While introducing the "Handy Genes" chapter about using DNA to find the evolutionary history of organisms, Shubin relates genetic information to physiological features (45).
How does this type of deduction lead to variations in phylogenetic trees? What can be misleading about tracing the past of an organism solely through DNA or solely through physiological features?
There are two types of phylogenic trees: molecular and morphological/developmental. A molecular phylogenic tree is mainly based on molecular data such as ribosomal genes, Hox genes, and other protein-coding nuclear genes, while a morphological/developmental phylogenic tree is based on morphological and developmental data, such as body symmetry and germ layers.
ReplyDeleteWe will use an invertebrate phylogenic tree as an example. Both the molecular and morphological phylogenic trees of invertebrate phylum start of the similarly, with metazoa. In the morphological tree, the metazoa branch off into eumetazoa and the phylum porifera, while in the molecular tree, metazoa branches off into eumetazoa and another class called porifera, which branches off further into two phylums: silicea and calcerea. There are further branch offs from porifera on the molecular tree because within the class of porifera, there is some variation/difference of ribosomal genes, Hox genes, or other protein-coding nuclear genes that cannot be identified solely through morphological/developmental data. As both of the trees continue to branch further out, it is apparent that molecular data is much more detailed and specific than morphological data, therefore there are generally more branches on a molecular phylogenic tree.
A morphological phylogenic tree alone can be misleading because there may be groupings in the tree that have similar morphological and developmental features, but their molecular features - including different types of genes or DNA portrayed may be very different - different enough to be put into separate groupings. A molecular phylogenic tree alone can be misleading because there may be some molecular similarities between organisms that have very similar morphological characteristics. For example: there may be two organisms with similar protein-coding nuclear genes but one may be bilaterally symmetrical and one may be radially symmetrical. One type of phylogenic tree alone cannot be very accurate.
Sujin Ko (sujinko93@gmail.com)
Source: Campbell
Shubin
First off, 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)
Phylogenetic trees are evolutionary diagrams that help us visualize the interealatedness between organisms. This information is seen as important because by understanding organisms that are related to each other and organisms that are related to us, we can understand how mechanisms that would be hard to experiment on in humans would work. An example of this is the ongoing research with the ZPA gene that is found in sharks and chicks. Due to the fact that we have been able to study the genetic sequences of these animals, we are aware of the similarities inherent between these organisms. Due to this, we are able to construct experiments around the use of these animals (humanely) and find ways around the dilemmas of human experimentation (http://www.philvaz.com/apologetics/TREE.gif) (http://www.ncbi.nlm.nih.gov/About/primer/phylo.html)
ReplyDeleteHowever, many deductions can be made through the physiological similarities between us and organisms. An example of this would be similarities seen between chimps and humans. The first theories of this were found before DNA testing had been truly perfected. However, by looking at similarities and differences between structure in us and old world apes, we were able to find almost identical structures in brain development between us and them. http://www.cs.unc.edu/~plaisted/ce/apes.html
However, both of these organizations come with their limitations. In the case of physiological organization, the problem can be seen in convergent evolution. Sandlace are fish belonging to the class Actinopterygii and have the main characteristic of independent eye movements (their eyes can move in different directions). Similarily, chameleons can move their eyes independently but are part of the class Reptilia. Through Evolution, both of these distantly related species were able to stumble upon the same evolved characteristic due to selective pressures. http://jeb.biologists.org/content/205/9/1241.full
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