Monday, April 6, 2009
Taxonomy
Contrast how the new modern genetic-based method of species classification and ancestry differs from the older version of taxonomy. Explain how genetic biologists are able to use "junk" DNA (use specific terms, hint: p.99) to establish new hierarchies and discover ancestry and relationships between different species. Also, give at least 2 examples of genetic-based discoveries that have led to the reorganization of taxonomic branches. Finally, explain your opinion on whether or not genetic based organization over classical body plan organization will affect your biophilia (in essence, will it increase or decrease your respect for the diversity of life, specifically in regards to our closest ancestors).
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Old taxonomic methods were forced to classify organisms based purely off of physically apparent characteristics; symmetry, presence of a body cavity, key body organs, etc (see Campbell Chapters 32-34 for a review). However, DNA evidence has allowed us to see connections between many species that are not obvious based on the organisms' bodies themselves. Junk DNA in particular has been useful.
ReplyDeleteJunk DNA, because it does not code for any active, transcribed and translated traits, is not under selective pressure. This has multiple implications. Because junk DNA is not under selective pressure, it will not change based off of divergence of species' due to habitat. For example, an organisms exons (coding segments) are under regulation and pressure due to the environment; these genes will change (or remain the same) based off of the niche of a species. However, the niche of a species is not necessarily reflective of its genetic lineage; sometimes evolution works "backwards", moving a species back into a niche that its ancestors may have held (ex: flatworms losing a body cavity). Because of this connection between environment and genes, exons are not a reliable source when determining genetic lineage. However, junk DNA does not code for proteins and is thus not under selective pressure. This also means that the steady rate of mutation on junk DNA is not filtered out by natural selection. From all of this information we can make various conclusions. Junk DNA is passed down genetic lineages; as it is not affected by selective pressures, changes in organism niches will not affect the junk DNA, thus helping to maintain the junk DNA down the genetic lines. Sean Carroll mentions this phenomenon briefly when discussing LINES and SINES as a method of determining genetic lineages, saying "once a SINE or LINE is inserted, there is no active mechanism for removing it" (Carroll 99). There is no removal mechanism because natural selection doesn't act upon junk DNA, SINES and LINES included.
Additionally, because of the steady rate of mutation, by looking at how much one segment of junk DNA in an organism differs from that same segment of junk DNA in another organism, we can roughly gauge the relative relatedness of species. Again, this is possible because junk DNA is not under selective pressures and mutations are not filtered out.
Genetic-based taxonomic research has led to myriad reorganizations of phylogenic trees. In class we studied in detail the phylogenic tree of invertebrates, both the new and the old classification systems. The old, based on body characteristics, had presence of a body cavity as a key separating trait. The new, based on genetic evidence, recognized that presence or absence of a body cavity did not signify genetic relatedness; instead this tree puts more focus on protostomatic versus deuterostomatic organisms.
DNA evidence has also cause us to create the new domain of archaea. It was originally thought that these organisms were more bacteria (Carroll 70). However, DNA analysis led biologists to realize that archaea were COMPLETELY different from bacteria, thus leading to the modern three domain, six kingdom system we use today.
It's funny that you ask about biophilia, because the biodiversity unit really did increase my appreciation of and camaraderie for my fellow animals. Having studied all the organisms out there, all the body plans, and all of the lifestyles, any mammal and really most vertebrates don't seem all that different from us humans. The DNA that we share only serves to strengthen the feeling that we are closer than we know. And I had this feeling for all mammals. Primates? Sheesh... we're practically brothers for all of the traits, both genetic and physical, we have in common.
The older phylogenetic tree based on body plan grades classified animals into aceolomates, pseudoceolomates, and coelomates. The coelomates were further divided into Protostomes, Lophophorates, and Deuterostomes. The new version of the phylogenetic tree is based on a combination of genetic compassion, “mainly on one, the gene for SSU-rRNA” (Campbell 642) and body plan grades. This new classification established the Protostomes, and Deuterostomes as the overarching grades. Two new divisions were created under Protostomes: the Lophtochozoans and Ecdysozoans. This new tree was able to resolve the placement of the phylums Nermertea, Broyzoa, Phoronida, and Brachiopoda, which had been ambiguous in body-plan tree. The classification of these phylums under Lophtochozoans was established by based on combination of morphology and 18S RNA. All phylums Lophtochozoans have embryos with a lophophore. The similarity of the 18S RNA also account for their relatedness. -www.ucmp.berkeley.edu
ReplyDeleteEvolution select for traits that would give an organism the most advantage their niches. This natural selection process could lead to the addition and deletion of traits. As a result, using body plan to classify animals would be unreliable because organisms that shared common ancestors may end up looking completely different due to their selection pressure in their environment. As a result, a more workable solution to taxonomy would be to analyze the junk DNA to determine the historical relationship between species. In genealogy, long interspersed element (LINES) and short interspersed elements (SINES) are types of junk of DNA that function as tracers since “once a SINE or LINE is inserted, there is no active mechanism for removing it” (Carroll 99). Organisms that diverged earlier share less SINES and LINES than organism that diverged later.
I agree with Eric Bersin’s assertion that the genetic based organization strengthens our brotherly bonds to other organisms. By classifying animals based on body plans, it creates a sense of segregation that makes us feel distant to those animals that do not physically resemble use. However, we must not forget that as diversify as life forms are, we are all affiliated to each other through our universal genetic code (Theme 7). By studying SINES and LINES, we can trace our steps back to the beginning. Life has diversified over billions of years but there are still a handful of immortal genes serves as proof that we have shared a common ancestor. The genetic based phylogenetic tree has allowed me to not only make a connection to our closest relative, the chimpanzees, but to every living thing on this planet. This sense of unity towards all of Earth’s life forms adds to my biophilia.