I don't think that the proportion of nucleotide bases as much to do with natural selection as the actual sequences themselves. DNA codes RNA which then codes for protein. The information for what amino acid to make when is determined by the three nucleotide sequence called the codon. There are only 20 common amino acids but there are 64 different codons so there is redundancy in the genetic code. A mutation can be caused from a single nucleotide being deleted, added or changed to another nucleotide. For example, you may have a DNA sequence like...
TACAAAGCGTTGAAACGCCGG. When spread out into triplets you get
TAC-AAA-GCG-TTG-AAA-CGC-CGG. These triplets will determine what amino acids will be made. If we delete a nucleotide,
TACAAAGC-TTGAAACGCCGG, then the triplets will be,
TAC-AAA-GCT-TGA-AAC-GCC-GG. This is a different sequence of codons and the new codons may code for a new amino acid, or they may not because almost all of the amino acids have more than one codon. The same thing can happen if a nucleotide is added, or a single nucleotide is changed. If the mutation causes a change in the protein function, then I would imagine it could impact natural selection. But I'm not sure if just by looking at the base proportions you can learn much about the organism and how it evolves.
There is some interesting things though. You guys talked some about G and C. I assume you all know that in DNA, A-T and G-C. Well what is cool is that G and C have a strong bond than A and T. So if you were to look at a bacteria who lives in thermal vents, I would expect you to find that its GC content was higher than the AT content since G-C bonds are more stable.
And there is a lot of repeated sequences in the genome. I forgot what they mean, but I remember learning that you can find CG repeating regions before some genes. Wounded Kind said that cytosines are often times methylated and its in these regions where a lot of that methylation occurs. Methylation actually causes genes to be inactivated, though.
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