Genetics and mutation...

I have been wondering for a long time- if you were to take a small creature that could normally not see in dark/dim light and could breed and reproduce quickly, and put it in a very dim enviroment until it reproduced, did the same with its young, what would happen over the generations? Would its great-great-great grandchildren have adapted to the dark and become blind, or have slowly grown larger eyes for better eyesight in dim light? I’m doing a research paper, and this is the subject for it.

If you’re doing a research paper your best bet would be to research the topic.

But I can tell you that whatever happens the answer will be primarily speculation and depend massively on what species and even what individuals your are starting with. Evolutionary changes of this type are the result of random mutations being selected for. If a random mutation leading to larger or smaller eyes never occurs then there is simply no way that the population as a whole can ever have significantly larger or smaller eyes. It’s that simple. And there is simply no way to predict what random changes are going to occur. That’s why they are random changes.

For one thing, it’s going to depend on how useful the ability to see is in this environment. If the test environment is an aquarium fully stocked with nutritious food sufficient for all the organisms, and with no predators, then better eyes aren’t going to carry much evolutionary advantage. If, on the other hand, the environment contains both food and poison chiefly distinguishable by their color, for instance, or predators one might be able to see and avoid, then better eyes will, all things being equal, carry an advantage.

There’s also the question of what the cost of better eyes would be. If the environment is truly completely dark (say, the depths of Mammoth Cave or Carlsbad Caverns), then it won’t matter how good your eyes are, or how useful sight would be, because you’re just not going to see anything, period. You might also, for instance, have an environment which isn’t quite that dark, but almost, such that for eyes to be useful, they’d have to be three times the size of the critter’s head. That’s unlikely to happen. Or, of course, you might have an environment which is only slightly dimmer than the creature’s original habitat, so the eyes only have to get a tiny bit more efficient to be effective. In that case, it’s much more likely that they’ll adapt.

I agree with the previous posting: do the darn research! I like SDMB but I’d hardly consider it authoritative! :rolleyes:

That said, I think the “laboratory” of the real world answers your question. Species that exist in complete darkness often have very poor to no eyesight. Moles are a good example of very poor eyesight; mole rats and species discovered in caves have none. A biologist might give the reason why. I only know that this happens to animals that are mostly in complete darkness.

On the other hand, species that operate in very low light, like nocturnal mammals, often develop specialization for low light. Many nocturnal animals have developed a mirror-like layer on the back of the eye, which helps improve the amount of light they receive. That’s the gleam you see in flash pictures of animals like cats. Or, look at lemurs (the most famous example of which is on the cover of the O’Reilly Learning vi book) and their oversized eyes. Nocturnal animals with keen eyesight certainly have an advantage since there’s enough light for photoreceptive chemicals to work, if the physical mechanism is adapted.

Just as a thought, I guess that human eyesight (nearsightedness aside) is ideally adapted for the sun/shade mixture found in sparse forests.

If the poor beasties need to be able to function in the dark, then the Intelligent Designer will force a change to allow them to do so. Weren’t any of you people educated in Kansas? :rolleyes:

Unfortunately for the OP, this kind of research is nearly impossible to conduct. We really do have to look across the natural world for these types of thing to occur.

Quite a lot of evo-devo genetics research has been done on the several lines of blind cavefish, Astynax. There are several different Astynax, including ones with sight and ones with intermediate sight and ones with no sight and no eyes. Lines of fish which move to caves seem to reasonably rapidly lose their sight. Interestingly, the mutations that do this seem to be in the same handful of genes (including the homolog of one that I did a PhD on).

There is no easy answer to your question. Vision is a pretty complicated thing, and if you take away the necessity for maintaining vision, like in cave-dwelling Astynax, the functional pressure on the genes necessary for developing eyes goes to nothing. Random mutations accumulate over generations and are not selected against.

That’s not to say this is what is going to happen in every case. For species that require vision, and are not moving into totally lightless environments, and will have huge fitness hits from losing their vision in their new environment, and they have enough genetic diversity in the population to allow population variability to emerge at a level required for selection, then you can expect to see some improvement in vision. Very good examples of this are seen throughout the animal world, comparing similarly dized diurnal and nocturnal species.