On page 45 of Your Inner Fish, Shubin explains how the scientists use DNA to help find out the past of organisms. What are the benefits to looking at an organism's DNA over body structure? Are there anything that could be missed by just looking at DNA? Explain how this approach led to the variations in the phylogenetic trees.
Jackie James
(jackie.james@comcast.net)
It is important for scientists to look at both DNA and body structures of animals to be able to make connections of the evolution of the animals. Looking at an organism's DNA can give some advantages, however. With DNA, scientists are able to perform experiments. Experiments are very helpful in making connections because the scientists are able to manipulate something in order to see results. Using DNA could help show that "the genes that build a fish's fin are virtually the same as those that build our hands" (45). This could not be done with just looking at fossils. This is because it is not possible to "do experiments on long-dead animals" (45). Looking at the DNA of organisms can help show connections between the genes in reptiles and mammals and other animals.
ReplyDeleteHowever, looking just at the DNA would be a mistake for scientists. Looking at fossils and the body structure of animals is also necessary in order to make the correct connections. Fossils can help to show evolutionary changes in organisms. For instance, humans walk upright, while creatures like the Tiktaalik have a more sprawled posture. Differences between us and this creature can be seen in fossils. The change in posture “came about by changes to the hip joint, pelvis, and upper leg,” (43) along with changes to the position of the hips. This information was found by looking at the fossils and the body structure of the Tiktaalik and would not have been seen by just looking at the DNA of both organisms.
So, it’s very important for scientists to look at both DNA and fossils like Neil Shubin, who splits his lab in two: “half is devoted to fossils, the other half to embryos and DNA” (45). The differences in what can be found in fossils compared to DNA are also seen in phylogenetic trees. A phylogenetic tree shows the inferred evolutionary relationships among species based on similarities and differences in their physical or genetic characteristics (http://en.wikipedia.org/wiki/Phylogenetic_tree). One tree is based on body structure, while the other is based on the DNA of the organisms. Scientists began by just looking at the body structures of animals to make connections and figure out evolutionary patterns. However, as is seen in the differences in the phylogenetic trees, just looking at the body structure wasn’t enough. But, since the DNA phylogenetic tree was so different from the body structure tree, it shows that scientists need to consider both methods while making connections in evolutionary patterns of animals.
Danielle Webb (dwebb456@gmail.com)
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ReplyDeleteLooking at an organism’s DNA is much more helpful than looking at the body structure when trying to find out about its past. For example, if one can find the DNA of a certain organism, they can relate that DNA to another organism’s. This is where phylogenetic trees come into play. A molecular phylogenetic tree relates certain organisms to each other through similarities in their DNA sequencing. Through these “trees” scientists are able to track evolution between certain species. This is possible because evolution really happens on the molecular level. Small changes in DNA are the reason for the evolution of a species. So as a scientist looks at a molecular phylogenetic tree, he/she can often trace this organism back to one of its earlier ancestors.
ReplyDeletehttp://www.ncbi.nlm.nih.gov/About/primer/phylo.html
But there are downsides to just looking at molecular phylogenetic trees. Although two organisms may appear to be similar in DNA structure, the characteristics of their appearance can be vastly different. A good example of this is the differences/similarities between chimpanzees and humans. Although the two species have 96% similar DNA, they look vastly different on the outside. This is why scientists also create morphological phylogenetic trees. Morphological trees are based upon appearance and body structure which is why these are usually easier for the average person to create. But even this system has its flaws. For instance, most people would say that identical twins would have identical DNA since they look almost exactly the same. But that is not the case due to CNV, or Copy Number Variation. This type of mutation is an alteration in the DNA sequence of a genome that results in a cell having an abnormal number of copies of a specific section of DNA. CNV can account for roughly 12% of human genomic DNA which explains why the DNA in identical twins is not identical.
http://www.sciencedaily.com/releases/2008/02/080215121214.htm
http://www.sanger.ac.uk/humgen/cnv/
Because of the pros and cons of these two phylogenetic trees, there will never be a “perfect” phylogenetic tree.
Matt Micucci (coochqbk@sbcglobal.net)
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ReplyDeleteThis question deals with the pros and cons of comparing animals based of their phenotypes and genotypes. Genotypes are the genetic constitution of an individual organism. Phenotypes are the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment. (Dictoinary.com)
ReplyDeleteAs Danielle mentioned above, genotypes are incredibly useful for finding out similarities between two different organisms. This deals with a very microscopic view of a cell, because it deals with which genes are turned on and which ones are turned off. Based off this, these genes create proteins through transcription and translation. The problem with this is that it doesn't take a look at the organism as a whole and what these protein do specifically in the body.
This is where the phenotype (or expressed characteristics) come in to play. This looks at the macroscopic view of the animal as a whole. This is usually done by bone examinations if the sample is millions of years old. As Danielle mentioned about, scientists view the change from walking on four legs to standing up on only two. "This change in posture came about by changes to the hip joint, pelvis, and upper leg: our pelvis became bowl shaped, our hip socket became deep, our femur gained its distinctive neck, the feature that enables it to project under the body rather than to the side" (Subin 43). The main problem with this use of grouping organisms together is that they may have absolutely nothing to do together in terms of niche or genetics.
So what is the best way to go around classifying groups of organisms? As both of the comments brought up before, scientists use phylogenetic trees to show both the genotypic similarities and the phenotypic similarities. http://www.nature.com/scitable/topicpage/reading-a-phylogenetic-tree-the-meaning-of-41956. One this this site brings up is that organisms of the same species "become more and more different over time." This brings up a problem for the genotype part of the phylogenetic trees. When does an organism change species completely relative to the others in that category?
Nikhil Pereira (nikhil.pereira3@gmail.com)