There are a number of species of blind cave-fish, all of which have non-functioning eyes. I find these of particular interest for evolutionary theory. How can the traditional evolutionary doctrine explain these fish?
I see only two possible hypotheses. The first is that eye-like structures can mutate into existence without having any adaptive value at all. This would be an expensive hypothesis for the theory, since it would allow for evolution even in the absence of survival benefit in the competition for continuing existence. In effect, making this hypothesis would be tantamount to giving up the doctrine of natural selection. Yet, in addition, if we accepted the idea that eye-like structures can mutate into existence without having any adaptive value at all, we would faced with the new question as to why that adaptive process did not produce fully seeing eyes. Why, it would have to be explained, did the process that produced the blind eyes stop short of fully seeing eyes?
The second, and more attractive, option is that these were at one time seeing fish who lost their ability to see through lack of use. The notion that leaps to the mind is that of atrophy. We all know the popular wisdom that if you don’t use it, you lose it, right? All that is required of evolutionary theory is that natural selection is taken to work backwards as well as forwards. This would mean that no longer adaptive features gradually cease to appear in the population since their utility is gone. Hmm.
This is promising. Sightedness once gave them an advantage, and thus all of them became sighted, the blind ones disappeared. Once in the caves, the sightedness lost its utility and … it disappeared? While the eye structure stayed? Why did it disappear? Were there still blind fish around? Let’s assume there were, why would their blindness be a sufficient advantage to make blindness the norm? And further, why is the eye structure still there? If the sightedness disappeared simply because it was no longer useful to survival, why wouldn’t the same be true of the eye structure? Why, for that matter, do we still have the appendix?
And if there were no blind fish around to compete in the dark, would sightedness still have disappeared (leaving the eye structure)? Why? How does natural selection explain this? But even if we use the atrophy argument instead of natural selection, there is still a problem. If you don’t exercise, yes, you will become increasingly weak and pudgy, but it does not follow that you will have weak and pudgy children as a result of your own weakness and pudginess. Yet, the fish who didn’t use their eyes not only lost their sight, but they bred children who were also blind! Jean-Baptiste Lamarck, these fish rehabilitate your theory! Acquired characteristics can be heritable!
I think you put your finger on some very important points here. Have you considered the possibility that maybe the development of the eyes occured alongside the development of something else, which was itself an advantageous mutation? The environmental factors that caused the eyes to form could have also caused superior gills to form. There need not be any necessary causation b/w the two effects, only correlation. Doesn't the theory of evolution posit that certain evolutionary steps occur simultaneously anyway? I admit I'm no expert, I just wanted to suggest a third alternative.
ReplyDeleteThanks, Asher, for your thoughtful comments. I can't quite see how the possibility you describe would address the puzzle. The issue, as I see it, is that of explaining the presence of eye structures (organs) in fish for whom those organs give no additional survival value. If it is natural selection that determines the presence or absence of biological features, then natural selection should be responsible for the presence of non-functioning eyes in these fish. Even if the eyes formed at the same time as useful gills, this still does not explain what caused them to happen.
ReplyDeleteSimplicius, I'm saying that there could be environment factor X, which causes biological byproducts P and Q to form on a given organism. P is advantageous, while Q is not (eyes with no sight). Natural selection in this case would be responsible for the non-functioning eyes, because the same process also caused something advantageous to occur. The reason it is natural selection is because all organisms with no eyes whatsoever also did not develop byproduct P, which was very advantageous.
ReplyDeleteI guess my overall point is that you have to take a species as a package to a certain extent. Many mutations may have happened at the same time, and if they did, there's no reason why each individual mutation is necessary for survival - all you need is one really good one in the bunch.
More likely than these theories is the case for non directed genetic drift. Instead of use it or lose it, a deletory mutation probably occurred in the lineage of cave fish (once they had moved to the caves). Being that the mutation was neither adaptive or non adaptive it would neither be selected for or against. However, natural selection is not the only way a population can change. Sometimes when there is no selective pressure a population can drift due to chance. These changes tend to occur very slowly and can drift back as well. It may even be that there is a higher probability of mutation at on of the genes determining cave fish sight, such that in the absence of positive selective pressure drift is more likely to occur.
ReplyDeleteThe argument that the whole eye would disappear does not really follow from the fact that sight disappeared if there was no negative selective pressure on sight.
Some people are born without appendices and it is possible that the frequency of those individuals will increase over time. On the other hand there is also a possibility that appendices still serve a small function in immune defense.
Jeff