Anna Leng (annaissbananas@gmail.com)
Thursday, March 17, 2011
Orderly Change
In chapter seven, Shubin describes the bodies of animals as being very orderly. He explains that "each of our organ 'knows' its size and place in the body. We grow in the correct proportions because the growth of the bones in our arms is coordinated with the bones in our fingers and skulls" (118). Animals are continually changing, yet they are still very organized organisms. Has this always been the case for animals, even early in evolution or was this something that evolved as an adaptation over time? Is this function entirely necessary to survival, or could we do without it? Use examples of living/extinct/mutated animals and incorporate gastrulation and cell development to help explain why or why not.
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Animals have always changed as they developed and development is essential for survival because it creates the animal that can survive on its own. However, the manner of development has changed immensely over evolutionary time.
ReplyDeleteOne organism that changes continually throughout development is the sea urchin, which was one of the first deuterostomes. The blastula of a sea urchin has an animal pole and a vegetal pole, the vegetal pole being the side of the urchin with the vegetal plate. As the blastula matures, it begins invagination, which is when the vegetal plate buckles and forms the archenteron (Campbell 1028). As a matter of fact, evolutionary developmental biologists think that the Hox gene changed over time to create deuterostomes since Hox genes determine where limbs and other body segments will grow in a developing embryo or larva (Miller 5). As a matter of fact, Pax6/eyeless, which controls eye formation in all animals, has been found to produce eyes in mice and Drosophila, even if mouse Pax6/eyeless was expressed in Drosophila (Xu and Woo 3). This means that both mice and flies use the same set of genes to create eyes, which means that that part of their genome is identical and must have come from a common ancestor. To continue with urchin gastrulation: after invagination starts, the filopodia contract and drag the archenteron across the blastocoel. Then, the fusion of the archenteron with the with the blastocoel wall forms the mouth, and the archenteron is now the digestive tube. This process shows the changes brought about by development and how the organism could not survive without this essential change.
Another example of an organism that undergoes massive changes during development is a frog, which came about later than the urchin in evolutionary history. In the early gastrula of the frog, the animal pole, defined by the fact that it will be the future ectoderm, begins to envelop the vegetal pole. The blastopore is created by invagination, and the sheets of outer cells then roll over the dorsal lip (which is called involution) and move toward the inside of the gastrula and form the archenteron. In late gastrulation, the endoderm-lined archenteron has completely replaced the blastocoel (Campbell 1029). This development is different from the urchin’s, but is just as successful, meaning that it has the same end result, assuming nothing went awry: an organism was created exactly as intended and is fully functional. Without this semi-complicated process, all that animals would create would be a zygote, which would mean that animals wouldn’t be able to survive.
benitorosenberg12@comcast.net
http://www.ncbi.nlm.nih.gov/pubmed/9006082
http://www.ncbi.nlm.nih.gov/pubmed/100000411686
One may argue that evolution itself is a highly organized process. Although the word “organized” may imply that it was accomplished with some sort of intention, it is important to note that evolution is completely random yet highly selective. Evolution can be random, depending on the various factors that cause sources to become scarce. However, evolution can also be highly selective, and therefore organized in the way that it is results in the differential survival and reproductive success of individuals who can survive in its competitive environment, and death to those who cannot.
ReplyDeleteFor example, a population undergoing an exponential population growth will eventually meet its carrying capacity, causing competition among its population for various resources. Whether it is for food, finding a mate, or for a territory, the individual organisms that can out-compete its competitors will most likely survive, while those who could not acquire the scarce resources would die. In this manner, known as the survival of the fittest, the organisms with traits that will increase the likelihood of surviving will survive and reproduce, allowing its offspring to inherit the qualities that it had. This process repeats until the inherited traits become completely distinguished from the traits that its ancestors had. Another important idea to note is that "[i]t is not the strongest of the species that survive, nor the most intelligent, but the one most responsive to change” (Clarence Darrow). The fiercest lion in the world would not be able to survive in the Arctic, and the fluffiest penguin would most likely not be able to survive in Africa (unless it lives in a zoo). This simple quote explains that what is most important for an individual organism is not to be strong, bulky, or fast, but to be able to express a phenotype that would effectively respond to an environmental stimuli.
There is never a clear-cut answer any kind of inquiry, and so I would like to say that the answer to your second question is both yes and no. With respect to embryonic development, an organized process would indeed be very important. If the entire process of embryonic development were completely disorganized, traits would not be inherited correctly and the offspring and mutations would be likely to occur. Because Ben Rosenberg has extensively covered the importance of having an organized system, I will focus more on the aspect of mutations.
ReplyDeleteMutations in the development of a human body is generally disadvantageous. Various the The sickle cell disease, which is an autosomal recessive genetic disorder, is where a mistake in the genetic code of an individual causes the production of irregular shaped red blood cells (http://learn.genetics.utah.edu/content/disorders/whataregd/sicklecell/index.html). These red blood cells, shaped like sickles, become stuck in the blood vessels, and cause various problems throughout the body. However, one may also argue that mutations are beneficial to bacteria. Because bacteria undergo such a rapid division of cells, they are often more prone to errors and mistakes in the genetic code. However, these mutations allow bacteria to invade other species with ease because the memory cells that were produced by the primary immune response no longer can be used against them. In addition, a research done by the University of Madrid has shone that a drug that causes more mutations to occur in infected cells can actually kill cells that have been infected by a HIV virus (http://www.popsci.com/science/article/2010-01/how-scientists-hope-turn-rapid-mutation-against-viruses). In this way, being organized has its pros and its cons, but what it most important, I believe, is that it is the most responsive to change. If mutations can allow your species to survive and reproduce, I say go for it.
Works Consulted:
Campbell Biology
Your Inner Fish
Clarence Darrow
http://learn.genetics.utah.edu/content/disorders/whataregd/sicklecell/index.html
http://www.popsci.com/science/article/2010-01/how-scientists-hope-turn-rapid-mutation-against-viruses
(Keigo Tanaka; tanakarus3@hotmail.com)