Muller’s Wheel

H.J. Muller is famous for (among other things) his argument for the evolution of recombination involving the purging of deleterious alleles (dubbed Muller’s Ratchet). In a nutshell, Muller observed that, in the absence of recombination, deleterious mutations will fix in populations because every chromosome will, eventually, obtain a mutation which decreases the fitness of the organism. Recombination allows for the movement of deleterious mutations off of chromosomes, which decreases the genetic load in a population (because fewer deleterious mutations will fix).

Much of the recent work in population genetics centers on the effect of recombination rate on the efficiency of natural selection. Regions of low recombination have lower effective population sizes; because natural selection is most effective in regions of higher recombination, more deleterious mutations and fewer advantageous mutations are expected to fix in regions of low recombination. (Please note that there is evidence suggesting that all that is required for selection to work is some recombination — and selection works equally well in regions of low and high recombination — but what follows can be best viewed as rampant speculation, so I will conveniently ignore the aforementioned reference.) We also know that selective sweeps greatly reduce neutral polymorphism around the site under selection (decreasing effective population size, as well).

So, given that natural selection is most effective in regions of high recombination (ie, large effective population size), and natural selection reduces effective population size, I present a new “tool”: Muller’s Wheel. Here’s my logic:

  • Natural selection is most effective in regions of the genome with large effective population size (high recombination rate).
  • Selective sweeps (which are fairly common if you believe some people) will reduce the effective population size surrounding a region under selection.
  • With a reduced effective population size, natural selection will be less efficient, thereby allowing recombination to increase the effective population size.
  • The large effective population size allows for more efficiency of selection, which leads to a selective sweep, which decreases effective population size, which decreases the efficiency of selection, etc., etc.

I need something to drink.


  1. #1 Ross
    June 27, 2007

    It seems intuitive enough.

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