a very short lived event:
p3
"Proliferating cells can be detected… **throughout the period of eye growth ** although the retina does not markedly increase in size during this period.
Not only that but fish with no eyes had other tissue savings aside from the retina
p5
"After obtaining a surface fish lens, the Pachon cavefish eye reversed its fate and began to grow and develop (Yamamoto and Jeffery, 2000). Eventually, the cornea and iris appeared, which are normally missing in cavefish, and the retina enlarged and became more organized. Further growth resulted in the presence of a highly developed eye containing all of the expected eye tissues, including the cornea, iris, and photoreceptor cells, in the adult Pachon cavefish host "
So by removing the eye the fish doesn’t have to waste energy on eye growth, it doesn’t need to waste energy constructing or maintaining the iris or the photoreceptor cells. It doesn’t waste energy constructing or maintaining cornea and presumably all the defenses that go with that. Yes it wastes just as much, but no more, energy with the development of retinal cells, but it saves in not needing to maintain the retina after the growth period.
Yet despite acknowledging that the eyes in blind fish are smaller and less organised with no wastage for photoreceptors, cornea etc. the authors seem to have concluded that there is no net energy saving because the retinal cells keep growing. If there is some reason for this startlingly conclusion the authors haven’t given it in that paper.
As I said, it seems like a total non-sequitur.
Ooooh. I never thought of that. That is a good point.
Medieval people, on average, were only an inch or two shorter than modern populations living in the same area. The myth that medieval people were stunted dwarfs is just that, a myth.
Since it would constitute a hijack of this thread to continue this discussion here, I’ll just say “I disagree”.
Then why, as I’ve always heard, were people in the time of the Civil War so short that anyone six feet tall or taller was an aberration?
Valete,
Vox Imperatoris
I would say that a trait is evolutionarily expensive if keeping it brings with it associated disadvantages, but the benefit of having it overcomes the disadvantages.
I think sexual selection characters make for great examples of this. Think of a peacock’s tail. It is inconvenient, energy expensive, makes you easier to catch and all around unhappy. It gets you laid, though and that makes it worthwhile.
To go along with the eyes, they are hard to make, expensive to operate and a pain to keep healthy, but putting up with that is better than going blind.
It’s not, it’s just that the picture has become confused.
First off there was never Medieval height. Heights in the in middle ages wandered all around, and was always higher for nobles than for peasants.
Secondly there is no modern height. There was a serious decrease in height during the industrial revolution, 3 or 4 inches as far as we can tell, followed by a rapid increase in the 20th century.
Finally the modern population has become meaningless because of immigration. The modern population of London or Paris contains as many Indians, Jamaicans and Chinese as it does Anglo-Saxons. That makes it very difficult to do a direct comparison.
The best studies I’ve seen suggest that the modern indigenous descended populations of Western Europe are about 4" taller than the lowest Medieval heights, and about 2" above the tallest medieval heights.
Because it’s true, but the American Civil War was fought somewhat after the middle ages. It was at the tail end of the Industrial Revolution, the absolute shortest period in human history s far as we can tell.
Here’s an interesting picture - Lincoln, in the middle (duh), was somewhere between 6’2" and 6’4". Them is some tiny little men. (I believe the average height for a Civil War soldier was around 5’7". That’s my height. I can’t imagine being an averagely-tall man and looking around at all the teeny tiny women.)
It seems to me to be very on-topic, and I’m interested.
To me, it seems plausible that even a partially-developed eye would have some resource savings over a fully-developed eye, and so selection pressure could easily fix in the population a mutation that leads to abandoning eye development at some point (while clearly, the earlier eye development is abandoned, the more resource savings, developmental pathways don’t always leave easy opportunities).
Since you’re usually very good on evolution issues, I must be missing something from your argument: can you explain?
I considered it a hijack because the thread is about the overall meaning of “evolutionarily expensive”, rather than the selective pressures which resulted in cave fish blindness. There’s no doubt that eyes are “evolutionarily expensive”, for example.
However, in order for them to be lost as a direct result of limited nutrients (which, really, is what we’d be talking about: “too expensive to maintain” means there aren’t enough nutrients available in the environment to support them in their given form – said nutrients would be put to better use elsewhere – and that said nutrient-poor environment imposes a direct selection pressure toward their loss), one would reasonably expect the developmental pathway to be shut down at an early stage - that is, simply do not develop the eyes. Energy can thus be spent developing other structures which would be of greater benefit.
However, said fish do in fact, develop eyes. As shown in the paper I linked to, the eye begins to develop normally, then switches to an apoptotic pathway in which new cells continue to generate but then die, while other cells surrounding the eye region continue to generate as well. Thus, it’s essentially a case of the rest of the fish growing faster than the eyes (rather than instead of), and eventually the eye remnants are covered over by the surrounding tissues. In terms of energy savings, it makes little sense to generate cells that will only die off before they are ever of any use. Note that eye development is never actually “abandoned”, it’s just overtaken by the development of surrounding tissues. Thus, there would seem to be little, if any, actual energy savings to be had.
I see no logical fault, therefore, in the authors’ conclusion that selective pressures related specifically to energy savings are unlikely to have been the impetus for the evolution of sightlessness in cave fish.
For what it’s worth, noted bioloigists such as PZ Meyers agree with this assessment.
This post is not supposed to imply I have any knowledge here, just questions regarding logic:
Does it have to be nutrient poor? If more energy is required to support X cells than X-Y cells, then it means more energy is spent acquiring the additional energy than if Y cells were not there. So it seems like, all things being equal, there is an energy advantage to shed Y cells.
I don’t know if the eye cells take up any extra energy compared to whatever replaced them, but it seems like good logic if the eye cells do use up more energy than their replacements.
