A well-known icon of evolution is the blind fish that lives in numerous caves in Mexico. At some time in the past, sighted fish living in the sunlit waters of Mexico entered lightless caves and, over time, lost the ability to see. To evolutionary biologists, this is proof that the individual fishes that entered these caves adapted to their environment and evolved into an entirely new species.
However, as more and more evidence is discovered regarding the incredible variability of the genetic code of all living beings, it is becoming clearer that every species possesses the ability to adapt to its environment by turning on or off certain genes and changing its inherent nature, sometimes even radically. Moreover, if the species' environment changes back to what it had originally been, that same gene (or gene sequence) can be just as readily switched off, and the original form of the creature will reappear.
Examples of the genetic on/off switch have been witnessed in bacteria since the 1970s. When bacteria undergo high levels of stress from their environment, they automatically undertake what is called the SOS response. As the April 2006 issue of Scientific American reported:
When bacteria are under extreme stress, they try various means of fixing the damage as an initial step. They then SWITCH ON GENES whose protein products precipitate a spate of mutations that occur 10,000 times as fast as those arising during normal cell replication. In essence, the cells undergo a quick identity change (Stix 2006: 82 [emphasis added]).
For example, when the well-known bacterium E. coli is subjected to severe damage from the antibiotic ciprofloxacin (known as cipro), it sends out a genetic SOS. This instant mutation prevents cipro from binding to its target, a protein in E. coli known as gyrase; if E. coli did not engage this on/off switch, it would die (Ibid.).
Studies have shown that this response works in higher animals as well. The Mexican cave fish is a remarkable example. According to the journal Science News, 'Fish and other creatures lose their sight after generations living in caves. Yet working vision genes from one parent can partly make up for defunct versions from the other parent, at least in young fish, reports Richard Borowsky of New York University' (Milius 2008: 21).
Although there are many species of blind cave fish around the world, the Mexican tetra is the favorite of biologists, because different forms of it will breed in captivity, and it has a sighted version that lives in open waters. Borowsky mixed and matched wild, blind tetras from 29 different Mexican caves; he then tested vision in the pure strains as well as in hybridized cross-breeds. When the fry (baby fish) were just over a week old, he performed a vision study on them and found that 39% of them could see (Ibid.).
Interestingly, the cave hybrids usually went blind as they grew up, but when Borowsky crossed blind cave tetras with sighted tetras from sunlit waters, 'some of the offspring did retain vision into adulthood,' according to Science News (Ibid.).
More studies need to be performed, of course. Nevertheless, Prof. Borowsky's findings are extremely fascinating and just as telling. The blind cave fish did not evolve into a new, separate species, as has always been assumed. Rather, their genetic programming allowed them to change their physical characteristics to fit their surroundings, but these changes were not permanent. Blind or sighted, the tetra is still the tetra.
Milius, S. 2008. 'Seeing Again.' Science News 173, no. 2.
Stix, G. 2006. 'An Antibiotic Resistance Fighter.' Scientific American 294, no. 4.