Wednesday, April 8, 2009
On page 136. Carroll states that "the making of the fittest is not necessarily a "progressive," additive process." This can be seen in the loss of the blood cells in the icefish that its ancestors had. He also states that there is a "pattern of gain and loss" seen in species' DNA due to natural selection. How can this be seen in other organisms? Why and how is Carroll's statement true? How does the environment affect this "pattern of gain and loss"?
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Evolution is not just a simple process of more and more adaptations that eventually cause great change. It involves fine-tuning some genes, getting rid of processes, duplications of genes, mutations that cause premature stop codons, etc. If evolution was always additive then the genome would be extremely long and species would have processes, organs, and genes that they no longer needed. This is not the case in real life. Natural selection is very useful in that it helps organisms adapt to their environment, the organisms they interact with, etc. But just as useful is the relaxation of selection which causes the fossilization of genes. If what I mentioned about adding more adaptations is “additive”, then fossilization of genes can be called “subtractive”. It is necessary for organisms to experience losses of genes that are no longer useful to them. For example, the ice fish evolved to not need blood cells anymore; if the ice fish continued to make blood cells even though they can survive successfully without them, it will just waste energy and not benefit them in anyway. Similarly, the blind mole rat’s lifestyle and dark environment does not require open eyes. If the mole rat’s genes continued to promote the opening of eyes, it would be useless. As you can see, evolution involves gains and losses of genes and processes to adjust to the environment. Some genes must be fossilized in order for other more useful adaptations to evolve. An article called “ ‘Fossil Genes’ Reveal How Life Sheds Form and Function” quoted Sean B. Carroll himself saying, "many people think evolution is always happening in a forward direction. The other side of the coin is that we lose things. Losses as well as gains make up the story of evolution." Why should preservation continue if there is no use for the genes anymore? This goes with the saying “use it or lose it” which holds very true to evolution. Fossilization must occur in organisms in order to respond to the changing demands of the environment.
ReplyDeletehttp://www.sciencedaily.com/releases/2004/09/040921081106.htm
http://www.trueorigin.org/pseudogenes01.asp
While this post does not directly answer the stated question, it is interesting and relevant. Juliya already explained the “subtractive” nature of evolution; if a certain set of genes codes for a body part or function that is no longer needed, that set of genes will be destroyed by natural selection over time. As she also stated, energy conservation is a concern; why keep an organ if it only takes energy from the rest of the body? Interestingly enough, humans currently have a slew of structures that are no longer needed. These might be done away with natural selection just like hemoglobin in icefish. Here are a few examples. Keep in mind that not all of these are definitely useless; they might serve a purpose unknown to us as of yet.
ReplyDeleteHumans get goosebumps when they are cold, scared, or angry. This is equivalent to a cat or dog’s hair rising and a porcupine’s quills raising. When it is cold, the rising hair traps air between the hairs and skin, creating insulation and warmth. Goosebumps a good response to fear for animals; they make the animal look larger. Humans no longer benefit from goosebumps; they are left over remnants of the past when we had no clothes and needed to scare our enemies not with guns or knives but with sticks. Natural selection removed our thick hair but left behind the mechanism for controlling it. This is an example of how studying the DNA of a creature can give clues to its past: studying the DNA for goosebumps indirectly tells us that we once had thick hair that we could use to create warmth and scare away enemies. Natural selection might eventually delete the gene that causes goosebumps.
Jacobson’s organ is another fascinating part our anatomy. In this case it tells us about our sexual history. This organ is located in the nose and is a “smell” organ that detects pheromones. Pheromones trigger sexual desire. This organ allows animals to track others for sex. In a time when humans could not communicate, they used this organ to locate mates. We don’t need Jacobson’s organ anymore; we can locate mates by going on dates, going to bars, flirting, etc. However, everybody is still born with this organ. In the future this organ will probably be whisked away from our anatomy.
