EvidenceforEvolution-07-web - Evidence for Evolution 1...

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Evidence for Evolution Phylogenies (cont.) Phylogeneticists use all the tools at their disposal to try to better understand relationships among organisms. The tools they use include morphology, anatomy, fossils, biogeography, and molecular data. They try to determine evolutionary homologies. I should note that we can never know with complete certainty that the phylogenies I’ve drawn are correct, because we haven’t observed the process of speciation. (Evolution probably just takes too long in most cases!) However, there are some cases, like these examples, where the evidence is so overwhelming that it makes little sense to think that the phylogeny is different from that which we’ve drawn. Another important, but sometimes confusing, point about phylogenies is that they can be twisted around the nodes but still convey the same information. < Figure 26.3 > Molecular phylogenies In the 1960’s it became possible to look at the molecular composition of proteins, and by the 1970’s it was possible to determine the DNA sequence of genes in organisms. Around this time, it became apparent that one could compare DNA sequences between organisms and use this information to determine phylogenetic relationships. This was an idea that was largely pioneered by Dr. Walter Fitch, a member of the faculty here at U.C. Irvine. The basic idea is as follows. Imagine that we’ve determined the DNA sequence of a gene from some different organisms. STEPS : Step 1 . Once we have some DNA, we have to compare the DNAs from different organisms. The first step to building a phylogeny is to “line the DNAs up”, or align the DNAs. Here’s an example of a DNA alignment between a cat and a dog gene. cat. a t cgatcatg t tggcgcat g cgattttgagc t cgagactacgaca c ata dog. a c cgatcatg a tggcgcat a cgattttgagc c cgagactacgaca g ata An important note: the DNA sequence has to be from the same gene in each of the organisms or else the comparison won’t make any sense! (If you compare one gene from one organism with a different gene from a second organism, it won’t be possible to align the DNA sequences.) One of the most commonly used genes is the ribosomal 1
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Evidence for Evolution DNA (rDNA) gene. Ribosomal DNA is a gene that carries the information for ribosomal RNA. The ribosomal RNA eventually becomes part of the ribosome. The ribosome is an important component of the machinery that makes proteins.
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