The loss of sight in cave dwelling species is widely known. We presume that since sight in utter darkness has no fitness value, the mutation of a gene critical to the development of the sense of sight is not selected against. Over time, any population living in darkness will eventually experience experience such mutations, and these mutations can reach fixation.
Astyanax mexicanus: Top is the surface, sighted form, bottom is the cave-dwelling, blind form. From the Jeffery Lab.Beyond this, we may hypothesize that a mutation “turning off” sight could be beneficial. By definition, an adaptation (such as sight) has a cost. When a trait that is adaptive is no longer adaptive, individuals with that trait “turned off” should experience an increase in fitness. It may also be the case, however, that such an increase in fitness is so small that it may be irrelevant. This line of thinking needs further investigation and what one finds in such an investigation may vary a lot from system to system. For example, a mutation that simply causes a particular protein to no longer be produced in what would have been a small quantity would save the individual with that mutation the use of a few tens of thousands of amino acids over some fixed period of time. This would have very little fitness value. But if a system is exploitable by a pathogen … such as a receptor site on a cell used by a common virus … turning that gene off may have enormous benefits. But this is a bit of a digression from the research at hand.
Borowsky, in his paper “Restoring sight in blind cavefish,” provides a test case for how we think evolution works. In Mexico, the species Astyanax mexicanus, is known to exist in 29 distinct populations. Genetic studies indicate that the turning off of the sense of sight in these fish has involved a deleterious (as in loss of function) of genes in at least three different lineages, or to put it a different way, sightlessness has evolved three or more separate times in these Mexican blind cavefish.
When Borowsky cross breeds some of these cavefish, crossing them between these populations, he gets a certain percentage of fish that have functional, if not fully developed, eyes.
This should not be at all surprising. Several different genes are involved in the development of sight, so by cross breeding strains that have experienced mutations in different genes, one would expect a certain number of offspring to have a set of functioning genes sufficient to make the sense of sight develop at least to some extent. When Borowsky breeds the blind cavefish with the non-blind version of this fish (“surface fish”) he gets restoration of the sense of sight in all of the offspring.
F1 hybrids between surface fish and cave fish have smaller eyes than surface fish, but are fully visual, even into adulthood … Thus, one surface allele at each of the population-specific eye loci is sufficient for restoring vision.
This is also expected, although not necessarily inevitable (This depends on the dosage required for each genetically coded step in the development and function of sight).
It seems to me that one could test the hypothesis mentioned above that turning off any fitness-free gene is adaptive. If simple production of unused proteins is costly, the rate at which particular genes are found to be turned off should be correlated with that cost. Perhaps the genes coding for longer proteins, or proteins that are produced more often in a particular system, should be more likely turned off. Or, some measure of the total mass of amino acids turned into proteins when a gene functions, should be correlated to the likelihood of having a gene turned off. At a most basic level, one would need to show that the mutant genes are in fact turned off and are not simply producing a non-functional protein.
In short, this study (and others by this and other research teams) demonstrates in empirical reality what is expected from commonly held evolutionary theory. Creationists often cite blind cave dwelling organisms as evidence against evolution, because, they say, it is “devolution.” This point of view is absurd, and relies on a teleological view of, in this case, teleost (bony fish) evolution.
Darwin wrote about cave blindness and disuse, and through various observations notes the potential complexity oif the problem:
It is well known that several animals, belonging to the most different classes, which inhabit the caves of Styria and of Kentucky, are blind. In some of the crabs the foot-stalk for the eye remains, though the eye is gone; the stand for the telescope is there, though the telescope with its glasses has been lost. As it is difficult to imagine that eyes, though useless, could be in any way injurious to animals living in darkness, I attribute their loss wholly to disuse. In one of the blind animals, namely, the cave-rat, the eyes are of immense size; and Professor Silliman thought that it regained, after living some days in the light, some slight power of vision. In the same manner as in Madeira the wings of some of the insects have been enlarged, and the wings of others have been reduced by natural selection aided by use and disuse, so in the case of the cave-rat natural selection seems to have struggled with the loss of light and to have increased the size of the eyes; whereas with all the other inhabitants of the caves, disuse by itself seems to have done its work.
[On the Origin of Species..., 1859, pp 137-138]
You might be wondering how these fish got into these caves to begin with. I can’t describe the exact process for the fish studied in this paper, but there is a general way in which this can happen. Underground lakes or streams in caves may be connected to each other during less arid periods, in some cases running from the deeps of large lakes that later try up almost entirely. In this way, a continuous population in a river or lake is broken into relict populations that are separate from each other and perhaps living in habitats that are different from the original, continuous habitat, and possibly different from each other as well. Under these conditions evolution’s just gotta happen.
BOROWSKY, R. (2008). Restoring sight in blind cavefish. Current Biology, 18(1), R23-R24. DOI: 10.1016/j.cub.2007.11.023