scigirl

Do you have proof of that assertion?

scigirl

Vanderzyden

Scigirl,

That was discussed earlier in the discussion. If you really doubt that an animal that has suddenly become blind will not go extinct, then please go back and read (somewhere in the middle of the thread).


Thanks,

John

Doubting Didymus

I reread the appropriate section, and I think I see the problem.

Allow me to elucidate the evolutionary explanation a bit clearer than I did last time:

Before the fish's eyes began to reduce, the population of the fish was already living in the dark caves. Because of this, small mutations that stop the eyes from developing properly would have a net affect of zero, seeing as the fish is not using its eyes in the first place.

So, seeing fish moved into caves (to avoid predation, perhaps, or some other reason).

Once this habitat change had occured, mutations that debilitated the eyes no longer affected the survival of the fish as they once would have. In the light, any mutation that had this affect would have been screened out by natural selection. However, in the dark, such mutations no longer have a bad affect on the fish, as they couldn't see anyway.

Thus, the fish would not go extinct when it occurred.

Vanderzyden

DD,

As before, I will ask:

What should lead us to believe that sighted fish, who are forced into caves, could survive at all?

Living in the dark is equivalent to being blind. Being forced into darkness for life is equivalent to having the eyes removed. Surely you agree that sight is the primary sense. Without it, the fish could not find food, especially in a new environment. Without food, the first generation will die, immediately. Any progeny that survive in the dark cave will die, immediately. This hypothetical new species never gets the chance to "evolve".

A note in anticipation of your reply: if there is any light at all, then the fish can see. However, if there is no food to see, then these will, again, die immediately. If there is food and there is light, then, all else equall, these fish will continue on like their ancestors.


John

[ October 31, 2002: Message edited by: Vanderzyden ]</p>

Jesus Tap-Dancin' Christ

Been there, done that. Fish don't entirely rely on sight to find food, evade predators, etc. Sound works DAMN fine that way, especially since underwater, sound travels much better. Covered waaaaay in the beginning.

Doubting Didymus

For a start, sight may be the primary sense in us, but in water, olfactory senses are far more useful than on land. I do not know which sense is more important in fish, but I do know that they have very powerful noses.

That is a gross oversimplification. Blind animals in the wild do not immediately die, they are only at a disadvantage. Secondly, and as above, the fish depends heavily on the olfactory sense.

There are a few factors you are missing. First, there is a large benifit to cave life, which is that there are no predators there, and the fish can insert themselves at the top of the food chain. Secondly, while the fish may have some problems finding food, there is nothing to suggest that it would be impossible. Remember that the life in the cave would be both sightless themselves, and unused to dealing with larger predators. Third, the fish's food supply may be stationary or very slow, such as anemones, starfish, etc. Such food would be easy to locate with smell.

I will grant that the fish may be less well off than they would be outside, but it is quite possible for the fish to be forced into the new environment by other factors. Under changing conditions, it may become the case that the fish are actually better off inside the caves, smelling their way around for slow moving food, then they would be outside, where there are too many predators or not enough food, or some other factor.

Doubting Didymus

I think smell would be more important. Sharks, for instance, can smell blood from miles away. That's a pretty impressive nose. You might even say that the sharks nose is more important to it than its eyes.

Jesus Tap-Dancin' Christ

Yeah, I forgot about smell. Between the two, eyes really aren't terribly necessary, though they ARE helpful.

scigirl

Yes I have read this entire thread. My question still stands - do you have any proof of your assertion?

scigirl

Vanderzyden

DD,

The olfactory sense does not appear to be primary in fish with vision. Ask a fisherman:



We must consider land predators and water predators. Land predators may be avoided just as easily by avoiding the shoreline as they can by hiding in caves. Water predators would follow the fish into the caves. Each would be unable to see. The water predator threat is greatly diminished, but still remains. If the water predators could somehow accidentally eat a few fish, they would find that their food supply would eventually diminish to zero and the predators would go extinct (locally). Furthermore, the predator would be unlikely to be able to eat enough food to sustain itself properly. Meanwhile, the fish population would rapidly decrease because it could not see to find food, if there is any food to find. All of this seems to contribute to the rapid extinction of the fish and the water predators.

I have also raised the issue of progeny. First, there is the difficulty for the female in finding a proper place to lay the eggs. Second, there is an additional complication in the male being able to fertilize the eggs. Remember, we are dealing with fish that until just recently were heavily dependent upon their sight! Will they "smell" their way to a suitable location in this strange, dark environment? For the sake of argument, let's say that somehow that new fish are able to hatch. Here is yet another problem: How will these young fish survive in the dark? How will they find food?

All things considered, it seems like we have a case of poor conditions and insufficient time for a sighted fish to become eyeless.


John