Explain the various genealogic trees that have been made, focusing on the more recent trees that have been approved by the scientific community. In doing so, explain the major differences between the trees and note the scientific reasoning behind the changes. Is there one tree that is more accurate than another? Why or why not? Why bother spending times on creating genealogic trees, especially when it will most likely be made obsolete in a few years? And of course, as this is AP Bio, use scientific terms and reasoning in your responses.
Thursday, April 2, 2009
Genealogic Trees
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Biologists have various methods of creating such trees through classification and division; there are four concepts through which biologists analyze species: the ecological species concept, the pluralistic concept, the morphological concept, and the genealogical species concept.
ReplyDeleteEcological Species Concept - "defines a species in terms of its ecological niche, the set of environmental resources a species uses," (Campbell 468).
Pluralistic Species Concept - "the factors that are most important for the cohesion of individuals species vary. In some cases, reproductive isolation may be a key unifying factor for a species. In other cases, adaptation to a specific ecological niche may be the main factor in species cohesion. In still other cases, the integrity of the species may depend on some combination of reproductive isolation and a unique niche," (Campbell 468).
Morphological Species Concept - "characterizes each species in terms of a unique set of structural features, remains the way we distinguish most species," (Campbell 468).
Genealogical Species Concept - "defines a species as a set of organisms with a unique genetic history - that is, as one tip on the branching tree of life. The sequencing of nucleic acids and proteins provides data that researchers are now using to define each species in terms of unique genetic markers," (Campbell 468).
With each of these concepts now established, let's turn to the main task at hand. Genealogical trees are made using the concepts as above. According to Sean Carroll, however, a better way to decipher a specie's relationship with another specie is through the genealogical species concept. "It also relies on DNA, but rather than being based on the degree of sequence similarity, it looks for the presence and absence of certain landmarks in specific places in species DNA. These landmarks are produced by accidental insertions of junk DNA sequences near genes. Particular chunks of junk DNA, called long interspersed elements (LINES) and short interspersed elements (SINES)," (Carroll 99). Using physical genetic markers established during mapping, as on Campbell page 389, biologists can compare LINES and SINES to that of other organisms. The steps for making a tree are outlined (and condensed here for the purpose of informing) on page 99 in Making of the Fittest.
To Make A Genealogical Tree:
1. Identify a set of SINES to be surveyed in one species.
2. Specific regions of another species are examined.
3. Analysis confers: if another species' DNA contains the same SINE as that in one species or close to the number of base pairs extension by that of a SINE, there is a kinship/relationship.
Most of the recent genealogical trees today are in forming the invertebrate/vertebrate trees. Take a look at the trees on page 636, 640, and 694 in Campbell. The traditional trees are based upon body-plan grades for page 636, but the next tree is based upon sequencing of small subunit ribosomal RNA (SSU-rRNA) - page 640. On the 636 tree, there are dotted lines to show the incongruities between the placement of phyla; however, with the newer version of the tree, there are no incongruities based on the SSU-rRNA. Similarly the 694 tree is shaped with such markers. The trees that are considered more accurate are those that are much more inclusive and rely upon the genetic markers since they carry the scars of evolution. The only way such trees can be made obsolete is to spend time creating genealogical trees. Without such time being spent, there would be no relationships and the biologists would have to sit around for their beards to grow. All in all, the majority of this discussion relates to the biological theme of evolution: using different means to come to a conclusion upon the relative kinship between species.
Finally: I urge readers to read the Campbell pages I have stated (389, 468, 636, 640, and 694) as well as Carroll p. 99.
As Danyal Sheikh has mentioned the different concepts of species classification, I will focus on the evolution of genealogic trees dependent on the physical features of the organisms to the DNA of the organisms. The first genealogic trees have been made based off the outward appearance and body plans of organisms. As we have learned in the invertebrates unit, the different grades of the animal phylum branched due to the body characteristics of the organisms. For example, the first branch from the ancestral colonial choanoflagellate was the clade Porifera, which was due to the absence of true body tissue (Campbell 636). The remaining branches were due to characteristics like body symmetry, body cavity, etc. Because this type of genealogic trees had areas where the relationships between organisms were uncertain, scientists adopted methods such as examining DNA or protein sequences to create the genealogic trees. The scientists searched for similarities in the nitrogenous bases that made up the organisms. Re-examining the genealogic tree of the animal phylum, the “nucleotide sequences in the small subunit ribosomal RNA” (Campbell 640) were examined for similarities. The scientists acknowledged that the relationship between organisms depended upon more than just the outward appearance and body planes of organisms. Even though the animals had vastly different outward appearances, there were similarities in their genome that unified them. This diversity of outward appearances and unification of DNA sequences relates to the theme of Unity and Diversity, which focuses on the similarities and differences of organisms. However, the method of examining the gene sequences of organisms was flawed due to the sheer number of genes that need to be examined in order to gather a sufficient amount of data. Because there are only four nitrogenous bases, the few genes that were examined could have contained dubious similarities that could be mistaken for a clear relationship between organisms. Now, the new method of creating genealogic trees focuses on the examination of “certain landmarks in specific places in species DNA” (99). The landmarks are called long interspersed elements (LINES) or short interspersed elements (SINES) and the elements are inserted near genes. Because there are not active mechanisms that remove the LINES and the SINES, the interspersed elements remain in the genome of the organism, serving as a continual tracer. The interspersed elements are passed on from generation to generation, which relates to the theme of heritable information, the “inheritance of biological information in the form of DNA molecules” (Campbell 22). The interspersed elements indicate whether or not two species have the same ancestor. If the two species share the same interspersed elements near the same gene, then the two species have descended from the same ancestor. The final method of creating genealogic trees is the most accurate, as the insertions of SINES and LINES into the genome is a rare occurrence, and the possession of the same SINE or LINE would indicate a clear relationship between species. The SINES and LINES can be easily detected because the detection depends on an already established SINE or LINE, and does not require the calculation of gene base similarity on a wide scale. I cannot agree with Danyal Sheikh that certain genealogic trees will become obsolete when replaced by another. All genealogic trees strive to show relationships between organisms, and different genealogic trees show different relationships. Even if a tree is updated with new information, the previous relationships will not be lost, therefore, a genealogic tree can never disappear. The trees provide clues and information on how life formed on Earth. The trees also allow people to better appreciate the interconnectedness of the species on Earth.
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