Stephen Jay Gould describes vestiges as "senseless signs of history" (56). Vestigial traits demonstrate long lasting evolutionary relationships. Coyne describes many examples such as flightless birds having wings, sightless mole rats that have remnants of eyes, and whales that have leg bones. Humans also have vestiges such as the coccyx. These are all quirks that provide a solid argument for evolution. However, some of our vestiges are not just "quirky". Appendixes can rupture. Wisdom teeth can become infected. These medical issues usually become hazardous later in a person's life (after they have passed on their genes).
Can you name any other deadly vestiges in humans or in other organisms? What are the possible evolutionary histories behind them? How do they affect the organisms ability to survive and reproduce? How may it be considered an evolutionary "roadblock"?
Feel free to discus wisdom teeth or the appendix.
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In talking about vestigial organs, I would first like to define what a vestigial organ is. Coyne defines vestigial organs as organs “that make sense only as remnants of traits that were once useful in an ancestor” (56). Coyne goes on to add that all vestigial organs are not functionless; rather it is a vestigial organ because “it no longer performs the function for which it evolved” (58).
ReplyDeleteOne vestigial organ in the human body is the appendix. Currently in the modern human body, the appendix can cause death if it becomes infected, which is called appendicitis (https://health.google.com/health/ref/Appendicitis). People who have their appendix removed generally are in good health.
One potential ancestral purpose was put forth by Charles Darwin. He hypothesized that the appendix was used for digesting leaves as primates, an ancestor to human beings. This belief can be seen by studying the koala. The cecum in the koala is very long, enabling it to host bacteria specific for cellulose breakdown; human ancestors may have also relied upon this system and lived on a diet rich in foliage (http://en.wikipedia.org/wiki/Vermiform_appendix). When people began to eat foods that are more easily digested, the need for cellulose-rich plants was reduced, and through evolution and natural selection the appendix in humans has degraded down to nearly nothing.
New studies argue that the appendix has a function in fetuses in modern human beings – a secondary function. Endocrine cells have been found in the appendix starting with the 11th week of life in the fetus; these cells contribute to homeostatic mechanisms (http://www.scientificamerican.com/article.cfm?id=what-is-the-function-of-t).
Lastly, the appendix has been found to serve as a lymphatic organ in modern human beings. Because the appendix is rich in lymphoid cells, which fight infections, it is suggested that the appendix can play a role in the immune system (http://en.wikipedia.org/wiki/Vermiform_appendix).
In conclusion, the appendix is no longer used for the function for which it evolved, which could possibly have been to digest leaves. However, as already stated, a vestigial organ simply means the organ does not perform the function for which it evolved; the hypotheses for new functions of the appendix do not change the fact that the appendix is a vestigial organ.
In addition to the potentially deadly effects of the appendix in humans, as Ryan mentioned, there are also many other potentially harmful vestigial traits or structures that can be seen in other organisms.
ReplyDeleteOne of these, mentioned by Coyne on page 65, are the side toe bones of modern horses. The ancestors of the modern horse we see today used to place their weight on three toes on each appendage, and this evolutionary past can be seen today with some atavisms that appear among some individuals. These toes may have provided extra stability for the horses' ancestors, improving their ability to stay upright perhaps in running away from a predator.
However, in modern times, these structures appear merely as thin structures alongside the now single toe base. Perhaps this gradual change over time may have evolved out of a need for more turning ability; the existence of structures along the sides of does inhibit the ease with which horses can perform sharp turns.
Whatever the case, it is apparent that today these structures can prove to be actually harmful for horses. One common negative condition of these vestigial toes is the 'splints'. This may occur when a horse is performing sharp turns or putting uneven load on its hooves. These seemingly normal actions for such an active animal can cause such swelling and pain for the individual organism, leaving them 'lame' and less able to move easily.
In fact, the bone growth can get so out of control that the vestigial 'toes' may interfere with normal knee function. In such cases, surgery by a veterinarian is required. Without surgery, the horse's normal functions would be extremely impaired, leaving it with far less of a chance of survival than its healthier peers.
