Monday, April 6, 2009
Fossilization of Organs
On page 129, Carroll writes about "when an entire organ or process falls into disuse". This would include very relaxed selection and extreme fossilization in which entire processes would stop to be included in future organisms. What is an example of an organism that had an entire ORGAN fossilized? What is an example of an organisms in which an important PROCESS fell into disuse and was therefor fossilized? How is this possible? Relate this to themes of Evolution and Natural Selection.
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There is actually a term used to describe the “fossilization of organs” – it is called vestigiality. Vestigiality describes homologous characters of organisms which have seemingly lost all or most of their original function in a species through evolution. These may take various forms such as anatomical structures, behaviors and biochemical pathways. This process is very gradual and occurs as organisms evolve and no longer require certain organs, behaviors, etc.
ReplyDeleteOne very commonly known example of this process occurring is that of the human appendix. The appendix has no function in modern humans. Normally, it goes unnoticed; however, if the appendix becomes inflamed, it is referred to as appendicitis and this is a very serious and deadly reaction that requires the surgical removalof the appendix. The appendix is attached to the large intestine. It was believed to have been part of the digestive system of human’s primitive ancestors. New theories, however, are beginning to arise. One such theory that has grown increasingly popular among scientists is that the appendix functions as a store-house for bacteria. Biofilms are very densely packed bacteria and they line the entire digestive tract and aid in digestion. According to Duke’s immunologist William Parker, the appendix serves as a place to store these biofilms so that this stored bacteria can replenish the intestine in the event of a bacteria-depleting illness. The evidence for this comes from the presence of an organ called the cecum in other mammals such as rabbits. This organ, while much larger than the appendix, is located in the same spot and serves the same purpose. This, of course, is only speculation and it has been shown that in modern humans the appendix is not necessary and provides so selective advantage. The evidence for this comes from the people who have had their appendixes removed and experience no side effects other than those from the surgery itself.
Two other examples of vestigial structures in animals are the eyes of moles and the wings of emus. Moles live underground in complete darkness. Their ancestors, however, lived above ground and had eyes because that provided them with a selective advantage and increased their chances of survival for obvious reasons – the ability to see allows organisms to seek out food, spot predators, etc. Eyes, however, require light in order to work. When moles began adapting to live underground as that also provided them with the selective advantage of providing a safer environment with less predators in which they could more easily raise their young, their eyes became dysfunctional. Their eyes no longer worked and the moles developed other methods such as a more acute sense of smell to compensate for their lost sight. Now, moles only have eyespots marking the place where their ancestor’s eyes once were. Like the mole, the emu also adapted in such a way that one of their physical features was no longer needed. The ancestors of emus at one time had the ability to fly and as such had fully functional wings. Emus are much too large to fly; instead, they have developed long, powerful legs that allow them to outrun their predators. This leaves their wings without a purpose and only serves as reminders of the emu’s evolutionary history.
This occurrence of vestigiality throughout the natural world is a perfect example of Carroll’s theory called “Use It or Lose It” (mentioned on page 130). Carroll proposed that if organisms no longer use a gene, that gene will eventually lose its functionality. In all of the previous examples, the organisms adapted in such a way that certain features were no longer necessary or helped increased the organism’s survival rate. This causes natural selection to stop selecting positively for the genes that controlled these features. Any mutations in the genes would go unselected, resulted in the loss of functionality over time.
Sources:
http://en.wikipedia.org/wiki/Vestigial
http://www.innerbody.com/image/dige03.html
http://discovermagazine.com/2008/jan/function-of-appendix-explained
http://www.talkorigins.org/faqs/vestiges/appendix.html
http://www.absoluteastronomy.com/topics/Vestigial_structure
http://www.eggscape.com/birds.htm
By using natural selection and by random mutations evolution is able to occur to allow a species to adapt in order to survive in a certain environment or survive with certain demands such as energy. In this way evolution brings in new processes that may cause older ones to be put aside and not used as frequently. When this occurs fossilization may occur rendering the sequence of genes useless. This occurs in those genes that are not expressed and that are more prone to mutations. This is because natural selection is constantly keeping an eye on those genes that are crucial to the organism and so those genes that are unused are overlooked and start to become broken down. Normally the fossilization of a gene is like a final say in the sense that there is no turning back from fossilization; such as the production of hemoglobin of the ice fish discussed on pages 21-26. Because the ice fish were placed into an altered environment where temperatures were below freezing, they adapted in order to allow for a more efficient way of survival. The environment around them, the cold water, is more oxygen rich than those of warmer waters and so ice fish are able to receive more oxygen from this environment. There were also mutations that by natural selection allowed the ice fish to adapt such as the scaleless skin in order for absorption of oxygen to take lay throughout the skin and also the gills with larger surface area for a higher oxygen intake. Another adaptation is the large heart of the ice fish. All these new characteristics render hemoglobin to have a declined use in the ice fish and so the genes that code for hemoglobin have become fossilized in that certain fish.
