Thursday, June 2, 2005

45. Can't see why they don't see

Nature's caverns often survive void of light, alive only in the shadow of obscurity. The absence of light in these caves postulate many inquiries into the life that it encapsulates. For the contemplation of those that live in total darkness augments the discussions of biological adaptations while quenching the appreciation for nature's diversity. Adapting to life in the dark solicits answers to the question- does one need to see? And if one does not need to see- for it is dark and nobody sees- does, one needs eyes?

The answer to this question, for many species is no- and in fact, in living in darkness for millenniums, not only do they not see, they no longer have eyes. Many species, including insects, crustaceans, salamanders, spiders and fish, have lost their eyes through adaptations to their environment. Thus, the real question for scientists and other interested parties, is how does this happen? The answer is not completely certain and, furthermore, not quite as easy as one might think (noting the temptation to enter into Lamarckian evolution). This fascinating discussion takes place in the June 2005 issue of Natural History (Why Do Cave Fish Lose Their Eyes), and provides much of the material that I base my discussion.

Lamarck proposed a theory of evolution based on the inheritance of acquired characteristics. That is, if an individual, over the course of his or her lifetime, acquired a trait (or talent), that trait would subsequently be passed onto his or her offspring. The most famous example cited is the long necks in giraffes, which Lamarck would explain as generations upon generations of giraffes reaching and extending their necks to reach leaves in the tallest trees, and then passing along that characteristic to subsequent generations. The temptation in our question is to apply the reverse, to propose that species that do not use a specific characteristic are susceptible to losing it. As Luis and Monika Espinasa describe in the article, "Lamarck believed that unused organs shrivel until they disappear. In short, use it or lose it."

As we know now, our genes care not if we use a characteristic- only that the genes survive to be reproduced at rates that are statistically successful. Hence, natural selection is the principle of nature selecting the most successful genes based on reproductive and survival rates. Adaptations are the successful reproduction of mutations- genetic alterations that are tested in nature's arena of "survival of the fittest."

Understanding this theory, in light of regressive evolution (the loss of a trait such as the eyes in cave fish), one can easily argue as to what advantage there is in not seeing- when fish with eyes cannot see either. In other words, how can nature "select" eyeless fish over fish with eyes when it is rather obvious that neither has a reproductive advantage over the other? Humans also have these "rudimentary organs," such as the appendix and the coccyx, which in the course of our evolution proved to be of use to our ancestors, but matter not to us now as a matter of survival. Darwin himself struggled with the issue, writing, "It is scarcely possible that disuse can go on producing any further effect after the organ has once been rendered functionless. Some additional explanation is here requisite which I cannot give."

There are two types of rudimentary organs- embryonic and those that develop into adulthood. Gill slits are an embryonic example of ancient relationships that never develops as the embryo does. Deciphering evolutionary lineages through embryonic development is just one of the things that makes embryology so interesting.

The appendix represents a rudimentary organ that does develop and that through comparative anatomy can be studied to reveal evolutionary lineages, homology and purpose. Another interesting biological field, comparative anatomy provides diverse insight into vestigial organs. Many other mammals have functioning appendixes, that, to be fair and to make matters more confusing, must be sorted out between evolutionary relationships and convergent evolution.

Biologists today, according to the article, offer two hypotheses for rudimentary organs: pleiotropy and neutral mutation theory. Pleiotrophic effects are the alterations to multiple characteristics, inherently unrelated, by a single gene. The best example, also described in the article, is sickle cell anemia. As a mutation, deformed red blood cells survived in certain, primarily African, populations because the parasite that causes malaria could not survive in the "sickle" cells. However, the consequence is that this adaptation can lead to other problems such as anemia and organ damage.

Neutral mutation theory is the negligible effect of a mutation on an individual or species. Thus, since the mutation does not hinder survival, it continues to be reproduced generation after generation- increasing its likelihood in the population.

In a series of experiments, only possible in the last few decades, the differing theories were tested by switching the (eye) gene of an eyeless fish with the (eye) gene of a fish with sight during embryonic development. What they found is that the gene for the fish with sight produced an eye in the eyeless fish and vise versa. In other words, the gene for sight in the eyeless fish is functional and must be "turned off" by some other mechanism. This discovery essentially rules out neutral mutation theory.

Further to the pleiotrophic hypotheses, scientists have isolated a "master control" gene that "whose modified expression leads to blindness in cave fish." More interesting is the fact that the smaller the eyes were, the more taste buds are produced. Moreover, when a fish develops without eyesight, the bones in the skull shift, allowing for a bigger olfactory pit and larger olfactory epithelium. Hence it is likely that the non-existence of eyes, which in the natural environment of cave fish are useless anyways, offers the opportunity for an improved sense of smell- which can certainly be a competitive advantage.

In conclusion, the authors note the "classic example" pleiotrophic effect and summarize that, "Natural selection is not acting on cave-fish eyes; it is acting instead to increase the fish's sense of smell." In essence, evolution has said, on the issue of eyeless cave fish, that if in the process of amplifying some senses (which can be a competitive advantage), the fish loses its ability to see (which in this case is not a competitive advantage)- then the fish species is better suited to compete in its environment.

Nature, it seems, is always up to something- always looking for the slightest advantage and the most remote opportunity. And to appreciate nature's intricacies, scientific exploration celebrates in allowing us to not only ask the questions, but to also "see" what others cannot.

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