Here it seems that a circuitous path could reasonably be taken to losing attributes, couldn’t it? It seems like it wouldn’t necessarily always be the most logical or efficient path, as long as there was some gain.
If it’s the first step in the overall evolutionary process, can it be ruled out for not being the most logical first step?
Finch – I haven’t read much more than the last paragraph of the paper and the PZ Meyers link. Particularly with the developmental details he provides, I do find the plieotropy explanation plausible, and PZ knows fish development far far better than I do, so he’s probably right.
However, I agree with what Raftpeople is saying, in that, without knowing any details of the organisms development, it’s logically possible that selective pressure to conserve resources is a possible cause even if the resource conservation isn’t perfectly efficient. Evolution is never perfect, and usually not even very good the first time around; given that we know that these fish are virtually the same species as other populations on the surface, they probably haven’t been genetically isolated for very long, and so we’d expect most adaptations to the cave environment to be imperfect.
That’s not to say evolution is the necessary explanation (as I said, it appears likely it’s not), but it’s still logically possible.
First off, we’re talking about a fixed “energy pool” for the developing embryo. Cave pools tend to be somewhat light on the nutrients (and, let’s face it: if cave pools aren’t nutrient poor, then energy savings are a bit pointless and the “energy conservation” argument falls apart from the start), so the energy available both to a developing embryo and the subsequent juveniles is limited.
Second, at the stages of development where all this is happening, one cell is pretty much the same as another cell in terms of energy requirements. That is, nothing is actually functioning in it’s eventual role yet, they’re just differentiating into their eventual final forms.
So, given that the energy budget is fixed, and that growing cells whose only “purpose” is to have no purpose and die takes energy from the fixed budget, and that other cells are still growing and eventually crowding out those dying cells, there is no energy advantage to be had that I can see. To argue for energy savings as an adaptation unto itself, one has to show that energy is, after all, being saved, versus a sighted fish.
Again, though, the hypothesis that is being discounted here is not that energy is saved, but that cave fish are blind because it is metabolically advantageous for them to lose their eyes. That is, that energy expenditures in eyeless fish provide a definite and direct selective benefit versus sighted fish which might find themselves in the same environment.
But, again, there is no evidence of actual energy savings at all. Cells grow and die, instead of not growing at all. The “dying” part is not because it is efficient to do so, but rather because other structures which would normally enhance their continued growth are absent. The absence is not due to any particular loss of genetic function, but the result of gene signaling changes which enhance other traits while the eyes are left to degenerate. And, again, while it is necessarily true that energy is thus being spent elsewhere, that does not argue for such energy conservation being the driving force behind eye loss. Rather, eye loss is the result of other developmental pathways being actively selected for (e.g., an enhanced feeding apparatus is selected for, thus resulting in smaller/no eyes as a side effect of altered developmental pathways, rather than eye loss being an advantageous trait being selected for unto itself).
This assumes facts not in evidence: that the current condition is a “first step”, rather than the current end result of several previous steps being chief among them. All populations are in a state of evolutionary flux, so there is no real “first step” to be had.
Consider the following thought experiment: a population of sighted fish are transplanted to a cave pool, and forced to compete with sightless fishes for food. Which of the following explanations is most likely for the almost-certain demise of the sighted fish:
a) The sightless fish have a lower energy budget because they lack eyes, and thus have an advantage over their sighted competitors.
b) The sightless fish have numerous adaptations which allow them to function in a lightless environment, which makes them many times more efficient in finding food and mates than their transplanted rivals.
The energy conservation hypothesis would require that individuals with smaller eyes would outcompete their brethren on the basis of having to spend less energy for eye development and maintenance, leading to eventual eye loss. The research indicates that other traits are being positively selected for, but, because developmental pathways for those traits are linked to eye development, eye growth suffers at the expense of those other traits. It is, perhaps, a subtle difference, but it is an important one, nevertheless.
Understand that I am not saying that energy expenditures are never selected for. I am saying that, based on the current research on cave fish, it is unlikely that energy expenditures are the driving force behind the eye loss found in various species of cave fish. Rather, eye loss is a pleiotropic effect of other traits being positively selected for.
And, again, I’m not arguing for perfection. I (and PZ Meyers, and the authors of the paper I linked to, of course) am arguing against a particular causal force for cave fish eye loss.
The eye loss is almost certainly the result of evolution by natural selection. But, what has been selected for is not the eye loss in and of itself. Other cases of degeneration may well be the direct result of energy costs. It simply does not seem to be the case for cave fish.
Traits that aren’t necessary, even if they are not costing the creature anything, will fade away if there’s nothing that reinforces their need.
Take domestic animals. Domestic cattle have very poor eyesight compared to their wild cousins. Farmers were not breeding cattle to have poor eyesight, they were breeding them to be big, disease resistant, and easy to manage - but since there is no evolutionary pressure for cattle to have good eyesight, the trait has been disappearing - cows with genes that gave them worse than average vision bred just as well as cows with genes with better than average vision, and since detrimental mutations are far more common than ones with a benefit, they brought the average down. In the wild, the ones with worse than average vision would have been more likely to die before passing on their genes.
This is probably why there are eyeless fish in caves.
Brain size != intelligence
Lest you believe a blue whale is smarter than you and there are no midgets in Mensa.
I see your point, and based on the wording would choose B as first priority, but wouldn’t both be happening simultaneously? Or is there data in the paper (I didn’t read it) that indicates the ratio of gains between B and A is so weighted towards B that A can be generally disregarded in this case?
if so, that certainly makes sense, I guess the trick then with questions like this is to quantify the advantage one attribute has over others?