Evolution’s Speed – How Common Are Substitution Point Mutations?

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This is part of a series: How Fast is Evolution?
Previous: Evolution’s Speed – What Kinds of Mutations Are There?

I might have figured out how to test for evolution’s limit in a lab, and I’m reading up on genetics and evolutionary biology to find if it’s practical.

Today, I’m learning about the most common kind of mutation: point mutations.  Specifically, I want numbers for substitution mutations, as opposed to single insertions or deletions.

I quickly found something interesting.  Two of the ways that geneticists give mutation rates are:

  1. Per individual base pair
  2. Per generation

As a general rule, single-celled creatures have nearly the same number of mutations per genome, no matter how big their genomes are.  That rate is usually 3-4 per 1000 creatures, per generation (don’t forget to click “view inline” or “view popup” to see the mutation table).  I’m quoting the figures for DNA-based creatures, since they’re what I’d probably use in my experiment.

How reliable are these numbers?  They’re probably in the right ballpark.  Here are some things to keep in mind about them:

  • They usually come from very small samples
    This experiment, for example, estimates the mutation rate of one phage (specialized anti-bacteria virus) by:

    • Starting with a known number of phages and hosts
    • Putting the phages in a situation where it’ll be very obvious if they have a mutation that breaks a particular gene
    • Extracting the now-obvious mutants
    • Sequencing the broken gene in a bunch of mutants
    • Extrapolating the mutation rate in this one gene to the entire genome
  • They probably don’t catch all mutations
    In the experiment above, they basically count the phages that can’t survive due to a particular genetic disease, and then sequence the part that broke.  But if that same gene had a mutation that didn’t break it, the researchers would never know.
  • There are mutation hotspots and cold spots
    As far as scientists know, some genes mutate much more than others.  There are even certain codons – 3-letter codes for single amino acids inside proteins – that are extremely “hot”, while everything around them is not.  They mutate so much more than most of the genome that they can throw off the average.  For example, in E. coli’s LacI gene (whatever that is), ~72% of mutations to that gene happen in a string of 13 nucleotides (genetic letters).

Also (getting a little ahead of myself here), according to one study, 80-85% of all mutations are substitutions.  He cited a study looking for whether heat-loving bacteria get rid of bad mutations better, but it doesn’t expressly give this percentage.  He probably calculated it from some of the numbers that the researcher did give.  To find them in that paper, search it for “35 single-base indels”.

This is part of a series: How Fast is Evolution?
Previous: Evolution’s Speed – What Kinds of Mutations Are There?

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