ReplyDeleteAn example of an organ that has become fossilized is that of the gills. As animals began to move to land they required a more efficient system to transport oxygen to their vital organs. Because fish have a decreased need for energy the gills are reasonable organs to which they receive their oxygen which in turn gives them energy. The surrounding environment also allows gills to be efficient source of energy. The surrounding water leaves gills wet allowing for oxygen and carbon dioxide diffusion. The capillaries are also placed at a countercurrent allowing a higher concentration of oxygen to enter the bloodstream to be carried throughout the body. Yet when animals began to grow in size and go to land where they could no longer swim but needed the use of limbs, such as legs, the energy demand was increased causing adaptations to occur. Since there was no longer water surrounding land animals they required a way to keep the organ in which oxygen diffusion occurred moist. Through mutations and natural selection the lungs were adapted and in humans the four chambered heart allowing for oxygenated and deoxygenated blood to stay completely separated. You may be asking why this adaptation from gills to lungs even occurred. Competition for resources can be seen as a main reason. With an expanding population with each generation, it is likely that there was competition for food in order for certain species to survive. Land, at the time uninhabited, had an abundance of resources that was being unused. This would mean very few competitors for survival. Also the oxygen in lakes became depleted and fish found themselves in need of a greater quantity. This lead some fish to swim to the surface and take a gulp of air from above water. Seeing that oxygen sources were seemingly unlimited on land random mutations and natural selection occurred and species adapted in order to survive for even small amounts of time on land. This can be seen with the lungfish which normally have two lungs except for the Australian lungfish who has one. The lungs of the lungfish have smaller sacs that allow for a greater surface area for gas exchange to occur. This increases the amount of oxygen to the body and allows for more energy to be produced. This is different from normal fish who have swim bladders that are one sac, unlike the lung which has many tiny sacs. With this characteristic the lungfish is able to go on land and dig into mud during dry seasons and survive with the use of lungs and estivating. By slowing down their metabolic rate the lungfish in a sense hibernates and uses less energy. From this adaptation to having lungs as well as gills to amphibians to mammals that are almost entirely land living, natural selection and evolution created the lung and with it a more efficient cardiac system. This rendered gills as useless for animals that lived almost entirely on land because gills did not even have enough moisture surrounding them on land to be rendered useful. In this case the genes encoding to create the organ of the gills was fossilized in land animals such as humans because they were no longer a selective advantage and were then allowed to be mutated and fossilized.
An example of a process that is fossilized would be that of the baker’s yeast, Saccharomyces kudriavzevii (130-131). In baker’s yeast the most common source of energy is galactose. When this is unavailable most baker’s yeast is able to convert this galactose into glucose through 7 gene codes, four of which are for enzymes that are used in the process while three are used for proteins that control the production of the four enzymes. This is no the case for Saccharomyces kudriavzevii which lives primarily on decaying leaves. This leaves a shortage of sugar-rich environments for the yeast to grow and so when tested the yeast was unable to utilize energy from glucose. When looked into scientists had found that the seven genes that are part of the process of deriving energy from glucose were missing various parts of the code. It can be seen that since the environment in which this specific yeast lived in had no use for such gene codes, they were prone to mutations and were fossilized. Interestingly enough the surrounding codes were untouched and still perfectly intact. This shows the susceptibility to mutation that codes that are unused have compared to those codes that are still functional.
http://findarticles.com/p/articles/mi_m1134/is_8_115/ai_n16807321/pg_5/
http://science.jrank.org/pages/5841/Respiratory-System-Respiratory-system-fish.html
http://answers.yahoo.com/question/index?qid=20080202091811AA5mxVE
http://en.wikipedia.org/wiki/Dipnoi#Lungs
http://evolution.mbdojo.com/evolution-for-beginners.html
A prime example of fossilization of an entire body part can be seen when one examines cetaceans (whales, dolphins, etc.). Cetaceans are mammals even though they live in marine environments. They evolved from an animal that used to live on land. There is much evidence of this fact. Cetaceans have to come up to the surface to breathe air. The bones in their fins resemble the bones of jointed hands in land mammals, and their spinal movement, which is vertical, has more resemblance to a running mammal’s spine than the horizontal movement of a swimming fish’s spine.
ReplyDeleteAll land mammals have legs. This begs the question: where did the legs of cetaceans go? The cetaceans use their tail and forelimbs to swim. The ancestor of the cetaceans probably did the same. The fact that it wasn’t using its hind limbs for swimming meant that they were no longer necessary for the animal’s movement. Hind limbs tend to be used for locomotion and serve no other function. As a result, the hind limbs were no longer under intense natural selection because they had no bearing on the animal’s ability to survive and reproduce.
Over the course of time, the genes that code for the hind limbs of cetaceans were allowed to accumulate mutations. These mutations resulted in degenerated forms of the leg. Although the degenerated leg would have been a selective disadvantage to a land mammal because it would make them slower and more prone to predation, in the marine mammal, it may have been a selective advantage. The reduction in size of the hind limbs would have reduced resistance on the cetacean while it was swimming, resulting in a faster animal. This animal would have been able to escape from predators easier, or catch its prey quicker. Selection may have begun to favor cetaceans with degenerated legs.
Over the course of time, these hind limbs have evolved to a state that renders them almost unrecognizable. All that is left in most cetaceans is a tiny bone in the mid-region of the animal. From this bone, we can see that some of the genes that used to code for the hind limbs still function. Overall, the vast majority of the genes are now fossil genes. Similar to a fossil gene, which still bears some resemblance to a functional gene, this fossil body part is no longer functional, but it still bears a small resemblance to the hind limbs of land mammals.
http://en.wikipedia.org/wiki/Evolution_of_Cetaceans
http://www-personal.umich.edu/~gingeric/PDGwhales/Whales.htm