(http://extension.missouri.edu/publications/DisplayPub.aspx?P=G2742)
All of these potentially negative effects are the direct effect of having the vestigial traits of 'toe' bones running along side the primary base. If horses simply had one solid base of support, there would be a far lower rate of occurrence of this negative condition that could, before the days of human intervention, greatly reduce a horse's chances of survival and reproduction.
There are many different human vestiges. There are those that can cause major damage and death, the appendix or infected wisdom teeth. However, there are also vestiges that have simply lost their selective advantages and are solely remnants of past evolutionary changes as opposed to structures that could be detrimental to us now.
ReplyDeleteOne example of a harmless human vestige are the erector pili muscles. When humans evolved from their primate ancestors, they lost the majority of their body hair. This could be because they had a semi-aquatic phase in which we lived partly in the water (evidence of this also comes with our webbed hands and feet) so hair/fur caused excess drag. Or this could be the result of combatting deadly parasites that would live in the dense fur. They would no longer have a home. Either way, hairlessness became a selective advantage, and then a sexual selective advantage because it would be a sign of fitness. However, erector pili muscles were used in situations when the animal needed to appear larger. Bumps would come up on the skin and push the hair out to make the animal bigger and more intimidating. Without hair, these muscles are useless, however, we still have them and they are used often. These are the muscles that give us "goose bumps."
http://science.discovery.com/top-ten/2008/organs/organs-04.html
http://www.nytimes.com/2003/08/19/science/why-humans-and-their-fur-parted-ways.html?pagewanted=1
There are also other vestiges, along with wisdom teeth and the appendix that are detrimental to our health. Adenoids and tonsils are often removed from young children because they trap bacteria and are prone to infection. If this infection spreads, it can be potentially deadly. Therefore, they are removed. Removing your tonsils and adenoids has no adverse effects on your body. It is theorized that adenoids and tonsils evolved to help the immune system deal with deadly parasites and infection. Their are both made of lymphatic tissue, so this is a likely theory. However, these parasites and infections are no longer around, therefore, they have no positive effect on the bodies immune system. They are remnants of structures that were necessary for survival.
http://www.pedisurg.com/PtEducENT/adenoids.html
Many vestiges are detrimental to our survival, but not all. Vestiges such as the coccyx and erector pili muscles have no effect on survival, however, at one time they were selectively advantageous structures. At least they are not like the appendix, a relatively useless organ that only causes death and death related symptoms...
An example of a vestige found in snakes, particularly pythons and boa constrictors, are tiny hind leg bones buried in muscles towards their tail ends. These bones can be considered to be vestigial traits because, as Coyne put it, "[they] no longer perform the function for which they evolved" (58). Nowadays, these limbs, which are formally known as pelvic spurs, are used to grasp and tickle during courtship and mating, as well as combat with other males in some species. According to Hussam Zaher, a scientist who uncovered a fossil of a snake with leg bones and hip bones in 2006 (Najash rionegrina), "morphologically, the legs are absolutely functional, although we cannot really say for sure how these snakes used their legs." He hypothesizes, though, that the snake used its leg bones to dig and crawl - it probably led a burrowing lifestyle similar to modern burrowing snakes. The pelvic spurs aid the snakes in surviving and reproducing - according to an article published in the Journal of the International Herpetological Society, the pelvic spurs are usually held in the "erect" position and used by the male (at least) to stimulate the female. The male at all stages of the mating act has a tendency to dig his pelvic spurs into the female's body. Whilst crawling over the female the male's spurs will scratch the surface of the female snake often making a distinct scratching sound. On other occasions the male's spurs will move rapidly in and out, up and down, again to stimulate the female snake. This stimulation exercise increases the chances that snakes reproduce, and it also increases the amount of eggs that a female snake ends up laying. Up until now, no disadvantages have been found in having pelvic spurs as opposed to not having them. Within the variance in size of existing leg-buds, there's no corresponding variance in survival. There's no reason to assume that snakes with smaller leg-buds have any reproductive advantage over snakes with larger leg-buds.
ReplyDeletehttp://www.amnh.org/exhibitions/darwin/evolution/vestigial.php
http://en.wikipedia.org/wiki/Pelvic_spur
http://www.livescience.com/animals/060419_snake_hips.html
http://www.freerepublic.com/focus/chat/2339339/posts