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Phenetic approaches

Phenetic approaches - no one molecular clock Different...

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Phenetic approaches : DNA hybridization, sequence divergence from sequencing, restriction patterns or restriction maps. In each case the data would be in the form of a single number indication the similarity or difference between the DNAs of each pair of species in the study. Cladistic approaches : direct sequencing, restriction maps and restriction fragments (fragments less desirable). In each case one would look for shared derived character states (nucleotide, restriction recognition site) among taxa. With restriction sites shared loss is unreliable as a uniting character because the nucleotide change could have occurred anywhere in the recognition sequence. Like refrigerator/pizza/invertebrate example. Molecular approaches to systematics for us to think about the rates of molecular evolution . If DNA or proteins evolved at a constant rate in all species, then one could use estimates of sequence divergence to build very reliable phylogenies. If there was a molecular clock we could determine the "true phylogeny". Fact is, there is
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Unformatted text preview: no one molecular clock . Different proteins and DNA sequences evolve at different rates. Why? Different functional constraints . Different proteins do different things and some can do their structural or functional job with any of several different amino acids at many of the positions ( fibrinopeptides ). Other proteins will not function properly with "any" amino acid changes ( histones : two amino acid differences between peas and cows!). Intron sequences less constrained than coding exon sequences, and hence introns tend to diverge faster than exons. Synonymous sites evolve faster than non-synonymous sites, again due to different functional constraints (i.e., some form of selection against "incorrect" sequences). Nuclear DNA tends to evolve slower than mitochondrial DNA (in vertebrates). Unit evolutionary period : time required to observe a given unit of divergence. A 1% divergence of vertebrate mitochondrial DNA takes about 250,000 to 500,000 years....
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