Monday, January 20, 2014

Can some genomes evolve more slowly than others?

I've been teaching my students about random genetic drift, phylogenetic trees, and the molecular clock. It's hard for undergraduates to understand that trees based on sequences are reflections of the fixation of nearly neutral alleles by random genetic drift. That's because they, like almost everyone else, think of evolution in terms of natural selection and adaptation.

It's even harder to grasp the idea of a molecular clock even though it's been around for fifty years. It was back in the 1960s that scientists like Emanual Margoliash noted that the rate of substitution of amino acids in every lineage was remarkably similar [The Modern Molecular Clock]. We now know that this is because the alleles are fixed by random genetic drift and that the rate of fixation by drift depends only on the mutation rate. It looks like the mutation rate is relatively constant in all lineages (bacteria, protozoa, plants, animals, etc.). This isn't a big shock since the vast majority of mutations are due to errors in DNA replication and the fundamental biochemistry of DNA replication and repair are similar in all species.

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