So I've found myself referring in an answer once again to the idea that complexity (insofar as it's a quantity for which "number of genes" is a proxy) has an upper bound, limited by Muller's ratchet-related processes. The idea being that if harmful mutations happen faster than natural selection can cull them, the genome will accumulate harmful mutations and the organisms' fitness will degrade over time; and that this imposes an upper bound on the size of a functional genome, because the number of harmful mutations that can happen obviously depends on the size of the functional genome (i.e. how many bases there are where a mutation would likely be harmful).

I knew about Muller's Ratchet, and when referring to it I usually refer to the Wikipedia page for it. But I first came across the concept that this defined an upper bound on complexity (as opposed to being a huge paradox of evolution and a plausible reason for sexual reproduction) in this Less Wrong post (it might have been edited since I first read it).

I've pretty much adopted this concept as a thing that happens; it makes sense to me, and I have seen other references to such an upper bound limited by mutation rates from biologists, for example most recently in this talk by Nick Lane (at 39:15).

However whenever I talk about this in discussions or debates and look for sources I cannot find an easy-to-link source that presents this argument, as seen from within the field of biology. The Wikipedia page on Muller's ratchet only talks about it in the context of asexual reproduction and LessWrong isn't a biology source. The post claims that George Williams makes this argument in his book Adaptation and Natural Selection, and that may also be what Nick Lane is referring to in the video, but a book is hard to refer to online and I would expect such a powerful concept to have more material about it anyway.

Is this concept actually the consensus in the field of biology, or evolutionary biology failing that? Either way, where can I find good, preferably linkable, sources explaining the concept or debating it if it isn't the consensus?

  • $\begingroup$ Muller's ratchet is a process that depends upon the population size. What population size are you willing to consider? The question will be a bit hard to address I am afraid but you might want to search the literature for the terms '[mutational meltdown]', 'mutation load' (see this post on the maximum number of babies possible) and 'drift load'. $\endgroup$
    – Remi.b
    Commented Mar 16, 2017 at 14:38
  • $\begingroup$ Btw, please realize that a proxy should be a correlated variable, not a definition. What I mean is that if you don't clearly define complexity, it sounds funny to consider having a proxy for it rather than considering that you define complexity as being the number of genes. Of course many people will not find intuitive to define complexity as the number of genes also that might make some crop botanist happy :) $\endgroup$
    – Remi.b
    Commented Mar 16, 2017 at 14:40
  • $\begingroup$ @Remi.b I know what a proxy is and I used the word correctly. I didn't define "complexity" in my post at all, deliberately so, but the most exact expression of what I meant would be "there are many concepts, some well-defined others vague, associated with the word 'complexity' in biology, a subset of which can be considered a quantity correlated with gene number such that gene number can be used as a proxy for it (whether specifically and vaguely), and when I use the word "complexity" here I am referring to any member of that subset". $\endgroup$
    – Oosaka
    Commented Mar 16, 2017 at 14:48
  • $\begingroup$ To clarify; you are specifically asking for sources that discuss Muller's ratchet in relation to an upper bound for complexity? $\endgroup$ Commented Mar 23, 2017 at 19:14
  • $\begingroup$ @fileunderwater Yes. I hope that is clear enough, if not I would still be happy with whatever you think I meant, if nothing else it would help me know what I need to clarify. $\endgroup$
    – Oosaka
    Commented Mar 23, 2017 at 19:52


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