My last example is the plantaris muscle, in the foot. This example is special, as it is already being selected for deletion by natural selection. The plantaris muscles is used by animals to grip and manipulate objects with their feet. Apes use this muscle a lot; that is why they can use their feet as well as their hands. Humans still have this muscle, but it is so underdeveloped that it is often taken out by doctors when they need to do tissue reconstruction in other parts of the body. 9% of humans are now born without it, making it a deletion in progress. This example proves without a doubt the powers of evolution and the double-sidedness (addition and subtraction) of natural selection.
There are many more examples, although some are debatable. The appendix, coccyx, third eyelid, wisdom teeth have all been subjects of curious study by scientists as to their recent benefits and functions.
http://en.wikipedia.org/wiki/Plantaris_muscle
http://en.wikipedia.org/wiki/Goose_bumps
http://en.wikipedia.org/wiki/Jacobsons_organ
http://www.bloggingwv.com/20-useless-body-parts-why-do-did-we-need-them/
As Carroll states in the book, there are “patterns of gains and losses” seen in a species. One example that Carroll states in the book is about the moths in England.. A prime example of a mutation that could be beneficial for the organisms could be found in the book on page 52 and peppered moths. In the industrial revolution there was a lot of pollution in England and North America. The pollution altered the “coloration and lichen growth on trees where the peppered moths rested” (Carroll 52). This pollution drastically altered the moths’ environments. With dark environments the population of the moths called for natural selection. In darker environments there the moths that contained more melanin were harder to spot by predators because they were blended in with the environment. This called for the moths with low melanin or lighter moths to be easier to spot by predators. Soon as predators feasted on the light colored moths, the dark colored moths were blending in with the environment as the dark form of the moth accounted for almost 98percent of the total population in some areas of England. Soon after the industrial revolution, clean air laws were forced into effect lowering the amount of emissions into the air and cleaning up the environments. This however was a disadvantage towards the dark moths as cleaner air meant lighter environments and dark moths were once again easier to spot for predators. Soon the population of the dark colored moths drastically decreased “from about 90 percent in some areas to about 2 percent”(Carroll 52). In conclusion the environment in which the moths resided in determined the rate of survival. This is a perfect example of the environment affecting the pattern of gain and loss. The gain is when the moths would be darker because of the environment and the loss is when the environment would be cleaned up and there would be a selective disadvantage towards the darker moths because they could be spotted easier.
ReplyDeleteThere is also a pattern of gain and loss in humans as well. The appendix in humans is an organ has thought to have no function in the human body. The appendix is present in many primates, and primarily used to aid in the digestion of cellulose. Located between the small and large intestines, the appendix and neighboring cecum slows down the body's digestive process. The human has lost this cellulose-digesting ability. One theory that is proposed by Darwin is that the appendix was used for digesting greens and leaves. Throughout the evolution of humans, we have been eating less and less vegetables and more and more meats. Our shape and form of our appendix has evolved to be smaller to make room for the stomach. The appendix may be a vestigial organ of ancient humans that has degraded down to nearly nothing over the course of evolution. Evidence can be shown in Koalas. The koala like us, have a cecum that is long and it is specific for the breakdown of cellulose. The ancestors of humans’ diet must have consisted of a hefty diet full of food that was rich in cellulose. As people began to eat more easily digested foods, they became less reliant on cellulose-rich plants for. The cecum was no longer needed for digestion of the cellulose empty diets. These alleles became more frequent and the cecum continued to shrink. After many years of evolution the long cecum that has helped humans digest cellulose shinked to what is now called the appendix. The appendix has not affected human life today in any way. People function normally when they have their appendix taken out and show no signs of change. Appendicitis occurs when there is an inflammation of the appendix. Obstruction of the appendiceal lumen causes appendicitis. Mucus backs up in the appendiceal lumen, causing bacteria that normally live inside the appendix to multiply. As a result, the appendix swells and becomes infected. The appendix is a perfect example of an evolution that has created no use for the people, and just a functionless organ. This is due to natural selection and how we do not care about these things.
http://en.wikipedia.org/wiki/Vermiform_appendix
www.webmd.com/digestive-disorders/digestive-diseases-appendicitis
www.innerbody.com/image/dige03.html